Below I discuss briefly major characters in the order in which they will be encountered in the node and family characterisations. The emphasis here is on providing entries both to the general morphological literature, but especially to articles where there is much information on variation of a single character; I also give indication of how good the sampling is. Definitions of the characters mentioned here and of related features are provided in the Glossary. Note, however, that these characters and the "states" I use in describing the variation within them should not simply be converted into data for phylogenetic analysis. Some, for example, fruit types, are logically connected with the nature of the gynoecium, others, such as seedling morphology and testa and tegmen types, might each yield a dozen characters, while in these and others (all too many) the states are arbitrarily delimited. Finally, it should be remembered that because the same term is used for similar features of two plants, it cannot be assumed that this entails a hypothesis that the structures in the two organisms had a common evolutionary origin, i. e., that they are homologous under one definition of the term. However, if botanical terms were reformulated so as to refer only to synapomorphies (shared derived features), changes in the terms we use would be almost innumerable!
There are of course many other features mentioned in individual family characterisations. For these, general distributions may be unknown or not really of much interest because the feature is varying in a phylogenetically informative fashion only locally, or the feature may be unique or almost so.
I have necessarily relied much on information in Airy Shaw (1966), Cronquist (1981), Engler (1900-onwards) and Engler and Prantl (1887-1915; ed. 2, 1924-onwards - cited under individual authors), Goldberg (1986, see also 2003), Hutchinson (1973), Mabberley (1997), Takhtajan (1997: beware of typographical errors!), and N. Smith et al. (2004). The meticulously-documented series of papers that make up the Generic Flora of the Southeastern United States (ed. C. E. Wood Jr., e.g. Wood 1983) and in particular the series of volumes edited by Kubitzki (1993a onwards) have been invaluable; most papers are cited separately in the individual family accounts. Much information is summarised in Rolf Dahlgren's diagrams (e.g. Dahlgren 1975) and is included in recent large-scale phylogenies (e.g. Hufford 1992: Rosidae; Nandi et al. 1998: all flowering plants). Zomlefer (1994) was helpful, especially early on. Information on many characters mentioned below is taken from these references and also from the general references mentioned at the beginning of each character group; this is definitely not the best method of documentation (see Introduction).
Habit/life form (see also inflorescence position). This is indicated in a very general fashion; for an interesting survey of plant life forms, see Raunkiaer (1934). For a survey of epiphytes, see Madison (1977). Groover (2005) discusses the multiple derivations of the herbaceous life form from the woody, and the reverse; the latter can often be detected by details of xylem anatomy, and is discussed under individual families. The growth of monocots, predominantly a perennial herbaceous group, can be interpreted as variation on a basically sympodial theme (Holttum 1955). Procheş et al. (2006) discuss the richness of the geophytic flora in the Cape region of South Africa, at the same time observing how hard it is to define what a geophyte was...
Leake (1994; see also Imhof 2007) surveyed the biology of echlorophyllous myco-heterotrophic plants, observing that they were commonest in monocots, while only broad-leaved angiosperms were echlorophyllous parasites (for the latter, see Barkman et al. 2007). Note that myco-heterotrophic and parasitic plants often lack stomata and are otherwise distinctive anatomically; they often also have parietal placentation, many ovules with long funicles, small seeds, little endosperm, an undifferentiated embryo, etc. The floral morphology of some parasitic plants in particular is often very distinctive and difficult to relate to that of their putative photosynthetic relatives. They have a distinctive physiology, too; for instance, being a parasite often seems to entail having a high rate of transpiration both day and night, and if they do have stomata, these are often permanently open (Stewart and Press 1990). For general literature on parasitic plants see Kuijt (1969: still useful), the Parasitic Plants website (Nickrent 1998 onwards), and also Heide-Jørgensen (2008).
Plant architecture. Hallé et al. (1978) list a number of models, growth patterns based on variation in a number of characters and to which plants may be assigned (see also Hallé 2004; Bell & Bryan 2008). These are rarely mentioned here except in passing since they are not often constant in taxa of any size. However, some of the individual characters that Hallé et al. (1978) discuss, e.g., ortho- and plagiotropy (the latter in particular comes in various flavours), continuous and discontinuous (rythmic) and monopodial and sympodial growth, are mentioned below. Indeed, I find thinking of plant growth in terms of these variables very helpful, much more so than thinking of models (see also Barthélémy & Caraglio 2007). León Enriquez et al. (2008) examined variation in plant models in the context of phylogeny in Phyllanthaceae, and found that whether growth was rythmic or continuous correlated well with the major clades recognised. Note that leaf insertion (see below) varies independently of plant architecture.
Dessication tolerance. Although this is basically a physiological character, a relatively few plants, usually more or less herbaceous, show various degrees of dessication tolerance, some being known as "resurrection plants", examples being Myrothamnaceae, many Velloziaceae (Alpert & Oliver 2002; Proctor & Pence 2002; Dickie & Pritchard 2002). Perhaps not surprisingly, a number of plants have seeds that can tolerate dessication, also surviving very high temperatures (they are anhydrobiotes) (Mertens et al. 2008), and dessication tolerance in at least some flowering plants seems be derived from propagule dessication tolerance, not other stress-tolerance genes (Fisher 2008). Indeed, some of the genes involved in seed development and maturation, the LEC1- type HAP3 genes, are inductively expressed under drought stress in non-seed plants like lycophytes and ferns - and of course the seed dries out during the maturation phase (Xie et al. 2008).
Hegnauer (1962 onwards) is the source of information, and Siegler (1998) is also invaluable. Gershenzon and Mabry (1983) can still be read with profit, Wink (1999) discusses the biochemistry of plant secondary metabolism that yields the probably in excess of 100,000 secondary metabolites found in flowering plants, the distribution of a number of which are of systematic interest, Kite et al. (2000) provide a useful survey of the distribution of some chemicals in the monocots, while Strauss and Zangerl (2002) summarise the diversity of the major classes of compounds involved in chemical defence in angiosperms (see also insect-plant and fungus-plant relationships mentioned at the end of this page where secondary metabolites are commonly involved). Waterman (2005) provides a general survey of the diversity of secondary metabolites. Note that sampling for chemical characters may be spotty (although often better than that for many embryological characters?), indeed, sophisticated screening methods are leading to the detection of some classes of secondary metabolites in several families from which they had not been recorded (Lapcík 2007). Thus myricetin occurs in Magnoliaceae and Annonaceae (Crawford et al. 1986), although its apparent absence there had earlier been a matter for comment (Kubitzki & Reznik 1966; Gornall et al. 1979). It also can be difficult to say that a compound is really absent; is an alkaloid present at 0.000001% concentration present or not (Waterman 2007, q.v. for the problems facing chemical systematics)? Furthermore, the part of the plant sampled may affect the metabolites that are detected (e.g. Webber & Miller 2008 and references). Almost all authors rightly emphasize the importance of the pathways by which compounds are produced; apparently distinctive substances like napthoquinones can have quite diverse biosynthetic origins, yet at the same time furanocoumarins in Ficus, Apiaceae and Rutaceae - quite unrelated - are produced by the same pathway (Berenbaum 1983 and references). In general, chemical characters are like others in terms of their value in phylogenetic construction; some, like the presence of glucosinolates, are very valuable, many others, much less so. Still others - presence of ellagic acid is an example - show a fairly resticted distribution, but are still very variable in terms of presence/absence (D. Soltis et al. 2005b). Monocots in general have a less diverse secondary chemistry than do other angiosperms. Some distinctive secondary compounds are in fact synthesised by endophytic fungal or bacterial associates of the plant (e.g. Tan & Zou 2001 and Gunatilaka 2006 [both surveys]; Markert et al. 2008); Convolvulaceae and especially Poaceae are distinctive in this regard.
Acetylenes. Acetylenes are relatively uncommon in flowering plants (see Bohlmann et al. 1973 for a survey, etc.), although they are notably common in some families in Asterales and Apiales, where they seem to substitute for iridoids.
Alkaloids. For a general introduction, see Hegnauer (1988; Aniszewski 2007). Alkaloids are extremely diverse (some 10,000 different kinds are known), and similar alkaloids can be formed via different biosynthetic pathways. Alkaloids can be classified according to the nature of the skeleton nucleus that they have, some of which are mentioned below. For a survey of the structure and distribution of the isoquinoline alkaloids, see Buck (1987) and Bentley (1998), while Liscombe et al. (2005) put the origin of these alkaloids in a phylogenetic context. Detection of the activity of (S)norcolaurine synthase, the gateway to benzylisoquinoline synthesis, may suggest that these alkaloids will be found in groups in which they are not currently known, e.g. Chloranthaceae. For the synthesis of benzylisoquinoline alkaloids in the opium poppy (they are synthesised in the sieve tubes!), see Bird et al. (2003); these are normally, but not always, derived from tyrosine. For pyrrolizidine alkaloids, see Ober and Hartmann (2000) and Anke et al. (2004), also Boraginaceae, Apocynaceae and Asteraceae. The distribution of some distinctive types such as the erythrina (Erythrina, Cocculus) and homoerythrina (Phelline, Dysoxylum, Schelhammera, some conifers) alkaloids is very scattered. For the distribution of calystegines (tropane alkaloids) see Dräger (2004), for ergot alkaloids, see Gröger and Floss (1998).
Aluminium accumulation. Useful surveys are found in Chenery (1948), Webb (1954), Kukachka and Miller (1980), and Jansen et al. (2002b). Broadley et al. (2001) summarizes patterns of heavy metal accumulation in general in angiosperms.
For calcium oxalate crystals, see below under anatomy.
Some information on the distribution of chelidonic acid is taken from Ramstad (1953).
Coumarins are involved in plant defense, some 800 different kinds being known form about 80 families (see also Berenbaum 1983).
Cyanogenetic pathways - see the surveys by Saupe (1981) and Lechtenhberg and Nahrstedt (1999) and comments by Hegnauer (1986), also Siegler and Brinker (1993) and Miller et al. (2006). Some 60 different kinds of cyanogenic compounds are known from about 130 families; the distribution of some, e.g. the cyclopentenoid cyanhydrin glycosides, is of particular interest (see Malpighiales, families near Achariaceae), and these are derived from the non-protein amino acid 2-(2-cyclopentenyl)glycine (e.g. Spencer & Seigler 1985; Webber & Miller 2008). Cyanogenic compounds are stored as ß-glycosides and are activated by a ß-glycosidase; ß-glucosidases activate both the "cyanide bomb" and the "glucosinolate bomb", and the topology of an (abbreviated) tree produced by a phylogenetic analysis of the enzyme sequence is similar to that of other trees of angiosperms (Morant et al. 2008).
Ethereal oils are more or less volatile and aromatic; they are often terpenes of some sort. They may be found in idioblasts, i.e. specialised cells, as in basal angiosperms, and leaf blades, tepals, etc., containing ethereal oil cells may be punctate when a light is shone through them.
Fats, whether saturated or unsaturated, and their corresponding fatty acids (see also polyacetylenes below) are widespread, and are often important components of seeds. Petroselenic (cis-6-octadecenoic) acid has a distribution of considerable systematic interest (see Garryales and Apiales in particular: Kleiman & Spencer 1982 for a survey).
Flavonoids. For general information about flavonoids, see Giannasi (1988), Bohm (1988), and Harborne and Baxter (1999), for minor flavonoids, see Bohm (1988), for isoflavonoids in particular (often phytoalexins), see Reynaud et al. (2005), Mackova et al. (2006) and Lapcík (2007), for biflavonoids, Geiger and Quinn (1975), and for flavonoid distribution in monocots, Williams and Harborne (1988). As Stafford (1990: p. 9) notes, "the use of duplicate or triplicate names for the same group of flavonoids is an unfortunate aspect of flavonoid nomenclature". Hegnauer (1986) suggests that flavonoids are of little use when looking at higher-level relationships. Anthocyanins are common pigments, and when found in leaves they are usually red, perhaps protecting the young leaf (Manetas 2006: insect herbivores cannot see red, and if they are green, they contrast with leaf color).
Fructose oligosaccharides are sometimes stored by the plant; families in which they have been detected are listed by Pollard and Amuti (1981). Such oligosaccharides are mentioned only when they occur reasonably consistently within a family - so they are not mentioned in Cornaceae, where they occur in a few species of Cornus, but not others, nor in Melianthaceae, where they are reported only in trace amounts. For the distribution of stem fructans in monocots, see Pollard (1982).
Glucosinolates are largely restricted to Brassicales (Fahey et al. 2001 for a summary). Some 100 different forms are known. They are broken down by the enzyme thioglucoside glucohydrolase (myrosinase) into glucose and isothiocyanates, mustard oils, which have a R-N=C=S arrangement (see Cyanogenetic pathways above for ß-glucosidases). Mustard oils are uncommon in flowering plants (see also Putranjivaceae - Malpighiales), and their presence is systematically important. For details of glucosinolates and their distribution, see Fahey et al. (2001: note that some of the more unexpected records of glucosinolates, e.g. in Phytolaccaceae and Pittosporaceae, are reported by Daxenbichler et al. 1981, however, there are no vouchers and the reports must be considered suspect - J. E. Rodman pers. comm.), for their ecological and agricultural implications, see Courtney (1986), Chew (1988), Brown and Morra (1997) and Rask et al. (2000), and for herbivores that are attracted to mustard oils, see Host-plant preferences below.
