LIGNOPHYTA

True roots +; lateral meristems: cork cambium producing cork abaxially, vascular cambium producing phloem abaxially and xylem adaxially.

EXTANT SEED PLANTS/SPERMATOPHYTA

Plant woody, evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignins derived from (some) sinapyl and particularly coniferyl alcohols, thus containing p-hydroxyphenyl and guaiacyl lignin units, (lignins derived from p-coumaryl alcohol, i.e. S [syringyl] lignin units); true roots present, apex multicellular, xylem exarch, and branching endogenous; arbuscular mycorrhizae +; shoot apical meristem multicellular, interface specific plasmodesmatal network; stem with ectophloic eustele, endodermis 0, xylem endarch, branching exogenous; vascular tissue in t.s. discontinuous by interfascicular regions; vascular cambium + [xylem ("wood") differentiating internally, phloem externally]; wood homoxylous, tracheids and rays alone, tracheid/tracheid pits circular, bordered; mature sieve tube/cell lacking functioning nucleus, plastids with starch grains; phloem fibres +; stem cork cambium superficial, root cork cambium deep seated; leaves with single trace from sympodium ["nodes 1:1"]; stomata ?; leaf vascular bundles collateral; leaves megaphyllous [determinancy evolved first, then ad/abaxial symmetry], spiral, simple, lamina with vein density up to 5 mm/mm² [mean for all non-angiosperms 1.8]; axillary buds associated with at most some leaves; prophylls [including bracteoles] two, lateral; plant heterosporous, sporangia eusporangiate, on sporophylls, sporophylls aggregated in indeterminate cones/strobili; true pollen [microspores, i.e. no distal pore for release of gametes] +, grains mono[ana]sulcate, exine and intine homogeneous; ovules unitegmic, crassinucellate, megaspore tetrad tetrahedral, only one megaspore develops, megasporangium indehiscent; male gametophyte development first endo- then exosporic, tube developing from distal end of grain, to ca 2 mm from receptive surface to egg, gametes two, developing after pollination, with cell walls, with many flagellae; female gametophyte endosporic, initially syncytial, walls then surrounding individual nuclei; seeds "large", first cell wall of zygote transverse, embryo straight, endoscopic [suspensor +], short-minute, with morphological dormancy, white, cotyledons 2; plastid transmission maternal; two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], nrDNA with 5.8S and 5S rDNA in separate clusters; mitochondrial nad1 intron 2 and coxIIi3 intron and trans-spliced introns present.

MAGNOLIOPHYTA

Lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, non-hydrolysable tannins, quercetin and/or kaempferol +, apigenin and/or luteolin scattered, [cyanogenesis in ANITA grade?], S [syringyl] lignin units common, positive Maüle reaction [syringyl:guaiacyl ratio more than 2-2.5:1], and hemicelluloses as xyloglucans; root apical meristem intermediate-open; root vascular tissue oligarch [di- to pentarch], lateral roots arise opposite or immediately to the side of [when diarch] xylem poles; origin of epidermis with no clear pattern [probably from inner layer of root cap], trichoblasts [differentiated root hair-forming cells] 0; shoot apex with tunica-corpus construction, tunica 2-layered; reaction wood ?, with gelatinous fibres; starch grains simple; primary cell wall mostly with pectic polysaccharides, poor in mannans; tracheid:tracheid [end wall] plates with scalariform pitting, wood parenchyma +; sieve tubes enucleate, sieve plate with pores (0.1-)0.5-10< µm across, cytoplasm with P-proteins, cytoplasm not occluding pores of sieve plate, companion cells from same mother cell that gave rise to the sieve tube; sugar transport in phloem passive; nodes unilacunar [1:?]; stomata with ends of guard cells level with pore, paracytic, outer stomatal ledges producing vestibule; leaves petiolate, lamina [formed from the primordial leaf apex], development of venation acropetal, 2ndary veins pinnate, fine venation reticulate, veins (1.7-)4.1(-5.7) mm/mm², endings free; most/all leaves with axillary buds; flowers perfect, pedicellate, polysymmetric, parts spiral [esp. the A], free, numbers unstable, development in general centripetal; P not sharply differentiated, with a single trace, outer members not enclosing the rest of the bud, often smaller than inner members; A many, filament not sharply distinguished from anther, stout, broad, with a single trace, anther introrse, tetrasporangiate, sporangia in two groups of two [dithecal], ± embedded in the filament, with at least outer secondary parietal cells dividing, each theca dehiscing longitudinally by action of hypodermal endothecium, endothecial cells elongated at right angles to long axis of anther; tapetum glandular, binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellar, endexine thin, compact, lamellate only in the apertural regions; nectary 0; G free, several, ascidiate, with postgenital occlusion by secretion, stylulus short, hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, dry [not secretory]; ovules few [?1]/carpel, marginal, anatropous, bitegmic, micropyle endostomal, outer integument 2-3 cells across, often largely subdermal in origin, inner integument 2-3 cells across, often dermal in origin, parietal tissue 1-3 cells across [crassinucellate], nucellar cap?; megasporocyte single, hypodermal, megaspore tetrad linear, functional megaspore chalazal, lacking sporopollenin and cuticle; female gametophyte four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; P deciduous in fruit; seed exotestal; pollen binucleate at dispersal, trinucleate eventually, germinating in less than 3 hours, pollination siphonogamous, tube elongated, growing at 80-600 µm/hour, with pectic outer wall, callose inner wall and callose plugs, growing between cells, penetration of ovules via micropyle [porogamous] within ca 18 hours, distance to first ovule 1.1.-2.1 mm, tube moves between nucellar cells; double fertilisation +, endosperm diploid, cellular [micropylar and chalazal domains develop diffently, first division oblique, micropylar end initially with a single large cell, divisions uniseriate, chalazal cell smaller, divisions in several planes], copious, oily and/or proteinaceous, embryo cellular ab initio, minute; germination hypogeal, seedlings/young plants sympodial; Arabidopsis-type telomeres [(TTTAGGG)_n]; whole genome duplication, ndhB gene 21 codons enlarged at the 5' end, single copy of LEAFY and RPB2 gene, knox genes extensively duplicated [A1-A4], AP1/FUL gene, paleo AP3 and PI genes [paralogous B-class genes] +, with "DEAER" motif, SEP3/LOFSEP and three copies of the PHY gene, [PHYB [PHYA + PHYC]].