Gums, non-crystalline mixtures of sugars and organic acids, are scattered in angiosperms, being notably common in Fabaceae, where they are often found in the seeds (Nair 1995).
Hemicelluloses, see polysaccharides.
Iridoids. Data are taken mainly from Bate-Smith et al. (1975), Hegnauer and Kooiman (1978), and especially Jensen (1991, 1992, 1999) and Jensen et al. (1975b). Iridoids are formed from two isopentane (isoprene) units, and they are cyclic C10 compounds, often (always?) with O in one ring; they are formed in two main ways; they may be route I (secoridoids) or route II (carboxylated and decarboxylated iridoids: see Glossary). Although a very useful character, presence/absence of iridoids may vary in closely related taxa (e.g. Jensen et al. 1975b). (Note that Inouye and Uesato [1986] question classifying iridoids based on their biosynthetic pathways.) Iridoids are produced by the mevalonate pathway, and although polyacetylenes have a very different origin, being derived from their corresponding fatty acids, in familes or groups of families of asterids one or other of these two classes of compounds dominate to the exclusion of the other. Families that contain iridoids are largely asterids, although there are a few in Saxifragales, etc.; all told, some 600 different structures are known. For further details of possible evolutionary pathways of iridoids, see Albach et al. (2001a). As glycosides, many have distinctive, often adverse, physiological effects on mammals, micro-organisms, etc. Bowers (1988) summarises aspects of the relationships between iridoids, plants and insects and Nishida (2002) examples of insects that sequester the compound; see also Host-plant preferences below.
Iron uptake. The roots of most plants extrude protons which lower the pH of the adjacent soil and solubilize iron. Those of grasses, however, produce siderophores which chelate ferric ions and are then subsequently taken up by the plant (Schmidt 2003). I do not know the exact distribution of this character.
Latex is mentioned only when it occurs (see also laticifers, below). Gutta has the trans- configuration of isoprene units, and is distinguished from latex, which nas the cis- configuration, in the descriptions.
Lignans and neolignans. Lignans are ß,ß'(8-8')-linked dimers of cinnamic acid residues or their biogenetic equivalents (diaryl propanoids), and are widespread, although they apparently do not occur in monocots. They are also found in gymnosperms. Neolignans, more directly linked, or at least other than ß,ß', are found especially in magnoliids (monomeric allyl- and propenylphenols are also found in the groups where they occur). For definitions (which are not consistent, and depend either on structure or on biosynthetic pathway), distribution, etc., see McRae and Towers (1984), Whiting (1985: structure) and Ayres and Loike (1990).
Lignins are complex and still poorly understood components of secondary cell walls. Their composition is of considerable interest, and under certain conditions they yield substantial amounts of aromatic aldehydes - various combinations of vanillin, syringaldehyde, and p-hydroxybenzaldehyde. In particular, the Mäule reaction, in which a reddish colour is obtained after treatment of the plant sample with potassium permanganate, then dilute hydrochloric acid, and then ammonia, signifies presence of syringaldehyde (Gibbs 1957 for an early but fairly comprehensive survey), with the ratio of syringyl to guaiacyl units being less than 2-2.5:1. The occurence (or otherwise) of this reaction is correlated with fairly major taxonomic groups; note that vascular tissue may not always be stained by this reaction (and even when it is, it does not always stain primary xylem). Vanillin is a much more widespread component of lignin, but p-hydroxybenzaldehyde is also somewhat restricted, being absent from broad-leaved angiosperms - at least from magnoliids and eudicots (Towers & Gibbs 1953). For a general discussion on details of the synthesis of lignin precursors, see Li et al. (2001), and for discussion on the distribution of lignin types, see Harris (2005).
Mannitol. This is a polyol (q.v.) particularly common in groups like Oleaceae, but the systematic significance of its distribution is unclear (see Stoop et al. 1996).
The metabolic pathway of nicotinic acid in tissue culture may be of systematic significance, although at some 50 species the sampling is poor (see Willeke et al. 1979). Gymnosperms and all flowering plants except asterids metabolise nicotinic acid into trigonelline.
Non-protein amino acids are mentioned only when they are abundant, as in families like Fabaceae and Cucurbitaceae; they are quite widely distributed, and some 600 different forms are known (see Fowden et al. 1979 for a review; I know of nothing later).
Phenols are widespread. Hydroxycinnamic acids such as caffeic acid, p-coumaric, ferulic and sinapic acids and their derivatives are widespread, and the distribution of disaccharide esters of caffeic acid and related compounds seems of particular interest (Mølgaard & Ravn 1988). The distribution of caffeic acid itself, perhaps involved in plant defence, seems to be of some systematic interest. It is especially common in herbaceous groups, and is produced from cinnamic acid; proanthocyanadins or non-hydrolysable tannins (see below under tannin) are also produced from cinnamic acid and tend to show an inverse distribution.
Phorbol esters are diterpenes, often with a toxic effect (Evans & Taylor 1983 for distribution; Evans & Edwards 1987 for physiological effects [for which also see other papers in volume 94 of the Botanical Journal of the Linnean Society]).
Photosynthetic pathways. There are three main pathways by which carbon is incorporated into the organism or, perhaps more accurately, is initially captured prior to being incorporated into metabolic cycles, the C3 and C4 pathways, also CAM (Crassulacean Acid Metabolism). Recent work (Surridge 2002; Sage 2004 for references) emphasises that C3 and C4 pathways may occur in the same organism, and the spatial separation of the different parts of the C4 photosynthetic cycle may be at the scale of the cytoplasm and chloroplast within a single cell, as well as in different cells organised as distinct tissues (see also Bowes et al. 2002; Voznesenskaya et al. 2003 - more examples of separation at the subcellular scale continue to be reported). For a general survey of C4 photosynthesis, see Sage and Monson (1999); although usually found in plants growing in rather dry and hot conditions, it sometimes occurs in plants growing in decidedly cooler conditions (Wang et al. 2008). Despite the recent discovery of dinosaur coprolites from the Late Cretaceous (71-65 mybp) of central India (Prasad et al. 2005) with contained fossils of derived grass clades, C4 photosynthesis in Poaceae seems to have originated in the middle Miocene, some 12.5 mybp, and it expanded greatly as recently as 9-4 mybp (Jacobs et al. 1999: palaeoecology of Poaceae; Osborne & Beerling 2006: the general evolution of C4 photosynthesis). However, there are a number of suggestions that C4 photosynthesis persisted through the Mesozoic (Kelley & Rundell 2003 for literature). CAM is especially commin in epiphytes, plants of arid areas, and sometimes in aquatic plants, and it has a number of variants; seedlings of CAM plants seem to have C3 photosynthesis (for literature on CAM, see Winter & Smith 1996; Keeley & Rundell 2003; Lüttge 2004, 2005;). Although distinctive pathways are rarely constant in the taxa characterised below and have evolved in parallel many times, they are particularly common in core Caryophyllales, Poales in general and Poaceae in particular (e.g. see Sage et al. 1999; Muhaidat et al. 2007 for the C4 pathway).
Phytoalexins, antifungal or antimicrobial susbtances produced by plants following infection, are very variable chemically. However, some families (Cucurbitaceae, Rosaceae) seem rarely to produce them, others (Brassicaceae) produce chemically very distinctive phytoalexins, while in yet others (Fabaceae) there is some infrafamilial systematic information in the type of phytoalexin produced (Harborne 1999).
It might seem that phytochrome behaviour is perhaps of interest phylogenetically; very limited evidence suggests that Pr is converted to Pfr, and then in core Caryophyllales, Pfr undergoes light-dependent decay, while in broad-leaved angiosperms it reverts to Pr in a thermal dark reaction (Kendrick & Hillman 1971). However, Sarah Mathews (pers. comm.) observes that light-dependent decay of Pfr is very widespread indeed in angiosperms.
Polyacetylenes are derived from their corresponding fatty acids. In asterids either polyacetlyes or iridoids dominate to the exclusion of the other even although they are biosynthetically unrelated.
Polyols are linear reduced forms of monosaccharides that are carbon transporters in the phloem of a few families. They are particularly loaded into the phloem under stress conditions caused by drought or high sodium chloride, although the families which are recorded as having them - [Celastraceae + Parnassiaceae], Apiaceae, Oleaceae, Plantaginaceae, etc. - are not particularly notable components of vegetation that is habitually water- or salt-stressed (see Pommerrenig et al. 2007 for the physiological literature).
Primary cell wall polysaccharides show potentially interesting variation within seed plants. Poaceae and perhaps a large part of the asterid I clade differ in xyloglucan hemicellulose composition from the rest, and Poaceae have a generally different hemicellulose/pectin composition from other seed plants (O'Neill & York 2003). There may also be variation in primary wall structure and composition at deeper levels in the land plants (Nothnagel & Nothnagel 2007). However, sampling in such studies leaves something to be desired.
Quaternary Ammonium Compounds such as glycine betaine commonly occur in plants growing in saline habitats, and are probably involved in maintaining the osmotic balance of cells and organelles (Rhodes & Hanson 1993). Tertiary sulfonium compounds may also be involved. However, details of the distribution of glycine betaine and related compounds are still not well understood, and they are also common in many Lamiaceae (Blunden et al. 1996) and Convolvulaceae - families not noted as being particularly halophytic.
Resins and substances either similar to or confused with them are quite widespread in plants; for their distribution, see see Nair (1995), and for a readable summary, Langenheim (2002).
Saponins. Previously defined as compounds that produced soap-like foams when shaken with water, they are now characterized as being derived from the 30-carbon oxidosqualene (this contains 6 isoprene units) to which glycosyl residues are attached (Vincken et al. 2007, who also provide a classification of saponins). By and large, the distributions of saponins with similar skeletons seems to be of little systematic interest (Vincken et al. 2007).
Sesquiterpene lactones a subclass of C15 terpenoids (= sesquiterpenoids, sesquiterpenes), bitter-tasting and toxic, sometimes with allelopathic effects (Abdelgaleil & Hashinaga 2007), that are derived via the mevalonate pathway from three C5 isopentenyl pyrophosphate units.
Steroids, compounds characterized by a nucleus of 17 carbon atoms in the form of four fused rings, three containing six carbon atoms and one containing five; for phytoecdysteroids, perhaps protecting plants against herbivorous insects by affecting moulting, etc., see Lafont et al. (1991).
For suberin, see Bernards (2002).
The sugar that is the main non-structural or storage carbohydrate in the plant varies. Although usually polymers of glucose, glucans, predominate, in some plants fructans are found. The distribution of plants with fructans is of systematic interest (for taxa involved, see e.g. Pollard & Amuti 1981; Hendry & Wallace 1993), even if not circumscribing a broadly delimited Liliales (Pollard 1982). It has been suggested that fructan accumulation is associated with low temperatures (taxa accumulating fructans are rare in the tropics), and is involved in osmoregulation and cell growth by expansion - perhaps seasonality in general is an important trigger (Hendry 1993; Hendry & Wallace 1993). Note that the taxa storing fructans are a rather heterogeneous group ecologically (a number of Ericales, Poaceae-Pooideae [but not all], a number of Asterales, etc.). Zimmermann and Ziegler (1975) list sugars and sugar alcohols found in exudates from sieve tubes; in many plants the disaccharide surose is the main component, but in others.
Sulphated/sulfated compounds are particularly common in plants growing in saline habitats, e.g. Juncaginaceae and relatives (Alismatales) and Frankeniaceae/Polygonaceae and relatives (Caryophyllales).
Tannins are either condensed and non-hydrolysable (built up of proanthocyanidins, for which see Dixon et al. 2005) or hydrolysable (ellagic acid, also gallic and caffeic acids, are the building blocks). The two kinds of tannins are not chemically immediately related, and although the unspecific terms "tannins" and "tanniniferous" occur in some of the characterisations, they are close to useless; the generic terms non-hydrolysable and hydrolysable tannins are to be prefered. Proanthocyanidins and caffeic acid tend to be alternativesand the latter is especially common in herbaceous taxa, e.g. the asterid I and II groups (Mølgaard 1985 and Mølgaard & Ravn 1986, which see for more details than have been incorporated here). Early surveys by Bate Smith (1962) and Bate Smith and Metcalfe (1957) are still useful.
Terpenoids are a diverse and ecologically important group of compounds (e.g. Harborne & Tomas-Barberan 1991) made up of isoprene building blocks (seee Bohlmann et al. 1998 for early steps in synthesis). They are found in all plants, and some 15,000 different kinds are known.
Terpenoids are related to steroids, etc.