Evolution. Divergence & Distribution. Possible apomorphies for flowering plants are in bold. Note that the actual level to which many of these features, particularly the more cryptic ones, should be assigned is unclear. This is because some taxa basal to the [magnoliid + monocot + eudicot] group have been surprisingly little studied, there is considerable homoplasy as well as variation within and between families of the ANITA grade in particular for several of these characters, and also because details of relationships among gymnosperms will affect the level at which some of these characters are pegged. For example, if reticulate-perforate pollen is optimized to the next node on the tree (see Friis et al. 2009 for a discussion), it effectively makes the pollen morphology of the common ancestor of all angiosperms ambiguous... For other features such as details of sugar transport in the phloem, their placement on the tree is frankly speculative. Finally, for features such as parietal tissue/a nucellus only one (Nymphaeales) to three cells thick above the embryo sac and a stylar canal lacking an epidermal layer, although plesiomorphous for basal grade angiosperms (Williams 2009), I am unsure where on the tree a thicker nucellus and a stylar epidermal layer are acquired.

NYMPHAEALES [AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]: vessels +, elements with elongated scalariform perforation plates; wood fibres +; axial parenchyma diffuse or diffuse-in-aggregates; tectum reticulate-perforate [here?]; ?genome duplication; "DEAER" motif in AP3 and PI genes lost, gaps in these genes.

[MONOCOTS [CERATOPHYLLALES + EUDICOTS]]: (veins in lamina often 7-17mm/mm² or more [mean for eudicots 8.0]); (stamens opposite [two whorls of] P); (pollen tube growth fast).   Back to Main Tree

Evolution. Divergence & Distribution. The age of this clade is perhaps 147-143 million years before present (Leebens-Mack et al. 2005), but fossil-based estimates are somewhat younger, ca 100 million years (Crepet et al. 2004: monocots sister to magnoliids). However, the recent fossil findings of Sun et al. (2011) would imply a substantially greater age for Ranunculales - and hence the whole eudicot clade - of some ca 152-140 million years, so this clade would be somewhat older... Indeed, Chaw et al. (2004: 61 chloroplast genes, sampling poor) date this node to 150-140 million years ago, but Moore et al. (2010: 95% highest posterior density) estimate an age of (142-)135(-127) million years and Davies et al. (2011: 95% credibility intervals) an age of (161-)137(-124) million years.

Chemistry, Morphology, etc. Details of the exact position and magnitude of changes in characters like leaf venation density and pollen tube growth are still provisional (see Boyce et al. 2008; Williams 2008 for more details). The stamen-perianth member pairing, as well as the fact that the bases of members of a perianth whorl do not completely surround the floral apex, are two features very common in monocots, but they are rather more scattered in the eudicot clades up to Gunnerales, after which they are pretty much non-existent. Lauraceae may also be interpreted as having this sort of flower (see also below), so where this feature is to be placed on the tree is a little uncertain. Authors (e.g. Chen et al. 2007) have drawn attention to the occurrence of dimery and A-T pairing in the grade Proteales to Gunnerales.

Phylogeny. Relationships between the lineages immediately above the basal pectinations in the main tree, the ANITA grade (Amborellales, Nymphaeales and Austrobaileyales here), have recently been clarified. The topology of the main tree in this area thus differs somewhat from that in A.P.G. (2003). For further information, see the discussion immediately preceding the Magnoliales, i.e. the magnoliid clade. Chloranthales, eudicots and monocots are the other clades involved. There is, however, acccumulating evidence that Ceratophyllales are sister to eudicots.

MONOCOTYLEDONS / MONOCOTYLEDONEAE / LILIANAE Takhtajan   Back to Main Tree

Plant herbaceous, more or less rhizomatous, growth sympodial; non-hydrolyzable tannins [(ent-)epicatechin-4] +, ellagitannins, neolignans, benzylisoquinoline alkaloids 0, hemicelluloses as xylans; root apical meristem?; root epidermis developed from outer layer of cortex; trichoblast in atrichoblast [larger cell]/trichoblast cell pair further from apical meristem, in vertical files, or hypodermal cells dimorphic; endodermal cells with U-shaped thickenings; cork cambium in root [uncommon] superficial; root vascular tissue oligo- to polyarch, medullated, lateral roots arise opposite phloem poles; primary thickening meristem +; vascular bundles in stem scattered, (amphivasal), closed [no interfascicular cambium developing]; vessel elements in root with scalariform and/or simple perforations; tracheids only in stems and leaves; sieve tube plastids with cuneate protein crystals alone; stomata parallel to the long axis of the leaf, in lines, brachyparacytic; leaves not differentiated into petiole plus lamina, main venation parallel, veins joining successively from the outside at the apex, developing both acropetally and basipetally from the base and converging towards the apex, intermediate [and other] veins basipetal from apex, endings not free, (margins with spiny teeth), Vorläuferspitze +, leaf base sheathing, sheath open, colleters [intravaginal squamules] +; prophyll single, adaxial; inflorescence terminal, racemose; flowers 3-merous [6-merous to the pollinator?], polysymmetric, pentacyclic; T in two whorls, each member with three traces, median member of outer whorl abaxial, aestivation open, members of whorls alternating, similar, [pseudomonocyclic, each providing a sector for the T tube when present]; stamens = and opposite each T member [primordia often associated, and/or A vascularized from tepal trace], anther and filament more or less sharply distinguished, anthers subbasifixed; G [3], with congenital intercarpellary fusion, opposite outer tepals [thus median member abaxial], placentation axile; outer integument often largely dermal in origin, parietal tissue 1 cell across; antipodal cells persistent, proliferating; fruit a loculicidal capsule; seed testal; embryo long, cylindrical, cotyledon 1, terminal, plumule lateral; primary root unbranched, not very well developed, adventitious roots numerous, hypocotyl short, (collar rhizoids +), cotyledon with a closed sheath, unifacial [hyperphyllar], both assimilating and haustorial; duplication producing monocot LOFSEP and FUL3 genes, [latter duplication of AP1/FUL gene], PHYE gene lost. - 11 orders, families, 60,100 species.

Evolution. Divergence & Distribution. Bell et al. (2010: note, monocots sister to Chloranthaceae, magnoliids, etc.) suggest ages for stem monocots of (156-)146(-139) or (138-)130(-123) million years depending on the method used. The age of the crown monocots has been variously estimated at ca 200±20 million years before present (Savard et al. 1994), 160±16 million years before present (Goremykin et al. 1997), 135-131 million years (Leebens-Mack et al. 2005), 133.8-124 million years (Moore et al. 2007), etc., all using molecular data, however, a fossil-based estimate is only ca 90 million years (Crepet et al. 2004), the stem age being ca 98 million years. Bremer (2000b) suggested that the split between Acorales and other monocots could be dated to ca 134 million years before present (147-121 million years), a date also used in a more recent and comprehensive analysis that formed the basis for dating the age of monocot groups in general (Janssen & Bremer 2004); Magallón and Castillo (2009, which consult for more details) suggest ca 177 million years for relaxed and 127 million years for constrained penalized likelihood datings of the same split - probably underestimates; while Moore et al. (2010: 95% highest posterior density) estimate an age of (129-)122(-117) million years for this clade. Indeed, recent molecular estimates range from (167-)156(-139) million years old (with eudicot calibration) to (191-)164(-141) million years (without: Smith et al. 2010, 95% HPD limits, cf. also their Table S3, slightly younger divergence-time estimates).

The oldest unequivocally monocotyledonous fossils (as pollen) are from the Late Barremian-Early Aptian of the Cretaceous some 120-110 million years ago and are assignable to Araceae-Pothoideae-Monstereae and perhaps also Araceae-Aroideae; Araceae are sister to other Alismatales (Friis et al. 2004, 2010: for fossil monocots, cf. Gandolfo et al. 2000 and Friis et al. 2006b).

Scattered in monocots are taxa with broad, net-veined leaves and also fleshy fruits (excluding things like arillate, ant-dispersed seeds). Both these features are adaptations to shady conditions and they have tended to evolve together but independently (Dahlgren & Clifford 1982; Patterson & Givnish 2002; Givnish et al. 2005, 2006b). A number of monocots with broad leaves are vines which live in shady habits for at least parts of their lives, and there may also be an association with unoriented stomata (see Cameron & Dickison 1998 for references for the latter feature). Givnish et al. (2005, 2006b) have suggested that net venation has arisen at least 26 times in monocots (and fleshy fruits 21 times); they were sometimes subsequently lost). These two features, although independent, showed very strong signs of tending to be gained or lost in tandem, a phenomenon described as "concerted convergence" (Givnish et al. 2005, 2006b).