Metcalfe and Chalk (1950; 2nd edition, 1979 onwards, incomplete) provide information on general anatomy, although characters like nodal anatomy are rarely mentioned in the first edition; classical literature remains useful (see e.g. Thouvenin 1890; Beauvisage 1920; Watari 1939 for taxa that used to be placed in Saxifragaceae). Gregory (1994) is an invaluable bibliography with the more important contributions conveniently indicated, Dickison (2000) is a very accessible introduction to general plant anatomy, and Carlquist (2001) has recently updated his survey of comparative plant anatomy (see also Carlquist 1988b); Esau (1977) is still very useful. Herendeen et al (1999b) surveyed wood anatomical features in basal angiosperms and compared their findings to the classical evolutionary trends (see below). Wheeler et al. (1989) provide a useful survey of terms used in wood anatomy, while InsideWood: An Internet Accessible Wood Anatomy Database http://www.lib.ncsu.edu/insidewoood/ is a developing resource for information about wood anatomy, also identification, etc.
Carlquist (e.g. 1988, 1998b, 1998c) has emphasised the linkage between features of wood anatomy and the environment. For this reason, and also because many characters have states whose limits need justification, especially in the context of angiosperm-wide surveys such as this, wood anatomy must be interpreted with caution (for wood anatomy and cladistic characters, see Herendeen & Miller 2000). However, more information on wood anatomy is gradually being added to the characterizations.
Mycorrhizae. For the evolution and ecological significance of mycorrhizae, see Malloch et al. (1980), Read et al. (2000), etc., and for an account with a summary of the distributions of the main mycorrhizal types within angiosperms, see Brundrett (2002) and especially Wang and Qiu (2006). Brundrett (2004) offers a classification of mycorrhizal types based on anatomical criteria regulated by the host plant; note that roots with well-developed mycorrhizae commonly lack or have only a few root hairs. Ectomycorrhizae form a Hartig net of hyphae investing the rootlets and penetrating between the cortical cells; the hyphae are septate and are not intracellular - with the exceptions of Ericaceae and Orchidaceae. Bruns and Shefferson (2004) provide a convenient summary of taxa with ectomycorrhizae, but more detail will often be found here (see e.g. Ducousso et al. 2008 and references). Although basidiomycetes are frequent ectomycorrhizal associates, Pezizales (ascomycetes) are also quite common (Tedersoo 2006, ascomycetes with a hypogeous life style are derived from them). Interestingly, ectomycorrhizal associations are found in plants like Graffenrieda emarginata (Melastomataceae) in tropical montane environments along with the next kind of mycorrhiza (Haug et al. 2004 and references). Vesicular-arbuscular mycorrhizae formed by an association of Glomeromycota (Schüßler et al. 2001) with land plants - indeed, they are found only in association with plants, whether sporophyte or gametophyte. There is evidence of such mycorrhizae in the fossil record from the Silurian/early Devonian ca 400 mya, but this association has almost certainly arisen a number of times (e.g. Baylis 1975; Redecker et al. 2000b; Duckett et al. 2006). Although the liverworts of the first pectinations are associated with Glomales (Kottke & Nebel 2005), this association may subsequently have been lost (Duckett et al. 2006b), indeed, in some cases the fungus involved may have moved to the liverwort from a tracheophyte (Ligrone et al. 2007). Within Jungermanniopsida, Porellales lack fungus associations, but within Jungermanniales associations with ascomycetes are very old, more than 250 million years (Pressel et al. 2008). Glomus itself is probably paraphyletic (or worse: Redecker et al. 2000a; Schwarzott et al. 2001) and Glomeromycota seem to be a very ancient asexual group (Pawlowska & Taylor 2004) whose basic genetic structure is not understood (Rodriguez et al. 2004). In vesicular-arbuscular mycorrhizae the aseptate hyphae are intracellular, often forming vesicles or branching structures called arbuscules within the cells (when the latter alone are formed, an arbuscular mycorrhiza may result). The various mycorrhizal associations of angiosperms are summarized by Malloch et al. (1980), F. A. Smith and Smith (1997), S. E. Smith and Read (1997), and Landis et al. (2002). For phylogenetic aspects of the associations, see also Trappe (1987), the two main variants of vesicular-arbuscular mycorrhizae described there (the Paris and Arum types) show little association with higher-level groupings, except perhaps when Asparagales are compared with Liliales (Smith & Smith 1997 - note also that several families have both types in different species, or intermediates). Interestingly, in mycotrophic plants, the pattern of fungal infestation seems always to be a variant of the Paris type of endomycorrhiza, with well developed intracellular hyphae (Imhof 2007 and references). In addition to variation in details of intra-plant fungal infestation, there is also variation in the proportion/amount of fungal biomass in the plant compared to that outside the plant that is linked with the taxonomy of the fungus (Maherali & Klironomos 2007). For details of the mycorrhizae in Ericaceae, see that family. Aquatic plants, hardly surprisingly, lack mycorrhizae, but the frequent absence of mycorrhizae in Caryophyllales, Proteales, etc., is systematically interesting.
In, say, the tundra habitat there are a number of plants that take up substantial amounts of amino acids directly from the soil, but the ability to do this is not obviously correlated either with mycorrhizal status or taxonomy. Thus in such habitats a number of Cyperaceae, which usually lack mycorrhizae, take up N predominantly in an organic form, but others take it up in an inorganic form (Raab et al. 1999).
There is a considerable amount of variation in fine details of root morphology, e.g. whether roots are aggregated into clusters, as in many Proteaceae, and/or whether the root is clothed in exceptionally long and dense hairs (Lambers et al. 2006; Shishkova et al. 2008).
Nitrogen fixation. Angiosperms that can fix nitrogen in association with bacteria are uncommon, yet are found almost exclusively in a group of four orders, the N-fixing clade (Fabales, Rosales, Cucurbitales, Fagales), where the association has evolved several times. Jeong et al. (1999) and Clawson et al. (2004) discuss this association in the context of the evolution of Frankia, the bacterium most commonly involved outside the Fabaceae, and within Fabaceae themselves (q.v.), there may have been several acquisitions of the ability to nodulate, very different bacteria being involved. There seem to be factors in common allowing both bacterial nitrogen fixation (both Frankia and Rhizobium) and vesicular-arbuscular mycorrhizal associations to be established (Chen et al. 2007; Markmann et al. 2008; Gheri et al. 2008).
Endophytic fungi are found in a number of flowering plants, perhaps particularly in Poaceae (q.v.) and Ericaeae (e.g. Petrini 1986 and other references in this volume; Saikkonen et al. 2004 and references), although they are probably much more widespread. Indeed, Arnold et al. (2001) found 418 morphospecies of endophytes in only 83 leaves of two species of tropical trees (Ouratea, Heisteria). The effect of endophytes on their hosts is little understood, perhaps protecting the plant against pathogens, affecting the water balance of seedlings, etc. (Arnold & Engelbrecht 2007, and references).
For rusts and their hosts, see OTHER at the end of this page.
Root apical meristem. Details of the organisation of the root apical meristem are known for relatively few plants, but whether or not the tiers of initials are closed, i.e. are clonally distinct, open, with the fate of the derivatives of the apical cells not being immediately obvious, or some more or less intermediate condition intermediate-open, may be of systematic significance (Groot et al. 2004). The character has been placed on the tree, as much to draw attention to it as anything else; monocots are notably variable. Heimsch and Seago (2008) provide a recent summary of the information about root meristems; patterns of development are clearly difficult to interpret. In BLAs, the root cap is made up of gravity-sensing
Root stele. The stele or vascular tissue in the root is central, and the xylem forms a solid mass, although it is sometimes medullated, perhaps especially in monocots. However, the root stele of a number of broad leaved angiosperms, e.g. Mimusops (Sapotaceae) and the primary roots of Clusiaceae and Dipterocarpaceae is medullated, while that of some monocots, e.g. sapromycotrophic taxa, some Alismatales, Sisyrynchium, etc. is not medullated (van Tieghem & Douliot 1888; von Guttenberg 1968). The proportion of the root occupied by the stele varies considerably, and in a small study it was found to occupy 20-48% of the area in carnivorous plants (and was sometimes medullated), ususally rather less than 34% in other plants (Adamec et al. 2006). Tracheids, and sometimes also phloem, are to be found scattered in the pith in some monocots (von Guttenberg 1968). In broad-leaved angiosperms there are commonly two to five xylem poles (di- to pentarch) alternating with phloem poles; the lateral roots arise on the xylem radii except when the roots are diarch. Diarch and tetrarch, etc., taxa commonly occur in the same family, but some larger groups (e.g. Rosales, possibly Brassicales) are overwhelmingly diarch. In diarch taxa the lateral roots arise on either side of the xylem poles, but in Pittosporaceae and their relatives origination in this distinctive position is found in taxa that are other than diarch (van Tieghem & Douliot 1888; von Guttenberg 1968). Monocots are more often polyarch (even the primary roots are not diarch), and in a number - but not all - of Poales the lateral roots arise opposite the phloem poles (van Tieghem & Douliot 1888).
Root hair development normally occurs in epidermal cells that are rather like cells that do not have hairs. However, root hair development in monocots differs substantially from that of most other flowering plants. Here there are trichoblasts, small, densely staining cells that give rise to root hairs, and these are clearly differentiated from other epidermal cells. The trichoblasts occur in vertical files, and alternating cells, usually the proximal cells (i.e. nearer the root apex), produce the root hairs, although there is infrafamilial variation in this (e.g. Poaceae, Hydrocharitaceae). In some monocots, on the other hand, especially those with a velamen, it is the hypodermal (not epidermal) cells that may be dimorphic (Kauff et al. 2000). The basic condition for angiosperms seems to be the absence of trichoblasts. However, in Arabidopsis and relatives hairs develop at the proximal ends of trichoblast cells (Schiefelbein et al. 1997), with trichoblasts arising from epidermal cells that are situated above the radial walls of cortical cells; these trichoblasts are in vertical files (Pemberton et al. 2001; Dolan & Costa 2001). There seems to be a relationship between the development of cutinized root hairs from outer cortical cells and the absence of secondary growth in the root (Pinkerton 1936: there is usually no secondary growth in monocot roots).
Root hair cellulose. Cellulose fibrils in the outer epidermal wall of the root elongation zone are normally laid down transverse to the root axis, but in some Poales they are parallel to the long axis (Kerstens & Verbelen 2002). However, Pothomorphe (Piperaceae) behaved differently, and sampling particularly in clades other than eudicots and monocots needs to be improved.
Root epidermis development. Whether or not the epidermis of the root develops from the inner layer of the root cap or the outer layer of the cortex varies at about the same level as root hair development (Clowes 2000). The inner epidermis of the roots is lost completely in monocots (and Nymphaeaceae!), the epidermis then developing from the cortex, although the inner layer of the epidermis remains attached in broad-leaved angiosperms (van Tieghem & Douliot 1888). Monocots in particular may have distinctively-thickened cells in the exodermis, and the development of a velamen is quite widespread, if sporadic (von Guttenberg 1968 for literature).
Shoot apical meristem. Hagemann (1967), Kaplan and Cooke (1997) and Bowman and Eshed (2000) discuss the basic organisation of the apical meristem. In angiosperms the apex of the stem is covered by layers of cells, the tunica, in which cell division is predominantly in an anticlinal direction, that surround the corpus, a central mass of cells in which the plane of cell division is not so restrained (this construction pattern is responsible for the distinctive variegation patterns in many taxa - see Tilney-Basset 1986). Tunica-corpus construction may be a synapomorphy for angiosperms (and a parallelism with Araucaria and Gnetales - see Johnson 1951, Fagerlind 1954, Pohlheim 1971, and Evert 2006 for gymnosperm meristems; Gifford 1954 for details of tunica in angiosperms). Although details of organisation of the shoot apical meristem are known for relatively few plants, the numbers of corpus layers may be of some systematic interest (Bowman & Eshed 2000); tissues in monocot leaves certainly come from three types of cells (Stewart & Dermen 1979), whether or not the tunica is two or three cells thick. Monocots examined have a primary thickening meristem (Rudall 1991 for a summary), perhaps located in the pericycle-endodermis area (de Menezes et al. 2005). There is considerable variation in the size of apical meristems, but those of Cactaceae seem substantially wider than the rest, being 400-1500 µm across, while in other angiosperms they are often <325 µm across, although up to 528 µm in Phoenix canariensis, and almost as big in Nymphaeaceae. Within both polysporangiates and gymnosperms there is also considerable variation, and in the gymnosperms the shoot apices of Cycadales are notably larger than the rest - 500-3,300 µm across, versus <400 µm (Gifford 1954; Clowes 1961; see also references in Gifford & Corson 1971: I do not know of any recent surveys of meristem size).