For endosperm evolution in monocots, see Tobe and Kadokawa (2010). However, the subtleties of endosperm development are not captured by the typology employed here (see especially Floyd et al. 1999; Floyd & Friedman 2000, 2001 for alternatives). For instance, if Tofieldiaceae are sister to the rest of Alismatales, then where the change from cellular to helobial endosperm should be placed on the tree is unclear. Either one gain (apomorphy for order) and one loss (Araceae), or two gains (Tofieldiaceae and above Araceae). But the initial division of the endosperm is highly asymmetrical in Araceae, with subsequent divisions initially occurring only in the micropylar chamber (Tobe & Kadokawa 2010), the chalazal cells sometimes becoming massive (e.g. Paremeswaran 1959). Asymmetry in endosperm development characterizes helobial endosperm development, so this could be characterized as an extreme form of helobial development, but note that Masheshwari and Khanna (1957) and Tobe and Kadokawa (2010) characterize Araceae as having cellular endosperm development alone. Acoraceae also differ in the development of the two endosperm compartments (Buell 1938).

Some features that are likely to be synapomorphies are in bold in the characterization above. If Ceratophyllaceae were sister to monocots, synapomorphies like the herbaceous habit, absence of vascular cambium, etc., could be moved down a node, but currently there is little evidence for such a likelihood (see Jansen et al. 2007; Saarela et al. 2007; Moore et al. 2007). There may have been an early whole-genome duplication in this clade (Tang et al. 2010). Note that over half the putative synapomorphies in Table 4.1 of Soltis et al. (2005b) are in fact unlikely to be synapomorphies. The nature of the anther-filament junction has not been optimised in this part of the tree.

Plant-Animal Interactions. Caterpillars of Castniidae skipper butterflies eat a variety of monocots (Forbes 1956; see Powell et al. 1999 for some other groups that prefer monocots). Larvae of the chrysomelid beetle group Galerucinae subribe Diabroticites are apparently quite common on monocots, where they feed on roots (Eben 1999), indeed, Hispinae-Cassidinae (6000 species), sister to Galerucinae (10,000 species) are the major group of monocot-eating beetles (Jolivet & Hawkeswood 1995; Wilf et al. 2000; Chaboo 2007). Wilf et al. (2000) thought that these beetles initially ate aquatic members of Acorales and Alismatales, the association of commelinids with Hispinae-Cassidinae being derived, but Gómez-Zurita et al. (2007) suggest that the two main clades of monocot-eating chrysomelid beetles are unrelated, and also that the chrysomelids diversified 86-63 million years ago, well after the origin of monocots.

The idea has been floated that monocots experience less herbivory in tropical lowland rainforests than do other angiosperms, in part because they are tough and in part because the leaves remain rolled up for a relatively long time (Grubb et al. 2008); many monocots also have raphides as their main crystalline form of calcium oxalate, and these may be involved in herbivore defence (e.g. see Araceae; Franceschi & Nakata 2005).

Bacterial/Fungal Associations. Monocots are practically never ectomycorrhizal, but myco-heterotrophy is disproportionally common here - this may be because there is no secondary thickening, a thick cortex, no primary root, etc. (Imhof 2010).

Ecology & Physiology. Monocot vegetative morphology, their ecology, and their physiology are all closely linked. It has long been noted that many of the distinctive features of monocots are compatible with an origin from aquatic or hydrophilous ancestors (e.g. Henslow 1893 and references: the style of comparison and suggested mechanisms are interesting!). The scattered vascular bundles in the stem, long linear leaves, absence of secondary thickening (all perhaps related to the biomechanics of living in water), clusters of roots, rather than a single, branched tapwoot (see nature of substrate - mud), even the sympodial habit, etc., are all compatible with such an origin (see Mangin 1882 for "adventitious" roots in monocots). Many members of the first two pectinations in the monocot tree, Acorales and Alismatales, are water or marsh plants or at least prefer to grow in damp conditions.

However, even if monocots were sister to the aquatic Ceratophyllales (which see for literature) and/or their origin can be linked to the adoption of some kind of aquatic habitat, it does not help much in our understanding of the evo-devo side of how the distinctive monocot anatomical features, etc., evolved. Indeed, monocots appear to be so different from other angiosperms that relating their morphology, anatomy and development to that of broad-leaved angiosperms is difficult (e.g. Zimmermann & Tomlinson 1972; Tomlinson 1995). It has also been suggested that vessels in monocots and other angiosperms evolved independently (Cheadle 1953 and references). Nymphaeales - another aquatic group, also once suggested as being close to monocots and now including Hydatellaceae, which until recently were considered to be monocots - and Ceratophyllales are scarcely less remarkable in their vegetative morphology, but the common ancestors of all these clades with other angiosperms are likely to be plants with broad, petiolate leaves and a woody stem with lateral thickening meristems (cork and vascular cambia).

Many monocots are sympodial (Holttum 1955) and form tufts of leaves in part of each growth cycle, and/or are geophytes; internode elongation in such cases is very slight. Not only are the plants sympodial, but the roots develop from the stem, and since there is usually no secondary thickening, the hydraulic systems of root and stem are not in direct contact (Carlquist 2009). There can be marked differences in the xylary tissues in root, underground perennating organ, stem, and leaf, with at least some plant organs being secondarily vesselless (Carlquist 2009).

Given that a vascular cambium in monocots is uncommon, yet many are plants of quite considerable stature, even tree-like, there must be considerable changes in the plant in the period between germination and the mature (flower-producing) stage, particularly in the size of the apical meristem. This period is often designated as establishment growth, and occurs without stem elongation (e.g. Tomlinson & Esler 1972; Bell & Bryan 2008). Indeed, Burtt (1972) noted that during germination in monocots the plumule is frequently carried below the surface of the ground, a tube formed by the cotyledonary sheath (a "dropper") has the plumule and radicle/root area at the bottom.

There is some variation of lignin composition within monocots (see also Poales), and also in the rate of litter decomposition. Thus Cornwell et al. (2008) noted that "graminoid" litter decomposed more slowly than that of forbs, and "monocot" lignin more slowly than that of other angiosperms - and at about the same rate as that of gymnosperms. Clarification of lignin composition and the rate of decomposition of monocot tissues is needed.

Vegetative Variation. The development of monocot leaves needs more study. The basic tunica-corpus construction in monocots in similar to that in other angiosperms and from this point of view their leaves are similar, although a 1-layered tunica, as in maize, is somewhat more common here (Stewart & Dermen 1979; Jouannic 2011 and references). The outer tunica layer can proliferate and produce a rather broad margin to the variegated leaf.

However, it is the relation between the blade of a monocot leaf and that of a broad-leaved angiosperm that is of the greatest interest. Leaves can be divided into a hyperphyll and hypophyll. In broad-leaved angiosperms the former gives rise to the blade, which develops in an acropetal fashion - that is, the veins are first formed at the base of the blade. The hypophyll, on the other hand, gives rise to the petiole, leaf base, and stipules. In broad-leaved angiosperms like Arabidopsis development at the base of the blade, the junction of the hyperphyll and hypophyll, proceeds in two directions. Cells there are cut off both distally towards the apex and proximally towards the base (Ichihashi et al. 2011). In monocots, most of the leaf is developed from the hypophyll alone and development (add....). In many monocots there is a "Vorläuferspitze", a usually small abaxial unifacial conical or cylindrical protrusion at the apex of the mature leaf; this represents the entire upper part of the leaf. However, in Acorus and at least some Alismatales in particular a bifacial blade may develop from this upper part of the leaf primordium. Plants with such leaves would be similar in development to broad-leaved angiosperms. In the "typical" monocot leaf the blade develops from the equivalent of the leaf base in broad-leaved angiosperms (ref.), and this kind of development might then be a synapomorphy of a subgroup of the monocot clade, perhaps the entire group minus Acorales and Alismatales. But the leaves of Scindapsus (Araceae), but not Arisaema, Orontium, and Zamioculcas in the same family, and even Acorus (Acoraceae) itself, may develop in a "typical" monocot fashion (Troll & Meyer 1955; Bharathan 1996; Doyle 1998b).

Net venation may have arisen at least 26 times in monocots (see above: Givnish et al. 2005, 2006b). Thus the "blade" of Hosta and that of Orontium may not be equivalent in other than a functional sense (Troll 1955). Indeed, monocot leaves that have petioles and a blade that is net-veined in fact are not particularly similar morphologically - taxa with such leaves include Smilax, Trillium (Liliales), Dioscorea (Dioscoreales), Lowia (Zingiberales - leaves with broad blades very common here), Stemona (Pandanales), etc. Some kind of midrib or central vein is common in monocots (Doyle et al. 2008, which see for further details of the venation of monocot leaves, etc.).

The terete, unifacial blades with stomata all over the surface that are found scattered in monocots may result from the elaboration of the unifacial Vorläuferspitze (e.g. Arber 1925; Troll 1955; Troll & Meyer 1955; Kaplan 1975 - Oxypolis [Apiaceae] and the monocots that are compared would seem rather distant), or from the middle portion of a bifacial leaf (see illustrations in Linder & Caddick 2001, which see also for a summary of the literature). Monocots that do not have bifacial blades with stomata on only one surface may have unifacial and terete leaves, or laterally flattened and isobifacial laeves that are also often equitant at the base. Although the latter appear to represent a normal bifacial dorsiventral blade that has folded and become connate adaxially, they may represent the elaboration of a midrib/costal region, or, developmentally both they and terete unifacial leaves may represent the genetic abaxialization of the leaf, the genes normally expressed abaxially alone being the only genes expressed, at least at the leaf surface (Yamaguchi & Tsukaya 2010). Indeed, the leaves of Acorus are unifacial and equitant, quite unlike the bifacial leaves of grasses, and Yamaguchi et al. (2010) show how in Juncus equitant leaves differ from terete leaves by the activity of the DL gene that elsewhere in monocots is involved in midrib development. The development of an isobifacial monocot leaf such as is found in Acorus can also be explained classically 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.