Iqbal (1995) provides an entry into the literature on plant cambia, or lateral meristems (secondary thickening meristems), and the cells that differentiate from them. There are two main kinds of cambia, cork cambia and vascular cambia. The products of cork cambium (or phellogen), are cork or phellem to the outside and a little phelloderm to the inside; together the three make up the periderm. There is variation in the tissues produced as the cork cambium differentiates, when the cambium is first produced (see e.g. Möller 1882; Esau 1965; Edwards & Donoghue 2006), and, perhaps most importantly from a systematic point of view (Weiss 1890), where it is first initiated. Cork cambium occurs widely in broad-leaved angiosperms and in gymnosperms. In the stem it tends to be either superficial, the common condition in angiosperms, being initiated at or immediately below the epidermis (I have not distinguished between these positions), or deep-seated, being initiated deep in the cortex, inside the pericycle, or even in the phloem (for still invaluable surveys, see Möller 1882 [much information on gymnosperms]; Douliot 1889, also Kuhla 1987; Waisel 1995; etc.). Initiation in the cortex is also quite common and there is of course no absolute distinction to be made between the superficial and deep-seated positions. There may be a connection between the position of initiation of the cork cambium and the development of an endodermis (Arber 1925 and references). Czaja (1934) discusses the cork of several taxa that have cork wings on the young stem, although such variation is not of systematic significance at this level. Etagenkork, suberised cells that are not produced by a cork cambium, are also to be found in some monocots. Polyderm is a distinctive cambial derivative that is rather infrequently found, but whose distribution often seems to be of systematic significance (Mylius 1913 for a survey); the cambial cells cut off alternating layers of cork and endodermal cells.
In broad-leaved angiosperms, cork cambium position in the stem and root of the same plant commonly differs, the cork cambium in the roots usually being deep-seated (see Waisel & Liphschitz 1975; Esau 1977; Fahn 1990). However, in plants with aerial roots of various kinds the cork cambium may be superficial (von Guttenberg 1968); thus Clusieae, alone of Clusiaceae, are often epiphytic and have superficial cork cambium in their roots (but this is not found in Clusiella, a Clusia look-alike that recent work has placed in Kielmeyeroideae). In monocots the cork cambium in the root, when developed, is usually to be found just under the exodermis, i.e. in a superficial position (Philipp 1923; Arber 1925). However, specific information on the initiation of cork cambium in the roots is hard to come by, especially in monocots.
Whether or not a vascular cambium develops, and the nature of the products of that cambium, are is of considerable systematic interest. Vascular bundles in which cambium does not develop are described as being closed; if a cambium develops, normally at the interface between the phloem and xylem, they are open. Climbers in particular show anomalous patterns of secondary thickening, with the number of cambia, and the nature of the tissue they produce, varying. In non-climbing Caryophyllales such as Phytolaccaceae successive cambia develop. Carlquist (2004, see also 2007b) studied the secondary tissue of Nyctaginaceae in detail, and suggested that there, at least, the use of the term "included phloem" was inappropriate; several taxa that had been described as having included phloem in fact have successive cambia. In plants with successive cambia, a so-called master cambium produces conjunctive tissue adaxially, in which new vascular cambia develop producing phloem externally and xylem internally. Each successive cambium and its products are surrounded by a narrow layer of conjunctive tissue, hence the phloem is not completely included or enclosed by xylem tissue. Here the term included phloem is used to describe the situation when phloem islands cut off internally by a vascular cambium are completely surronded by xylem cut off by that same cambium, and internal phloem the situation where phloem is found internally to the xylem in the primary stem (this latter condition is commonly associated with bicollateral vascular bundles in the leaves). The terms "interxylary phloem" amd "intraxylary phloem" are confusing and should not normally be used. A normal vascular cambium is absent in monocots, although there may be traces of cambial activity (Stant 1970 and references). In some monocots a distinctive cambium cuts off tissue to the inside in which separate vascular bundles embedded in ground tissue differentiate; this is scattered in Asparagales (Rudall 1995b for records and literature) and has recently been reported from Eriocaulaceae (Poales: Scatena et al. 2005). There is no vascular cambium in monocot roots (Arber 1925). In extant angiosperms other than lignophytes, a vascular cambium is found only in some Ophioglossaceae.
Data on cambium storying are taken mostly from Carlquist (1988b); if not mentioned, the cambium is unstoried. One or both the kinds of cambial initials (ray, fusiform) may be storied, but only rarely are ray initials alone storied, as perhaps in Lauraceae (Paul van Rijckevorsel [pers. comm.] noted problems with this character).
For information on the nature of the vessel element perforation plate in broad-leaved angiosperms, see Bierhorst and Zamora (1965), for monocots, see Wagner (1977, sampling needs to be extended/newer literature integrated). There has been discussion whether vessels in monocots and in other angiosperms arose independently (Cheadle 1953), although this seems unlikely; the distribution of vessels in the monocot plant body may be of systematic interest (Cheadle 1944; Wagner 1977). There is much information in the literature of the nature of scalariform perforation plates, e.g. the number of bars, and in the past the nature of the vessel perforations has been treated of very great phylogenetic importance, with the trend running tracheids only -> scalariform perforation plates with many -> few bars -> simple perforation plates; the cells also became shorter and wider and the end wall less oblique (e.g. Bailey 1944, 1954). I have not tried to fit all this information to the trees, and certainly the evolutionary significance of this variation is less than was thought, even if the Baileyan trends are more or less evident in the fossil record (Wheeler & Baas 1991); Herendeen et al. (1999b) note that there is polymorphism in perforation type (scalariform/simple) in no fewer than 54 families of broad leaved angiosperms, although uncommon in five of these! Pit membrane remnants are turning out to be quite common in broad-leaved angiosperms (e.g. Aextoxicaceae, Bruniaceae, Hydrangeaceae, families near Ericaceae, Schisandraceae, Quintiniaceae, Rousseaceae), but the functional and systematic significance of this feature is unclear (e.g. see Schneider & Carlquist 2003, 2004a; Carlquist & Schneider 2004).
Some information is included about the presence of vascular tracheids and wood fibers, and the presence and distribution of xylem parenchyma (see Hess 1950), but these features are not treated for all families.
Pits. Vascular pits are in general quite variable in morphology and arrangement, although the distinction between some of the pit "types", such as bordered and non-bordered pits is arbitrary (Herendeen et al. 1999b note that this varies in some 45 families of broad-leaved angiosperms). The nature of intervascular pitting may be of systematic interest, but I have not included many details of this character. Hacke et al. (2004) describe how the torus-margo pit membranes of gymnosperms work; their absence may be an angiosperm apomorphy (see also Sperry & Hacke 2004). However, a torus, sometimes lignified, occurs occasionally in angiosperms (Coleman et al. 2004), and again its distribution may be of systematic interest. Vestured pits have close and minute sculpturing of the surface of the cell wall in the pit, and their distribution is of systematic interest (for surveys, see Jansen et al. 1998, 2001). Nearly all taxa with vestured pits have vessels with simple perforation plates, and such pits may promote the functioning of these vessels (Jansen et al. 2003).
Reaction wood. The amount and to a certain extent the anatomy of the xylem produced at the branch-stem junction, the reaction wood, varies, although in some taxa it may not be produced at all. Although it is often suggested that angiosperms and gymnosperms differ in the nature of their reaction wood, angiosperms producing cellulose-rich tension wood and gymnosperms lignin rich compression wood, a summary of the information available for angiosperms (Höster & Liese 1966) suggests that this is a gross oversimplification (see also Westing 1965, 1968 and Timell 1986 for gymnosperms). Tension wood cells also have gelatinous fibers immediately internal to the cell wall; they contain pectic mucilages and arabinogalactan proteins and may generate the active contraction force of such wood (Bowling & Vaughan 2008).
Wood fluorescence. Data are taken largely from a survey of some 10,600 species summarised in Avella et al. (1988). I am unclear as to the significance of this character.
Unlignified cell wall fluorescence is caused by the presence of ferulic and/or coumaric acids. Unlignified walls of most angiosperms do not fluoresce, and only those taxa with fluorescence are mentioned (data largely from Hartley & Harris 1981). Other information on the composition of primary cell walls, especially in monocots, is given by Smith and Harris (1999) and Harris (2000).
Details of vascular ray "types" are rarely mentioned, partly because it is difficult to reconcile the definitions of the terms as they have been used over the years (see Kribs 1935, 1959), partly because of the typology involved, and partly because it can be difficult even to define what a ray is (Carlquist 2003); in general systematically interesting information in this feature is probably best recorded in descriptive terms, i.e., describing the morphology of the cells involved. The short discussion by Chalk (1983) is particularly useful. Particularly distinctive rays are found in some Malvaceae (e.g. Chattaway 1933a, b). Ray width in both xylem and phloem may vary considerably. Broad phloem and xylem rays are a valuable field character for identifying families, and when the phloem rays are wide, tangential cuts of the phloem show a net-like arrangement of the paler-colored, anastomosing rays (Keller 1996). More precise indication of ray width can provide systematically interesting information (see Herendeen et al. 1999b for some "basal" angiosperms are relatives).
Phloem. For a good survey of phloem micromorphology, etc., see Behnke and Sjolund (1990). In gymnosperms sieve cells are involved in carbohydrate transfer; these are nucleated and have endoplasmic reticulum traversing the sieve pores, or, rather, the sieve area is composed of several small sieve pores which join to form a cavity in the region of the middle lamella (e.g. Behnke 1990a; Iqbal 1995). The sieve cells are associated with Strasburger cells (albuminous cells) which, although closely linked functionally with the sieve cell, are not formed from the same mother cell. In angiosperms sieve tubes carry out the same function; they lack nuclei but are closely associated with nucleated companion cells that are derived from the same immediate mother cell. There is little variation in sieve tube morphology easily usable at higher levels. Sometimes sieve tubes or sieve cells have substantially thickened, lamellate and nacreous secondary walls; these seem to be sporadically distributed, although within gymnosperms they may be resticted to Pinaceae (Behnke 1990b: cycads, etc.; Schulz 1990b: conifers; Evert 1990b: "dicots"; Eletheriou 1990b: monocots). Zahur (1956) described substantial variation in many characters, albeit it seems to be overlapping (Iqbal & Zahur 1995 list trends of specialization). Zahur (1956) found a weak correlation between presence of secondary septae and sympetalous families, but his data show variation within orders. Turgeon et al. (2001; see also Gamalei 1991) summarise variation in companion cell anatomy and delimit a number of "types" in minor veins. Type I or open phloem probably has symplastic connections between bundle sheath and phloem, there being 10-100 plasmodesmata/mm2 between bundle sheath and companion cells, while Type II or closed phloem probably has apoplastic connections, there being only <10 plasmodesmata/mm2. Intermediary cells are characterized by having numerous plasmodesmata that branch in the outer part of the walls adjacent to the bundle sheath cells; there seems to be a correlation between the presence of intermediary cells (see Lamiales) and the presence of stachyose in the translocate. However, whether or not such characters are more or less constant within families is not obvious, and the link between type of carbohydrate translocated and details of phloem anatomy is not entirely clear (see also Goggin et al. 2001; Turgeon & Ayre 2005). Zimmermann and Ziegler (1975) list sugars and sugar alcohols found in exudates from sieve tubes in broad-leaved angiosperms.
Transfer cells, metabolically very active cells with more or less labyrinthine intrusions of the walls, occur in various parts of the plant (Gunning et al. 1970; Pate & Gunning 1972), e.g. the heads of the glandular hairs in insectivorous plants, associated with phloem in vascular bundles (they are one type of companion cell), especially in the xylem and at the nodes, and in seeds (see below).
Stratified phloem, secondary phloem in which there are bands of fibres alternating with ordinary ploem tissue, is developed in groups like Malvales, Annonaceae, etc. It is easy to recognise using a hand lens, or even with the naked eye, in phloroglucinol-stained hand sections. Plants with such phloem typically have bark that can be pulled off in long strips.
An important character is the structure and composition of sieve tube plastids. Data on the ultrastructural morphology of the starch and protein in the plastids are taken from the work of Behnke (e.g. 1969, 1981b, 1989, 1991a, 2000, 2001). The basic condition for angiosperm plastids is to include starch grains alone, but nearly all monocots have cuneate protein crystals alone and core Caryophyllales have peripheral protein fibres. There are many other less striking variants with various combinations of starch and protein, and these may characterise small clades such as [Erythroxylaceae + Rhizophoraceae], whilst the absence of both starch and protein is common in parasitic groups, but also in Rosales, [Malpighiaceae + Elatinaceae], etc. The sizes of the starch grains and fine details of the protein crystals in the plastids may provide still more evidence of relationships, although I have not always included this information. Note that Pistia is the only monocot known to have starch plastids (Behnke 1995)!
P-proteins are proteins in mature sieve elements of angiosperms that vary in origin (nuleus or elsewhere) and appearance aggregated [non-dispersed] or not) and also behaviour, e.g. changing their morphology when the turgor of the sieve tube changes (Knoblauch et al. 2001). Information on this character, varying mostly in broad-leaved angiosperms, was obtained largely from Behnke (1981a, 1991b); P-proteins are absent from at least some monocots, e.g. Poaceae, but they are present in e.g. Iridaceae (Tóth & Sjölund 1994; Sabnis & Sabnis 1995).