The ligule in at least some cases demarcates the Vorläuferspitze from the rest of the leaf - note that in Zamioculcas there is a ligule very near the base of the petiole... Indeed, ligules may be paired structures; they are scattered throughout the monocots and are born either at the base (e.g. Potamogetonaceae) or top (e.g. Poaceae) of the petiole or sheath. Smilax has paired tendrils near the base of the petiole. Note, however, that such paired structures, whether tendril or ligule, are practically never called stipules because monocots are supposed not to have stipules.... 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 arising from adaxial cross meristems, a sort of intercalary meristem, in the transition zone between hyperphyll and hypophyll (see also Ichihashi et al. 2011). 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 - or perhaps all should be called ligules (see also Colomb 1887)!

Truly compound leaves are rare (Zamioculcas is an example), but cell death may result in the leaves appearing to be compound (a few Araceae) or having distinctive perforations (some Araceae and Aponogetonaceae). In palms, a process related to abscission causes the leaves to become dissected and appear compound (Nowak et al. 2007, 2008).

Genes & Genomes. Lee et al. (2011: cf. sampling and topology) found that genes involved in cell fate commitment, auxin metabolism, etc., tended to cluster at this node of the tree.

Salse et al. (2009) suggest that the common ancestor of monocots had five protochromosomes.

Chemistry, Morphology, etc. Some monocots (Amaryllidaceae, Araceae) have benzylisoqinoline alkaloids, but it is unclear if they are produced by the same biosynthetic pathway as these alkaloids in broad-leaved angiosperms (Waterman 1999).

Cork cambium in the roots is superficial in origin, developing just beneath the exodermis (Arber 1925); I am unclear as to how common it really is, but at best it seems rare. For the primary thickening meristem, see Rudall (1991, a summary), de Menezes et al. (2005) and Pizzolato (2009). This meristem is quite variable in details of its origin and the tissues to which it gives rise (initials in endodermis gives rise to cortex, those in pericycle, the vascular system), and de Menezes et al. (2011) suggested that there is no distinct primary thickening meristem in monocots. Vascular bundles in a number of monocots may have a sort of cambial layer, but it never amounts to much (Arber 1919).

Cheadle (e.g. 1944 and references) and Wagner (1977) surveyed vessel types in the vegetative parts of monocots; although there appear to be large-scale patterns, these data need to be extended. Cheadle (e.g. 1944) noted that there could be substantial variation in vessel morphology between closely related (congeneric) species. Cheadle also found considerable variation between different organs on the same plant (also Carlquist 2009), which he used to establish his evolutionary trends (see also Cheadle 1955, 1964, 1969a, b, 1970; Cheadle &Kosakai 1971). Tomlinson and Fisher (2000) noted a correlation in climbing monocots between presence of simple perforation plates in the metaxylem vessels and absence of direct protoxylem/metaxylem continuty and of the presence of scalariform perforation plates and the occurrence of direct protoxylem/metaxylem continuty. Amphivasal vascular bundles are common in monocot stems, although they are absent in some groups (e.g. Jeffrey 1917; Arber 1925). Botha (2005) discusses distinctive thick-walled late-formed sieve tubes that lack companion cells that are to be found only in monocot vascular bundles (but the sampling is not that good); they are found close to the tracheary elements in the bundles.

Many monocots, although not the old Helobieae (here in Alismatales) have thin-walled bulliform cells in the adaxial epidermal and/or in other tissues that cause the leaf to curl as they lose turgor (Löv 1926). Paracytic (and tetracytic) stomata are common in monocots, and variation in how they develop may characterise major clades, although there is much variation within them (Poales [Commelinales + Zingiberales]]: cf. Tomlinson 1974, which, however, see for data, also Paliwal 1969; Pant & Kidwai 1965; - many more observations are still needed: Rudall 2000). Zonneveld (2007) suggests that stomata occur in general epidermal cells in monocots, but not in other angiosperms; I have been unable to confirm this observation.

When the leaf is differentiated into petiole and lamina, any feature of the vernation noted in the characterizations refers only to the latter. Monocot leaf teeth, when present, are more or less spinose, never glandular. Colleter-like structures ("intravaginal squamules") may be a synapomorphy of monocots or of independent origins in Acorales and other Alismatales, within Araceae, for instance, they seem to be known only from very much phylogenetically embedded genera such as Philodendron, Cryptocoryne and Lagenandra (M. Carlsen, pers. comm.; see also Wilder 1975).

It is interesting that monosymmetric flowers in monocots are very frequently presented in the common angiosperm position with the median sepal adaxial; the main exceptions are the monosymmetric flowers of most Zingiberales. It is unclear why this should be so (well, one can come up with stories, e.g., the abaxial tepal that acts as a landing platform is partly supported by the two adjacent tepals of the outermost perianth whorl [and in those Commelinaceae where the abaxial tepal is very small, perhaps the well developed inflorescence bract serves the same purpose], whereas if the landing platform were a member of the outer whorl, there would not be the same support...). Indeed, floral orientation as a whole in the monocots is quite variable, and in part depends on the presence and position of the prophyll/bracteole, and also on the existence of other structures on the pedicel (see e.g. Eichler 1875; Engler 1888; Remizova et al. 2006b). Remarkably, although flowers on the one inflorescence of Crocosmia X crocosmiiflora were all monosymmetric, in some the odd member of the outer whorl was adaxial, and in others it was abaxial; patterning, etc. of the other floral organs was adjusted accordingly (pers. obs. vii.2009). Stuetzel and Marx (2005) also note the variability in the position of monocot bracteoles; they think that this may be because what appear to be axillary flowers in fact represent reduced racemes.

Monocots and "dicots" were often distinguished in the past by the 3-merous flowers of the former and the predominantly 5-merous flowers of the latter, even as it was realised that some of the "primitive dicots" might have more or less 3-merous flowers. With our current knowledge of phylogeny and floral development, it seems that a 3-merous perianth in particular is widespread and may even be a synapomorphy for a clade [[Chloranthaceae + magnoliids] [monocots [Ceratophyllaceae + eudicots]]] (Soltis et al. 2005b and literature cited). The two perianth whorls in monocots are often similar and both are petaloid, being called tepals; however, there is usually a slight difference between the members of the two whorls. The tepals of each whorl generally have open aestivation, and the flowers themselves are functionally six-merous. The stamens are individually opposite members of each whorl, stamen-tepal primordia being common, indeed, the individual perianth whorls may not completely encircle the floral apex and the outer whorl of stamens may even come to lie outside the inner perianth whorl... (Endress 1995b; Remizowa et al. 2010b and references); note that common petal/stamen primordia occur in some Zingiberales, but there each perianth whorl completely surrounds the meristem. In connection with this arrangement, monocot flowers often appear to be 6-merous, not 3-merous. All in all, these "3-merous" monocot flowers are rather highly stereotyped and usually pentacyclic: Pentacyclic 3-merous flowers are at best extremely uncommon in broad-leaved angiosperms and are here considered to be an apomorphy for monocots (cf. Soltis et al. 2005b; Bateman et al. 2006b).

It is unclear how the anther wall develops in Acorus (Rudall & Furness 1997), although it inclines to the monocot "type" (Duvall 2001). Given the diversity of carpel development in monocots in, or near the base of, the basal pectinations in the monocot tree here - whether or not the basic condition for monocot carpels is to be free or somewhat connate is unclear (e.g. Chen et al. 2004; Remizowa et al 2006a). Remizowa et al. (2006b) summarize variation in gynoecial morphology in some of these monocots. Septal nectary morphology (e.g. Daumann 1970; Schmid 1985; van Heel 1988; Smets et al. 2000; Rudall 2002; Remizowa et al. 2006a) is rather variable and is difficult to categorise when the carpels are more or less free.

Along with other aspects of monocot morphology, there has been much discussion about the evolution of the single cotyledon that characterizes the clade - by connation, or by suppression (see e.g. Haines & Lye 1979; Burger 1998)?

For monocots, in addition to references in the notes on the Characters page and under individual orders and families, there is much interesting information in Arber (1920, esp. 1925), Dahlgren et al. (1985) and Tillich (1998); Tomlinson (1970) outlined monocot morphology and anatomy, emphasizing the woody groups. Volumes III and IV of Families and Genera of Vascular Plants, edited and with useful outline classications by Kubitzki (see especially 1998a, c), also contain a great deal of information. For the morphology of sieve tube plastids, see Behnke (1981a, 2000, 2003), for floral evolution, see Vogel (1981a: somewhat outdated), for a summary of monocot embryology, see Danilova et al. (1990a) and Rudall (1997), for seed and fruit morphology and anatomy, Takhtajan et al. (1985), for evolution of monocot seeds, see Danilova et al. (1990b: now somewhat outdated), for androecial variation, see Ronse Decraene and Smets (1995a), for information on dimorphism in the cells of the root epidermis and hypodermis, see Kauff et al. (2000), for rhizosheaths, known from many Poaceae (distribution poorly known - certainly in other Poales, but rare in broad-leaved angiosperms?), see McCulley (1995), for endosperm development Floyd and Friedman (2000, cf. topology of tree used for optimisation of characters) and Tobe and Kadokawa (2010, conventional endosperm "types"), for incompatibility systems in monocots - quite common, many uncharacterized, but at least some gametophytic - see Sage et al. (2000), for the distribution of operculate pollen, see Furness and Rudall (2006b), for pollen variation in "basal" monocots, see Furness and Banks (2010), for the development of callose plugs in the pollen tube - quite often complete and regularly spaced in broad-leaved angiosperms, incomplete and irregularly spaced in monocots, see Mogami et al. (2006), for nuclear DNA content, see Bharathan et al. (1994), for seedling morphology, see Takhtajan et al. (1985: compilation) and Tillich (2007), for discussion on the evolution of the berry, see Rasmussen et al. (2006), for antipodal cells, see Holloway and Friedman (2008), for gynoecial morphology and evolution, see Remizowa et al. (2010b), and for inflorescence morphology, see Remizowa et al. (2011a).