In the primary stem vascular system there can be separate bundles (an eusetele), a (pseudo)siphonostele, with the vascular tissue forming an apparently complete cylinder, or a complete cylinder. However, from Benzing's work (Benzing 1967b) there seems to be no sharp difference between siphonosteles and eusteles in angiosperms. Moreover, the use of such terms may be inappropriate when describing the vascular system there (Benzing 1967a; Namboodiri & Beck 1968c; Howard 1979a), which can often be interpreted as a series of sympodially branching bundles; variation within families may be quite extensive (Jensen 1968). Finally, interpretation of the primary vasculature in terms of axial or foliar theories yields very different results (Al-Turki et al. 2003) - and of course the theories suggest very different evolutionary pathways for the plant body. See Schmid (1982; also Namboodiri & Beck 1968a, b; Beck et al. 1982) for a discussion of stelar variation and terminology in both angiosperms and gymnosperms. Beck et al. (1982) suggest that there is a correlation in broad leaved angiosperms between closed primary stelar patterns and distichous or opposite phyllotaxy and between open patterns and spiral phyllotaxis.
In monocot stems the vascular bundles are usually scattered, and I mention them only when they are in rings. In general, cauline vascular bundles are collateral, with xylem to the center of the stem and phloem to the outide. Variants of this are interesting, and are mentioned when they occcur; this variation is often reflected in the general arrangement of tissue types in the vascular bundles of the petiole and midrib. In monocot rhizomes in particular, but not in the annual stems, the scattered bundles are frequently amphivasal, i.e., they consist of phloem surrounded by xylem (Jeffrey 1917; Arber 1925).
For nodal anatomy see especially Sinnott (1914), Dormer (1946), Marsden and Bailey (1955), Bailey (1956), Balfour and Philipson (1962), Cutler and Gregory (1998), Howard (1970, 1974), Beck et al. (1982), and Metcalfe (1987) for information. Note that the behavior of protoxylem strands in the young stem may not correlate with node type (e.g. Beck 1962; Kumari 1963; Benzing 1967a, b; Gibson 1994), and treating flowering plants as if they had a modified siphonostele (see above) may obscure information (Keating 2000). In particular, although I refer to "leaf gaps" in these pages, Beck et al. (1982) in particular argue that this is inappropriate when talking about the steles of seed plants. Indeed, when looking at nodal anatomy in stems with some secondary thickening, it may be very difficult to detect nodes in which there are separate traces from adjacent sympodia. Also, it should be remembered that the stem, node and leaf form a continuum (see especially Howard 1974, 1979a), and there can be differences in nodal vascularisation when cotyledons and foliage leaves are compared (e.g. Stone 1970). In the descriptions I use formulae like 1:1, 5:5, etc.; the first number is the number of gaps, the second is the number of traces or separate vascular bundles leaving the gaps. These gaps are often recognizable as interruptions in the vascular cylinder formed by the early stages of secondary thickening. Split laterals occur in some taxa with opposite leaves and are single traces that send branches to both leaves, while the related flank-bridges and variants are found in some Dipsacales and Rubiaceae (e.g. Neubauer 1982), etc. The cotyledonary node often has 1:2 nodal anatomy (Thomas 1907), whatever the nodal anatomy of the later leaves. However, there can be quite a lot of variation at the cotyledonary node; it may be multilacunar in Juglandaceae, or 4:3 in some Magnoliaceae (with split laterals!, e.g. Sugiyama 1976a), and the basic nodal condition for seed plants may be 1:1 (Kumari 1963). Although many asterids, especially Lamiales, etc., are commonly reported as having 1:1 nodes, Neubauer (e.g. 1977, 1978) emphasized that there the single trace often divided immediately into three or more (e.g. see Linderniaceae, Scrophulariaceae, also Cordia [Boraginaceae]); nodal anatomy may repay attention in Lamiales (also Melastomataceae, etc.).
Leaves that have stipules often have blades with serrate margins and are not often unilacunar. Celastraceae seem to be an important exception to this latter feature, however, their stipules can be the most evanescent of structures (see also some Myrtales), while both Perrottetia and Bhesa, recently removed from Celastraceae (and now in Huerteales and Malpighiales respectively), have more complex nodes - and more obvious stipules, especially in Bhesa. Rosaceae tend to have 3:3 or more complex nodes, and are generally stipulate; the estipulate Spiraea has 1:1 nodes; there are a similar relationships between the normally trilacunar and stipulate Elaeocarpaceae that includes the unilacunar estipulate Tremandra and its relatives. Rubiaceae have unilacunar nodes, but lateral bundles immediately split off and form a vascular ring around the stem from which the stipules are innervated (Majumdar & Pal 1958). Many plants with 1:1 nodes have leaves with entire margins (Sinnott & Bailey 1914). Monocots have broad bases into which several bundles proceed, but Sinnott and Bailey (1914) noted that in the petiolate Potamogeton and Smilax there was a tendency for three main bundles to enter the leaf base.
Gross patterns of nodal anatomy are often easy to confirm in the field using a razor blade or sharp knife and a hand lens, and so can help in identification. Thus Celastraceae have 1:1 nodes, different from most Malpighiales with which they can be confused vegetatively.
In monocots a number of vascular bundles proceed in to the leaf, and details of nodal vasculature are not generally recorded. In aquatic angiosperms like Nymphaeales nodal anatomy is very complex, with a vascular plexus forming at the nodes and the internodal vascular tissue being very distinctive, even if ultimately the vasculature of the leaves can be interpreted in conventional angiospermous terms (cf. Schneider 1980; Moseley et al. 1984). Gymnosperm nodal anatomy is also described in terms of gaps and traces, although at the very least the behaviour of the traces can be unexpected from an angiosperm perspective, as with the girdling traces of the foliage leaves in Cycadales.
The pericycle is the outermost layer of cells of the stele, and is made up of a single layer of cells. In at least some angiosperms, cells of this layer early differentiate into cells that can subsequently divide and initiate lateral roots and those that cannot (Parizot et al. 2008). There are often lignified fibers immediately external/peripheral to the phloem in the stem, the pericyclic sheath. This sheath is sometimes absent, or it may be made up of sclereids. Gibson (1993) discussed the need to distinguish between fibers that are derived from phloic tissue and those that are extra-phloic in origin (see also Blyth 1958). Although simple presence/absence and nature (if present) of this sclerified sheath may be of systematic interest, as in Schlegliaceae, Dipsacales, etc., distinguishing between its different origins may also be of value.
The endodermis consists of one or a few layers of cells that surrounds the central stele and sometimes also vascular tissue elsewhere. In the root the endodermis is often marked by a single layer of cells with casparian bands, but this is uncommon in above-ground stems and even more so in the leaf, although it may occur there, perhaps especially in herbaceous taxa, some Bonnetiaceae being a notable exception (see Gutenberg 1943; Metcalf & Chalk 1979; Lersten 1997 for references). In the stem a ring of starch-containing cells (the starch sheath) may be found in this position, although casparian bands may develop when the stem is placed underground (Van Fleet 1961). Details of the distribution and nature of cauline and foliar endodermes in particular may be of systematic significance, see e.g. Alseuosmiaceae.
Cortical and medullary bundles, with vascular bundles in the cortex and pith respectively, are mentioned only when they occur, see Col (1904) for an early discussion and summary of information.
Petiole anatomy has been recorded rather sporadically. Radford et al. (1974) describe and name some of the main variants, although other sets of terms for describing petiole vasculature have also been proposed (e.g. Watari 1939; see also Howard 1979b). Col (1904) is a source for information in several groups. Families like Clusiaceae show great variation in this feature, and it is usually of systematic significance at levels lower than those emphasized here. However, Kim et al. (2003) have recently drawn attention to variation in how much adaxial surface is evident in petioles; when there is none the petiole bundles form a circle, or there is a single annular bundle, in both cases without any adaxial bundles (the "flange bundles" of the descriptions). Vascular bundles in the petioles of monocots are usually scattered, although sometimes they form a ring; only the latter feature is recorded.
Various tissues in the plant contain distinctive secretions. Latex is generally secreted in laticifers, rarely in isolated cells. The former may be unarticulated, branched or unbranched but not anastomosing syncytia that originate from single cells in the embryo. Articulated laticifers, generally associated with the phloem, are often anastomosing tubes that develop from cells whose common end walls have broken down. Gregory and Baas (1989; see also Dickinson 2000) provide a useful survey of mucilage cells, while Matthews and Endress (2006) distinguish a distinctive type of mucilage cell with thickened inner periclinal wall and distinct sytoplasm; they catalogue the distributions of this and of other kinds of mucilage cells.
Leaf blades containing volatile ethereal oil cells may be punctate when a light is shone through the leaf. In glands of taxa like Rutaceae and Myrtaceae material is secreted into schizogenous cavities; truly lysigenous glands may not occur in plants, reports of such structures seeming to refer to artifacts of preparation (Turner et al. 1998; Turner 1999). Cells containing ethereal oils and those with mucilage are similar in their early development, as in basal angiosperms (Bakker 1992). The main differences between the two is that oil cells often have a suberised layer between cellulose layers and plastids secrete the oil, while mucilage cells lack the suberised layer and mucilage is usually secreted in golgi vesicles. Other oil bodies, especially in leaf mesophyll cells, may contain neutral oils, triglycerides (triacylglycols). This other kind of oil body is widely distributed in angiosperms, although they have not been reported from the ANITA grade or the magnoliids (Lersten et al. 2006).
Sclereids (sclerenchymatous idioblasts) found free in the cortex, the mesophyll of the lamina, or elsewhere are variable in morphology; they are mentioned when they are common in a family (Rao 1991 for a survey). Sclereids occuring in the pith are rarely mentioned. Sclereids intergrade with fibers, very elongated and unbranched lignified cells. Distinctive sclereids of various kinds, including tracheoidal cells, may be associated with vein endings; such sclereids are rarely mentioned below because they are of significance at lower levels. Again, leaf blades with sclereids may be punctate when a light is shone through the leaf, and even fibrous tissue is often evident, either in such transmitted light or in surface view.
Cristarque cells are distinctive cells with U-shaped lignification and with calcium oxalate crystals or druses. They are mentioned in only those taxa in which they have been reported.
Calcium oxalate may crystallise in distinctive forms such as raphides (needle-like crystals), styloids (narrowly oblong crystals), crystal sand (very small crystals that appear granular), or single rhomboid crystals little longer than broad; crystals of any sort are rarely entirely absent. All these conditions are mentioned. Calcium oxalate crystals are usually in the form of druses, irregular aggregations of more or less radially arranged crystals, but then their presence is not mentioned. Note that two or more forms of crystal may occur in the one plant, e.g. druses and raphides occur together in some Araceae, but again, druses are not mentioned. Raphides can sometimes be seen in herbarium material when using a dissecting microscope if the tissue containing them is cut; the raphide sacs are then evident as small white patches. Individual raphide crystals vary in details of their morphology, e.g. shape in transverse section (Horner & Wagner 1995), and are especially common in monocots. Many data are taken from Metcalfe and Chalk (1950); Prychid and Rudall (1999, 2000) provide surveys for monocots, Lersten and Horner (2005) discuss styloid morphology and distribution, and Franceschi and Horner (1980) review the subject from a more physiological perspective. Calcium oxalate may crystallise as whewellite or calcium oxalate monohydrate [CaC2O4.H2O - COM] or weddellite, calcium oxalate dihydrate [CaC2O4.2H2O - COD], the latter being the less common form (Monje & Baran 2002), but I do not know what the general distribtutions of these forms are (but see Cactaceae); Hartl et al. (2007) suggest that more attention be paid to this variation when discussing crystal morphology. Not only does the crystal form of calcium oxalate vary, but also the particular tissues in which the crystals accumulate, and the combination of the two seems to be of systematic interest in the Apiaceae-Araliaceae area (Rompel 1895; Burtt 1991; Liu et al. 2006). Vacuolar crystal formation associated with membranes and paracrystalline bodies with widely spaced subunits is found in eudicots, while in monocots there are no membrane complexes and the paracrystalline bodies have closely spaced subunits (Horner & Wagner 1995; Evert 2006). A few taxa produce soluble, rather than crystalline, oxalate, such as potasssium oxalate or oxalic acid itself (hence the pleasingly sharp taste of some Polygonaceae and Oxalidaceae). Some taxa show little or no accumulation of oxalate in any form (Hodgkinson 1977; Zindler-Frank 1976). Little is known about why oxalate is accumulated, or accumulated in one form rather than another. Raphides have certainly been implicated in protection of the plant against herbivores, and their may be connections with the type of carbon fixation, the aquatic habit, etc. Franceschi and Nakata (2005) suggest that oxalate fomation is connected with calcium regulation, plant protection, and metal detoxification.
Cystoliths are moderately common and their morphology and distribution is quite often of systematic interest. They are concretions of calcium carbonate developing on an intrusion of the cell wall, silica may also be involved; the cells containing cystoliths are known as lithocysts. Cystoliths can sometimes be seen in the leaf blade when it is held up to the light, as in Acanthaceae and Urticaceae; they can also be visualized in spodograms, basically what remains after the leaf is calcined (see, e.g. Bigalke 1933).
Crystalline Silica (SiO2) is sometimes present, either in the wood of families like Chrysobalanaceae, or in the leaf, as many Poales. It may be present as sand, or as larger, more organised bodies of distinctive shapes. Presence or absence of silica bodies, where they are deposited in the plant, and the morphology of the silica bodies provide especially valuable characters in the monocots (see Prychid et al. 2003b for a summary). Silica-containing cells or other parts of plants are commonly preserved in the fossil record, where they are called phytoliths (see Piperno 2006 for a good summary); phytoliths are especially important in archaeology. Ma and Takahashi (2002) summarize the literature on silicon concentrations in plants. Although most broad-leaved angiosperms and non-commelinid monocots have low concentrations while commelinids have high concentrations, some commelinids, including members of Commelinales and Juncaceae sampled, also have low concentrations, but these groups also lack SiO2 bodies. Currie and Perry (2007) provide an introduction to the biocemistry of silica in plants.
For a useful general discussion of variation in the leaf surface, see Wilkinson (1979). Hydathodes, extra-floral nectaries (see e.g. Schmid 1988), domatia, and even lenticels may be found on the leaf surface, although they rarely mentioned in these pages because their variation is mostly at a level lower than is the focus here. Hydathodes are particularly well supplied with tracheids, not with phloem; extra-floral nectaries show the reverse relationship (for the expression of the CRABS CLAW gene in floral but not extrafloral nectaries in core eudicots, see Lee et al. 2004; but cf. Krosnick et al. 2008a). Extra-floral nectaries may of course be found on various parts of the plant - stem, leaf, the outside surface of the calyx - and a distinction is sometimes made between such nectaries that are intimately involved in pollination, as in those of the cyathia of Euphorbia (nuptial nectaries) and extra-nuptial nectaries that have nothing immediately to do with pollination, but may attract ants that help protect the plant against herbivores or destroy young stems of vines that would otherwise use the plant as support. For a survey of extrafloral nectaries, see Elias (1983).
Epidermis. Epidermal cells may contain silica bodies (see above) or their walls may be lignified. In many monocots, but not, apparently, Helobiae (Metcalfe 1960), there are bulliform cells, large, thin-walled cells on the adaxial epidermis, that help cause curling and uncurling of the leaf blade as their tugor changes; subepidermal cells may be involved as well (Löv 1926; Linsbauer 1930). Epidermal cell shape is usually of little interest at the genus level and above.
Stomatal type. The stomatal types mentioned are those delimited by Van Cotthem (1970: slightly modified by Baranova 1983, see also Baranova 1987, Wilkinson 1979), however, there is much disagreement in the literature over stomatal morphology, even that of smaller families. Stomatal "type" is variable, even when definable, but further confusing the issue is the tendency for each author to use a different set of terms or to interpret a particular configuration of cells in a different way (Carpenter 2005 recently introduced nine new types in an exhaustive study of stomatal variation in the ANITA grade, but the forty four types recognised by Patel in 1979 [and largely ignored since] is probably the record). As Payne (1970) observed when introducing his helicocytic and allocytic stomatal types, what may seem to be a distinct stomatal type to one person can be considered a mere modification of an existing type to another person. Baranova (1987) is a useful guide through the nomenclatural chaos, as she noted "The distinctions among these types, although conceptually useful, are often difficult to draw. We should not let the necessity to categorize the diversity among stomatotypes obscure the fact that varition is continuous." (Baranova 1987: 64). The patterning of cells around hair bases, epidermal oil cells, etc., can also be described using the same set of terms, even if the idea that all these structures are homologous seems unlikely (cf. Carpenter 2006). Stebbins and Khush (1961) provide an early stomatal survey for monocots, although Tomlinson (1974) qualified their account, emphasising the importance of stomatal development (see also Rudall 2000). Note that stomata with different ontogenies occur in Poales as circumscribed here (Tomlinson 1969). Illustrations in Conover (1991) are useful for ascertaining stomatal morphology of some monocot groups. Inclusions in the guard cells may be of systematic interest (see Tiemann 1988).
Stomatal orientation also varies. In monocots with parallel venation (see below), the stomata are usually oriented with the long axes of their apertures parallel to that of the blade, however, in those monocots with reticulate venation orientation is often random (Conover 1983). Elsewhere, random orientation is common, although there are exceptions. Butterfass (1987) summarises information on taxa that have tranversely-oriented stomata.
Cuticle waxes. Data are taken from Barthlott and his coworkers (e.g. Fehrenbach & Barthlott 1988; Ditsch & Barthlott 1994, 1997; Barthlott et al. 2003). Barthlott et al. (1998) standardised the terms used in describing wax crystals, and noted that unoriented platelets, often with more or less irregular margins, are common; indeed, their distribution at and above the familial level seems of little systematic significance. Below I usually mention only other forms assumed by the waxes and their chemical composition (for which, see Meusel et al. 1999; Barthlott et al. 2003). In general, wax crystalloid type is more important in monocots (e.g. Barthlott & Fröhlich 1983), but even there the distinctive types are often disconcertingly sporadic in their occurence. Wax crystals, cuticular folds (Barthlott & Ehler 1977) and also trichomes (Barthlott et al. 2003) rarely occur together on the one plant, with wax crystals predominating in "basal" angiosperms (but they seem to provide little information of systematic interest in the ANITA grade) and monocots, while a well-developed indumentum, and so conversely poorly-developed wax crystals, predominate for example in Lamiales, Solanales, and Asterales.
Still extremely useful are compendia such as those of Arber (1925) for monocots, and Arber (1920) and Sculthorpe (1967) for aquatic plants, Goebel (1931, 1932) and Troll (1935-1971); Bell and Bryan (1991, 2008) provide an attractive short survey of plant morphology. Vegetative characters have been emphasised where possible, especially in the thumb-nail sketches of families, and here Hallé, Oldeman & Tomlinson (1978), Keller (1996), van Balgooy (1997, 1998, 2001) and Schatz (2001) have been valuable sources of information. However, many vegetative characters are best used in a more local context, identifying members of the local flora; for identification using vegetative characters, see e.g. Keller (1996), van Balgooy (1997, 1998, 2001), Schatz (2001), Rejmánek and Brewer (2001) and Spichiger et al. (2005).
The vegetative body
Hair type. Only particularly distinctive hair types are mentioned, and these rarely suggest relationships between families, although they are quite useful at rather lower levels. There is the same problem here as with stomatal morphology, etc.: "Because of the intergradations between trichome types, and also in consequence of the use of imprecise and varied morphological and histological terms, it is often difficult, from the voluminous published descriptions that exist, to determine exactly what type of trichome is being discussed, unless there is an illustration." (Theobald et al. 1979, p. 41). For terminology, see Payne (1978), Hewson (1988), Stearn (1992), etc.
The vegetative bud can be perulate (scaly) or "naked", that is, lacking morphologically modified leaves enclosing the apex. Information on this character is only sometimes mentioned. The number of axillary buds, and their arrangement (collateral, superposed) may be of interest. Branching is not always axillary (Fisher 1978), and a few monocots have dichotomous branching (Tillich 1998 for references.
Branching. Branching is generally axillary, but a number of aquatic Alismatales show vegetative bifurcation of the vegetative axes, perhaps precocious axillary branching; strictly dichotomous branching is very rare in seed plants, occuringg only if Hyphaene (Arecaceae), Strelitzia (Strelitziaceae), and a few other examples (e.g. Wilder 1975). Keller (1996) distinguished between immediate branching, with branches coming from the currrent flush, and delayed branching, with branches developed on the previous flush. This is close to the distinction between prolepsis and syllepsis, whether branches develop from the shoot with (prolepsis) or without (syllepsis) evident rest. However, attributing such features to taxa can be difficult, since branching behavior on sucker shoots may differ from that elsewhere on the plant. Immediate branching is relatively rare in temperate taxa, but may occur in the local representatives of otherwise largely tropical families that commonly have such branching (e.g. Lauraceae). Buds developing from the axils of the last pair of leaves of the previous flush (e.g. Clusia, Garcinia) present problems for such distinctions, since they may grow out simultaneously with the elongation of the main axis, but after a period of rest. Branching in vines and annuals may also depart from any family norm. Although this character is mentioned quite frequently in the characterisations, sampling is poor.
Prophyll number and orientation. Prophylls are usually either paired and lateral (the basal condition) or single and adaxial, although Piperaceae in particular seem to show variation in this. Whether or not prophylls are basal on the branch is often, but not always, associated with whether branches are proleptic or sylleptic; in proleptic branches the prophylls are usually basal, while in sylleptic branches they have a well-developed internode, the hypopodium, below them. Exceptions have both biological and systematic significance. Thus most Ulmaceae, Cannabaceae, etc., appear to have basal prophylls whatever the dynamics of shoot growth, and buds in the axils of these prophylls may be very conspicuous; inflorescences are often axillary in pairs, or paired and borne at the bottom of branches (they come from the axils of the prophyllar buds). Taxa with axillary thorns or branch tendrils - these are usually sylleptic - may also show distinctive patterns of variation. Prophylls that are not basal on the thorn (e.g. Gleditsia) are often associated with the development of serial buds in the axils of leaves of the main stem (these provide replacement meristems for the main axillary shoot = lignified and dead thorn), while thorns with basal prophylls (e.g. Crataegus) have functional buds in the axils of these prophylls and lack serial buds in the axils of the leaves of the main stem. Note that bracteoles (see below) are simply the prophylls of the flowering shoot. Especially in monocots, there is extensive discussion as to what prophylls "are" - one leaf, two connate leaves?; Blaser (1944) has summarised the often exasperating early literature on the topic.
Leaf insertion or phyllotaxis. I record the insertion of leaves on flowering branches, not on non-flowering axes (where these are distinct); the variations mentioned are spiral, 2-ranked (distichous), 3-ranked (tristichous) and opposite (including bijugate and whorled). Mabberley (1997), too, makes the important distinction between spiral and distichous (see also Corner 1946a), which are too often lumped together and ambiguously referred to as 'alternate'. For systematically informative variation in leaf insertion on the main stem/trunk when leaves on branches are invariant in their insertion, see Johnson (1993); note that this variation does not affect architectural models. I make no distinction between opposite and whorled leaf insertion in the family characterisations since this commonly varies within a family or genus; most families in which opposite leaf insertion is common also have taxa with whorled leaves. For the control of phyllotaxis, see Reinhardt et al. (2003) and Kuhlemeier (2007), for a good brief discussion on phyllotaxy and its measurement, see Rutishauser and Peisl (2001).
Leaves are typically made up of petiole and lamina, also a leaf base with which the stipules may be associated. Petiole and lamina in monocots and other angiosperm are equivalent only be designation (Tillich 1998). For details of general leaf development, whether the lamina develops from the leaf base (monocots) or leaf apex (broad-leaved angiosperms), see Kaplan (1973), Bharathan (1996), Tillich (1998), Rudall and Buzgo (2002) and Nardmann et al. (2004); Sinha (1999) reviews leaf development in angiosperms. The terete, unifacial blades with stomata all over the surface that are found in monocots may result from the development of the unifacial "tip" or "Vorläuferspitze" of the developing leaf (e.g. Arber 1925; Troll 1955; Troll & Meyer 1955; Kaplan 1975 - Oxypolis [Apiaceae] and monocots that are compared in this last paper would seem rather distant), or from the middle portion of a bifacial leaf (see illustrations in Linder & Caddick 2001, for which see also for a summary of the literature). Nevertheless, patterns of leaf development may vary at high levels. "Unifacial" leaves in broad-leaved angiosperms may have stomata on both sides of the blade (amphistomatic), but the blade is still dorsi-ventrally flattened, i.e., they are isobifacial. In some monocots, such leaves are often equitant at the base; they, too, are isobifacial, but they are laterally flattened. They appear to represent a normal bifacial dorsiventral blade that has folded and become connate adaxially, or they could be developed from a "midrib", or, developmentally both they and terete unifacial leaves may represent the genetic abaxialization of the leaf, the genes normally expressed abaxially being the only genes expressed at the leaf surface (Yamaguchi & Tsukaya 2007). Much more work is needed on this suite of characters: the development of an isobifacial monocot leaf such as are found in Acorus can be explained in quite different ways - is it a hyperphyll, and so equivalent to a broad-leaved angiosperm leaf, or does it originate from an intermediate zone between hyperphyll and hypophyll (Rudall & Buzgo 2002)? - but of course such descriptions are separate from those that result from developmental genetic studies. Similarly, the "blade" of Hosta and Orontium may not be equivalent structures in other than a functional sense (Troll 1955), with the ligule (see below) in at least some cases in a monocot demarcating the Vorläuferspitze from the rest of the leaf - note that in Zamioculcas there is a ligule very near the base of the petiole...
Leaf type. The main distinction is between simple and compound leaves (see Doyle 2007 for such variation and phylogeny). However, this distinction may not be so clear-cut as it seems. Bharanthan et al. (2002) show how in many leaves dissection in general - whether resulting in simple lobing, or in fully compound leaves - is associated with expression of the KNOXI (Class 1 KNOTTED1-like) gene. Normally it is expressed in the stem apex, but not in leaves as they are initiated. Some simple leaves may show KNOXI expression early, but lobing is obliterated by inner blade growth; all simple leaves are not identical. KNOXI is normally expressed during the development of compound leaves, even secondary unifoliate ones (Bharathan et al. 2002; Champagne et al. 2007); exceptions to this can be of phylogenetic interest (see Fabaceae). Many taxa that commonly have compound leaves have some members with apparently simple leaves; there, however, the blade may be joined to the petiole by an articulation, i.e. the leaves are reslly unifoliolate, not simple. A number of taxa with pinnate leaves also have palmate leaves, however, Kim et al. (2003) have recently made a distinction between peltately palmate and non-peltately palmate leaves. In peltately palmate leaves the petiole bundle is strictly annular (or the bundles form a circle), there being no adaxial surface, whereas in non-peltately palmate and pinnate leaves the petiole bundle is more or less obviously dorsiventral, the adaxial surface being evident. Monocots, with the exception of a few Araceae and Dioscoreaceae, lack truly compound leaves in which the the leaflets arise from separate primordia along the leaf, the activity of the blastozone having become restricted to those areas that later develop into leaflets (see Gunawardena & Dengler 2006 for a useful review). Apparently compound leaves such as those common in Arecaceae, as well as fenestrate leaves, as in Araceae and Aponogetonaceae, are caused by localized cell death (e.g. Gunawardena et al. 2004).
Blade size. Ecologists in particular can find it useful to divide the size of the leaf or leaflet blade into a number of classes (Raunkiaer 1934; Webb 1959). Rather small, narrow leaves with very strongly revolute margins, ericoid leaves, are quite common in taxa that grow in warm and dry climates.
Lamina ptyxis (vernation) - terms and some data are taken from Cullen (1978) and Keller (1996: note that his convolute is Cullen's supervolute). In descriptions of ptyxis of compound leaves, the reference is to the folding of the leaflets. The sampling of this character is poor, the delimitation of the states needs attention, and variation may turn out to be rather too much for comfort when used at the level of this site. Indeed, Charlton (1993) shows how the plane of flattening of conduplicate leaves changes during development, and there is quite commonly variation in the plane of flattening within the individual, while in Begonia the ptyxis of the prophyll is different from that of the other leaves. When leaves remain rolled up as they elongate, the lower surface in particular of the fully expanded blades may have faint but distinct longitudinal lines; these often help in familial or generic identification of sterile material. Three generalisations can be made about ptyxis type: first, in water plants with floating, peltate or cordate leaf blades, involute ptyxis is very common, second, the leaflets of compound leaves tend to be conduplicate, and third, ptyxis in monocots is supervolute or supervolute-curved.
Lamina margin. In the category "toothed" are all margins that are serrate or dentate, even minutely so; the margin may even appear to be smooth, but close inspection will show small, more or less regularly spaced dots or glands. A number of tooth morphologies have been described for the leaves of broad-leaved angiosperms (Hickey & Wolf 1975), but these are difficult to recognise on simple inspection of a leaf. Intermediates between these morphologies are known, as in Fagales (Hickey & Taylor 1991), where two "types" and their intermediates may occur on the one leaf, and elsewhere (Doyle 2007). However, tooth morphology may yield information of systematic significance if analysed more carefully than it has been treated here (see also Doyle 2007). Note that nearly all families described as having leaves with teeth have some members with entire margins. In monocots, leaf teeth are never glandular but can be more or less spiny.
Punctations, often "glands" of various kinds that are embedded in the mesophyll, are evident when the leaf is viewed in transmitted light, provide evidence of these anatomical features and are useful in identification. Note that magnoliids with ethereal oils tend to have ± punctate laminas (e.g. van Balgooy 1997), as well as the text-book examples of Rutaceae and Myrtaceae, where the glands are often much larger and more organised. Other taxa secreting oils, tanniniferous substances, etc., or with cystoliths or sclereids (see below) may also be punctate, while punctations may also be evident in the calyx, corolla, etc. Punctations are mentioned only when they are conspicuous.
Lamina venation. A distinction is drawn between leaves in which two or more major veins leave the midrib at or almost at the base of the lamina (palmate), those in which these veins leave the midrib along its length (pinnate), and those in which there is no distinction between a midrib and secondary veins, all veins being similar, parallel, and proceeding to the apex (parallel, obviously especially common in monocots); further details of venation are rarely given. Doyle (2007) considers such variation in a phylogenetic context. Klucking (1986 onwards) provides a comprehensive survey of leaf venation and Christophel and Hyland (1993) is another useful source of information, however, the detail shown by Ettingshausen (e.g. 1858) has hardly been improved upon. Hickey and Wolfe (1975), Hickey (1979) and the Leaf Architecture Working Group (1999) define terms used in describing venation. Conover (1983) and Inamdar et al. (1983) discuss leaf morphology and architecture in broad-leaved monocots with reticulate venation; such leaves have evolved several times and are associated with particular ecological conditions (e.g. Givnish et al. 2004c). In many broad-leaved monocot taxa the parallel main veins are linked by well developed ladder-like cross veins. Nelson and Dengler (1997) surveyed the development of leaf venation. Unfortunately, sampling within broad-leaved angiosperms was largely restricted to Rosids, and within these, Arabidopsis (Brassicaceae) differed in having basipetal development of tertiary venation (it is normally acropetal in Rosids). Taxa such as Liriodendron (Magnoliaceae) have an intermediate pattern of initiation. Venation density varies considerably, and this has been placed in an evolutionary/phylogenetic context by Boyce et al. (2008).
The leaf base is commonly sheathing in monocots; the sheath may be open, with free edges, or closed and continuous around the stem. It may bear distinctive, adaxial flanges or ligules at the very base or some way up the sheath; auricles, lateral projections of the petiole" or sheath/blade junction, also occur. The ligule in at least some cases in a monocot demarcates the Vorläuferspitze from the rest of the leaf (see above).
Stipules in broad-leaved angiosperms are more or less foliaceous, vascularised, often paired outgrowths at the base of the petiole or on the stem lateral to the petioles; they are a part of the leaf. They may be petiolar, clearly inserted on the petiole (and falling off when the leaf falls, if they have not fallen before), or cauline, largely inserted on the stem. Intrapetiolar stipules are inserted between the petiole and stem, sheathing stipules (= ochreae) surround the stem in a continuous tube, whether or not they are more or less scarious, as in Polygonum, or thick and robust, as in Coccoloba and some species of Fagraea, while interpetiolar stipules are single, often lobed stipules borne between opposing leaves. No attempt has been made to distinguish between "true" stipules and other structures; some things called stipules in families like Burseraceae and Sapindaceae are clearly modified colleters or leaflets, although when this is obvious the characterisations are qualified. Useful studies of stipules and stipuliform structures may be found in Weberling (e.g. 1968, 1970, and especially 2006 [largely a summary]) and Weberling and Leenhouts (1965); see above for correlations between nodal anatomy and stipule presence/absence. Tyler (1897) reviewed some of the older literature.
Some suggest that stipules ex definitio do not occur in monocots. However, as has been pointed out by authors like Roth (1949) and Rudall and Buzgo (2002), the developmental origins of monocot ligules and at least some stipules of broad-leaved angiosperms seem not to be fundamentally different; both arise from adaxial cross meristems in the transition zone between hyperphyll and hypophyll. Although I have not used the term "stipule" in the monocot characterisations, it is probable that there are structures there that have at least as good a title to the name as some of the things called stipules in broad-leaved angiosperms.
Other aspects of leaf morphology are mentioned only occasionally. These include the presence of abaxial cavities or domatia, which tends to be a character varying at the level of species or sometimes genera.
Colleters are multicellular, more or less glandular but usually unvascularised structures secreting resin and/or mucilage and usually found in the axils of petioles or stipules - and quite often the calyx, too; the secretion they produce may be involved in the lubrication of the tightly-pressed young parts of the plant as they slide over one another, or they may have a bacteriostatic function (Klein et al. 2004). The "intravaginal squamules" of monocots are included here, their only difference from colleters (a heterogeneous class of structures in the first place) seems to be that they are flattened (Doyle & Endress 2000). A useful survey is provided by Thomas (1991), but the distribution of colleters is definitely under-recorded (see Lersten 1974 for distinctive colleters in some Psychotria perhaps involved in the bacterial leaf nodules there). They may occur in other than the axillary position, e.g. several Apocynaceae, Rubiaceae, and Caricaceae.
The breeding system predominating in the family is often mentioned. Any sizeable family recorded as being monoecious or dioecious is rarely consistently so, conversely, families predominantly with "perfect" flowers (the common condition, and not mentioned when it occurs) nearly always have some taxa with other flower types. When staminate and carpellate flowers occur on the same inflorescence, the carpellate flowers are usually topologically basal; this is true of both monocots (e.g. Tomlinson 1982; Rudall 2003; Ambrose et al. 2005) and broad-leaved angiosperms (e.g. Krosnick et al. 2006). Exceptions in the latter group include Begonia (Begoniaceae) in which the carpellate flowers are produced in the cymose inflorescence after the staminate flowers, Ricinus (Euphorbiaceae) in which the carpellate flowers are in the top part of the inflorescence, Pteleopsis (Combretaceae), and even the fossil Archaefructus, at least under some interpretations of its morphology (Friis et al. 2003b). Protandry is supposed to be the default option in dichogamous plants since stamens would seem likely to mature before the carpels in flowers in which floral development was centripetal (Kalisz et al. 2006), however, protogyny is probably the plesiomorphous condition in angiosperms.
Inflorescence. For a generally accessible introduction to inflorescence morphology, which can be very complex and with a correspondingly complex and daunting terminology, see Weberling (1989, also Troll & Weberling 1989). Briggs and Johnson (1979) suggest a "more logical (but not theory-limited)" approach to inflorescence description and comparison, but it is difficult to apply to a study at this level, while Claßen-Bockhoff (2000) discusses previous attempts to describe and classify inflorescences (and suggests a general evolutionary schema for flowering plants herself). If one decomposes the basic growth processes that are likely to be going on in inflorescences, the variables are encompassed by thinking of an axis that is terminated by a flower, or not, that keeps on growing, or dies, and there are inflorescence bracts/prophylls that subtend branches or do not; the same goes on at all orders of branching. Add to that the degree of elongation of the internodes, and that is basically it (Kellogg 2000b). Benlloch et al. (2007) discuss inflorescence architecture from the point of view of floral gene expression.
Here I refer mainly to racemes or racemose (polytelic) inflorescences where the main axis does not terminate in a flower, the flowers being single in axils along the axis, and cymes of various kinds, when there is no extended main axis and all lateral axes are immediately terminated by flowers (for a discussion of the various kinds of monochasial cymes, see Buys & Hilger 2003). In a thyrse the axes main axis may or may not be terminated by a flower, but the branches are cymes, and in a panicle ("Rispe") the inflorescence is branched, there are no simple, bracteate cymes, but all axes are terminated by flowers (extreme polytely). Capitulae, corymbs and umbels are also mentioned in the characterisations below. Leins (2000) distinguished between indeterminate (open) and determinate (closed) inflorescences. The former included racemes, etc., as well as thyrses, rather confusingly defined as inflorescences in which the main and sometimes the lateral axes might not be terminated by flowers, while the latter included "Vorblatt-Inflorescenzen" (cymes), panicles, etc. [This is all very unsatisfactory.]
Inflorescences may be axillary and/or terminal, or they may be borne along the branches or even at the base of the trunk. A few taxa are geocarpous, with at least some inflorescences being underground (Kaul et al. 2000); the terms used to describe this and other similar behaviours are complex (Barker 2005). Epiphyllous inflorescences that arise from along the petiole or lamina, tend to be sporadic in occurence (see Stork 1956; Dickinson 1978), but cf. Dichapetalaceae and Phyllonomaceae + Hellwingiaceae.
Bracts and bracteoles are usually mentioned only when they are absent; the position of the latter is normally the same as that of prophylls on vegetative shoots (see above), since they are simply the prophylls of shoots that bear flowers. Nevertheless, in many monocots that have single adaxial prophylls on vegetative shoots the bracteoles are depicted as being lateral, or paired, or even abaxial and often associated with flowers with inverted or oblique symmetry (see below: Eichler 1875, 1880; see also Bruhl 1995; Buys & Hilger 2003); prophyll number and position in monocots will clearly repay further study. For genes controlling floral prophyll development, see Masiero et al. (2004). Bracts and bracteoles cannot be recognised when the flower is single and terminal.
Pedicel articulation may be a useful character, especially in monocots (for information there, see Schittler 1953; Kubitzki 1998a); there is a line across the pedicel where non-fertilised flowers will abscise. The part of the pedicel above the point of articulation may be considered different from the part below, perhaps being more strictly part of the flower proper; the term "pericladium" has been used.
The description of the flower proceeds from the outside in, from the sepals or perianth to the carpels, ending up with mention of the stigmatic surface. The only general positional terms used here to describe the positions of the parts of the flower are the ad/abaxial and median/lateral pairs. Particularly valuable sources of information for basic floral morphology are Eichler (1875-1878), Engler's Die natürliche Pflanzenfamilien, and the great Flora brasiliensis (Martius & others 1840-1906). Batygina (2002) has edited the first of three volumes on terms used to describe flowers and floral embryology, and there are a number of useful summaries of aspects of floral morphology and development, including those of Leins (2000) and Endress (2005a [especially development], 2005c). I have not grappled with the complexities of floral, particularly carpel, vascularization, although the vascular supply to individual organs may be mentioned.
Pollination syndromes are mentioned only occasionally mainly because variation is at a finer scale than is the focus of these pages (for a summary, see Faegri & van der Pijl 1979; cf. also Waser et al. 1996). In general, similar syndromes have evolved many times, indeed, in the past over-reliance on similarity in pollination syndromes as an indicator of relationships has caused problems from the ordinal level down (e.g. Amentiferae, Bignoniaceae, Ericaceae). Variation in pollination mechanisms is of course extensive and biologically and taxonomically interesting, even if characterization of the syndromes of characters that are supposed to characterise e.g. ornithophilous flowers may be overly simplistic (Waser & Ollerton 2006, for references; cf. in part Fenster et al. 2004). Even within individual pollination categories, pollinator variation may be extensive, thus the large flowers of Catalpa speciosa are pollinated by several species of both day- and night-flying insects (Stephenson & Thomas 1977). Photographing the flower under u.v. light (see above) may help clarify exactly what the pollinator sees. Barth (1985) provides a very readable summary of the interrelationships between insects and flowers. Comments on particularly distinctive or pervasive pollination mechanisms are sometimes made after the family characterizations, for example, when there are oil flowers or wind pollination (for which, see Friedman & Barrett 2008). Aquatic angiosperms may develop very distinctive pollination mechanisms (Cox & Humphries 1993; Philbrick & Les 1996; Les et al. 1997). Seymour (2001) lists taxa in which thermogenic flowers (i.e., with respiratory heat production) have been found (see Onda et al. 2008 for the different systems involved, which may co-exist in a single plant). Deceit pollination - no particular morphological syndrome, of course - is perhaps surprisingly common, especially in Orchidaceae (e.g. Renner 2006a; Lunau 2006; Ledford 2007) and floral mimicry, neither well understood nor easy to categorize in terms of the classic Batesian/Müllerian dichotomy, has been reported quite often (Roy & Widmer 1999 for a review; Benitez-Vieyra et al. 2007). Of course pollinators do not care about the morphological nature of the parts of the flower that attract them or provide them with nectar, and in many cases the inflorescence functions as a single flower (Asteraceae are a good example), or a single flower may appear to be several (e.g. Iridaceae).
A few structures that look to all intents and purposes like single flowers, even on close inspection, in fact represent the aggregation of several highly reduced flowers - a good example is Cercidiphyllum. Such extremely reduced floral structures are called pseudanthia (e.g. Claßen-Bockhoff 1990, 1991: Endress 1994b). However, there are all intermediates between this condition and the inflorescences in species of Viburnum, which have enlarged flowers on the margin of a flat-topped inflorescence all flowers which open simultaneously, and the capitulum common in Asteraceae that is made up of numerous small flowers which similarly may be functionally a single unit when it comes to atracting the pollinator (see above). However, in both these latter cases individual flowers are perfectly easily recognisable (see especially Claßen-Bockhoff 1990 for images). There has been a recent resurgence of interest in pseudanthia, focusing particularly on taxa like Lacandonia (Triuridaceae) and on members of Alismatales (e.g., Rudall 2003b; Buzgo et al. 2006 for literature), and the definition of pseudanthia has sometimes been broadened to include structures that are part flower, part inflorescence (Rudall & Bateman 2003; Sokoloff et al. 2006). These latter, which may be terminal (Sokoloff et al. 2006), can perhaps be confused with peloric flowers (Buzgo et al. 2006: see below).
Floral development. Many aspects of floral development, whether at the genetic or gross morphological level, are integrated into individual characterisations and throughout the hierarchy and are also discussed separately below. In terms of general patterns, development follows the sequence in which the parts are borne in the flowewr (from outside in) sepals - petals - androecium - gynoecium, with the primordia, at least those of different whorls, all being separate. However, within the androecium is particular there is substantial variation, e.g. centripetal (oldest stamens outside) versus centrifugal (oldest stamens inside: e.g. Corner 1946b, see below) and diplostemonous (sepalline whorl outside the petalline whorl) versus obdiplostemonous (vice versa) development. However, obdiplostemous androecia develop in a variety of ways (Hardy & Stevenson 2000b). In a few cases the androecium is initiated before the corolla (Bello et al. 2004 for references) or the two are initiated as common primordia. Finally, Lacandonia (Triuridaceae) is the only angiosperm in which the carpels surround the stamens (see Rudall 2003 and Ambrose et al. 2005 for literature: if the flower is really a pseudanthium the exception goes away, but something like heterotopy is a more likely explanation).
Buzgo et al. (2004) in their study of Amborella note that distinctions between different kinds of floral parts can be hard to make, even the distinction between perianth and prophylls; the numbers of parts and their arrangement also vary here and in other members of the ANITA grade and magnoliids in particular (see also Taylor et al. 2008; Endress 2008a). As Endress (2005c) emphasises when discussing petals, there are a number of features - position, function, development, shape, anatomy, histology, gene activity, and relationships to other taxa that clearly have petals - that "make" a petal a petal, so if a structure does not have one of these criteria, is it thereby excluded from petalhood? Or if it does have one of these features, is it to be included? Thus Maturen et al. (2005) recently found that floral organ diversity genes (B and C) were expressed in the large, white dogwood (Cornaceae, Cornales) bracts, and Geuten et al. (2006) suggest that heterotopic SEP3-like gene expression - in bracteoles and calyx in extant members - was present in the common ancestor of Balsaminaceae et al. (Ericales) clade... Complications also arise in cases where the petals are believed to be modified stamens (e.g. Lauraceae, Caryophyllaceae), whether or not these modified stamens are also functional nectaries (Ranunculaceae); such complications are discussed where they occur, although it may be noted here that in some classical theories of floral evolution it was thought that the petals of eudicots at least, if not broad-leaved angiosperms as a whole, represented sterilised and modified stamens, but Ronse de Craene (2006) suggests that in core eudicots they may more often be modified perianth/bract members. Ideas that some flowers may really be pseudanthia (see above), or that evolution of floral parts has been mediated by homeotic changes (e.g. Ronse De Craene 2003; Rudall 2003), further confound our attempts to peg specific terms to the parts of the flower. The general plasticity of perianth parts compounds the general difficulty of mapping of perianth evolution (for example) on the tree (e.g., Endress 2004; D. Soltis et al. 2005b). Finally, there is a kind of "pull of the present" in operation. We commonly talk about the evolution of petals because core eudicots, which are so diverse and on which so much work is being carried out, have them, but what has evolved may be more a sharp distinction between organs that function as sepals alone and those that function as petals alone from organs in which these functions were not so sharply distinct.
The analyses of floral development made by Payer (1857) can still be consulted with profit.
Cortical vascular bundles in the flower are mentioned only when they occur (for information, see Ronse Decraene 1992).
Flower part insertion. Flower parts are most commonly whorled, but they may be spiral or even chaotically arranged (Ronse Decraene & Smets 1998a). The distinction between a whorled and a spirally initiated androecium is not always sharp (see Ronse De Craene et al. 2003 for a survey of variation in this character in "basal" angiosperms; Buzgo et al. 2004, but c.f Endress 2008b - in ANITA-type angiosperms transitions between spiral and whorled arrangements are common, but intermediate types are not). Even in a multistaminate, spiral androecium, the outermost stamens may be initiated in pairs, at least when it comes to timing of initiation (e.g. Ronse Decraene & Smets 1993).
In many monocots, although much less frequently in the commelinids than elsewhere in the clade, each of the six stamens is borne opposite a tepal, and the bases of the two tepal whorls do not each separately completely surround the flower; such flowers are as it were pseudo-uniseriate when it comes to the perianth (see also below). Here, unlike the situation in broad-leaved angiosperms, a stamen may be strongly adnate to each petal-like tepal of both whorls, both when each tepal is more or less free (Androcymbium [Colchicaceae] is a particularly striking example) and even when the tepals form a tube. Posluszny et al. (2000) discuss a perianth-androecium module in the context of the floral evolution of Alismatales (see also Buzgo 2001; cf. Rudall 2003), and I have (rather uncritically) applied a similar idea to non-commelinid monocots in their entirety. In Liliaceae it has been found that both A and B class genes are expressed in both whorls of the perianth, which are similar and petal-like (Kanno et al. 2003), whereas in Commelinaceae only A class genes are expressed in the outer whorl - which is a sepal (Kanno et al. 2004). Although Endress (1995b, Fig. 6) described petals and stamens arising from a common primordium in Zingiberaceae, as also occurs in some groups of core eudicots, e.g. Myrsinaceae and their relatives (see Ronse Decraene et al. 1993 for a summary; de Laet et al. 1995 for discussion), this is perhaps a different situation. In these cases there is a clearly completely biseriate perianth, unlike the condition that is suggested here to be plesiomorphic condition in monocots. A further complication occurs in Acorales, some Alismatales and perhaps Piperales. There the bract and the median tepal of the outer whorl are similar developmentally, the median (abaxial) tepal of the outer whorl being bract-like (Buzgo & Endress 2000; Buzgo 2001).
Meristicity. Especially when the parts of the flower are whorled, members of the different whorls, particularly the calyx and corolla, commonly also the androecium, and rather less frequently the gynoecium, have the same basic number; 2- or 3-carpellate gynoecia in otherwise 4- or 5-merous flowers are common in core eudicots. Meristicity of basal angiosperm clades and the eudicots (but less in core eudicots) is notably labile, and a trimerous perianth - and less fequently androecium - is widely scattered (e.g. Kubitzki 1987; Endress 1987; Drinnan et al. 1994; Albert et al. 1998; Soltis et al. 2003; Zanis et al. 2003; P. Soltis et al. 2004; D. Soltis et al. 2005). Monocots are overwhelmingly trimerous in all five floral whorls, unlike the situation in other angiosperms (some Aristolochiaceae are close), even if the simplistic distinction (which I used to teach) of monocots - overwhelmingly 3-merous flowers; dicots - often 5 merous flowers, needs qualification. In many core eudicots there is inter- and even infraspecific variation in meristicity although a particular family may be predominantly either 4- or 5-merous.
Floral Orientation. The main variation in the orientation of the flower is whether the median ("odd") sepal, or the median member of the perianth or outer tepalline whorl, is adaxial or abaxial. The plesiomorphic condition for the angiosperms seems to be to have the median member adaxial, although ascertaining this is complicated by the haplomorphy of the flowers of some of the basal pectinations. In the monocots the median member is abaxial (this is associated with variation in prophyll and bracteole position, q.v., i.e. if this/these are absent, the median tepal may switch positions 180 degrees; see also Hofmeister's Rule); Spichiger et al. (2005; see also Eichler 1875-1978) suggest that Agave, Tillandsia (but this is a mistake), Allium, Galanthus, Dioscorea, and Lilium all have the median member adaxial, that is, in this reversed position. Indeed, exceptions to the rule in monocots are very common often systematically and biologically interesting, often - but by no means always - being associated with various forms of monosymmetry (see below). Stuetzel and Marx (2005) note the variability of the position of monocot bracteoles/prophylls, perhaps because axillary flowers may in fact represent reduced racemes. For additional discussion of tepal orientation in monocots, see also Eichler (1875), Engler (1888) and Remizova et al. (2006b); the orientation of the flower may depend on the presence of a prophyll, also on the existence of other foliar structures on the pedicel.
Inverted flowers may arise in three ways. 1. The distinctive orientation of the flower may be evident from the beginning of development (Bremer et al. 2001 describe variation in the corolla orientation in monosymmetric flowers of euasterids that includes this). 2. Inversion may be secondary, by resupination. Here the pedicel and/or ovary (when inferior) may twist and become resupinate (as in Orchidaceae), or the corolla itself may twist (Daniel & McDade 2005), as in some Acanthaceae, or the flower may basically just flop over. This latter condition occurs in Balsaminaceae, for example, which has rather slender pedicels and strongly monosymmetric flowers where most of the mass of the flower