Phylogeny. Both molecular and morphological data strongly support the monophyly of monocots. However, monophylyy was not recovered in some morphological studies such as those by Hay and Mabberley (1991); Araceae were independently derived from broad-leaved angiosperms, perhaps from Nymphaeales. Similarly, in an analysis of fifteen chloroplast genomes, the five monocots included were not always monophyletic... (Goremykin et al. 2005; see Duvall et al. 2006 for other studies in which monocots appear not to be monophyletic - the 18S gene is implicated in producing this topology).

The analysis of morphological characters alone in monocots tends to produce trees with little resolution and little support for those branches that are resolved (e.g. Li & Zhou 2006: support only for Alismatales minus Aracaeae and for Zingiberales). Relationships within monocots as outlined in molecular studies by Chase et al. (1995a, 1995b, 2000a, 2005), Tamura et al. (2004a, b), Chase (2004), Janssen and Bremer (2004: rbcL only, but 878 genera from 77 families), Graham et al. (2006: to 16 kb chloroplast DNA/taxon examined), Givnish et al. (2006b), Chase et al. (2006), Li and Zhou (2007) and Soltis et al. (2011) are followed here; further comments may be found at various nodes within the monocot tree. Acorus seems to be sister to all other monocots (see also Duvall et al. 1993a, b; Soltis et al. 2007a; Moore et al. 2010; Morton 2011), a relationship recovered in most studies. Givnish et al. (2005: ndhF gene alone) found very much the set of relationships in the tree here, although Pandanales grouped with Liliales (low support) and Dasypogonaceae were sister to [Commelinales + Zingiberales].

Note, however, that Stevenson et al. (2000) suggest a rather different set of relationships - Acoraceae + most of Alismatales form a clade sister to all other monocots, Araceae a clade sister to the remainder. Davis et al. (2001) found a similar clade of Acoraceae + Alismatales (as delimited here) sister to other monocots. Davis et al. (2004) noted that this latter set of relationships was not found when rbcL sequences were analysed alone, but it appeared when mitochondrial atpA sequences were analysed, both alone and in the combined analysis (see Davis et al. 2006: four genes, two nuclear and two chloroplast, matK also supports this relationship). Characters of floral development also seem to be consistent with an Acoraceae-Alismatales relationship (e.g. see Buzgo 2001). Interestingly, a three-nucleotide deletion in the atpA gene is found in Acoraceae and Alismatales as recognised here, although in the latter it is not found in Cymodoceaceae nor Tofieldiaceae (Davis et al. 2004). Acoraceae show a substantially accelerated rate of molecular evolution in at least some mitochondrial genes (G. Petersen et al. 2006b), although they were nevertheless sister to remaining monocots in their combined trees. In another analysis of mitochondrial genes, again, with a much higher rate of change in Acoraceae and Alismatales, although not in Tofieldiaceae and Araceae (G. Petersen 2006c), Acoraceae linked with the fast-evolving group. Finet et al. (2010) found that Acorus and Asparagales formed a clade sister to all other monocots, but this is probably a sampling problem; no members of Alismatales were included.

Indeed, G. Petersen et al. (2006b) found mitochondrial data in general to be rather incongruent with plastid data, for instance, Orchidaceae grouped with Dioscoreaceae and Thismia, and the positions of Liliales, Asparagales and Dasypogonaceae in particular were very labile. Although G. Petersen et al. (2006b) suggested that the incongruences "could equally well refute the phylogenies based on plastid data" (Petersen et al. 2006b: p. 59), this seems unlikely; problems caused by distinctive properties of the evolution of the mitochondrial genome seem more likely. Similarly, Qiu et al. (2010) found Asparagales to be sister to all monocots other than Alismatales, although support for this position was not very strong and Petrosaviales were not included.

Previous Relationships. Lindley (1853) thought that the monocots that had leaves with reticulate venation, which he called the dictyogens, were intermediate between the exogens (dicots) and the endogens (other monocots). Indeed, some morphological cladistic studies have also placed net-veined monocots as sister to all other monocots, suggesting that this leaf venation was plesiomorphic in the monocots (Stevenson & Loconte 1995, see also Dahlgren et al. 1985; Yeo 1989; Li & Zhou 2006, etc.: Chase 2004); this is largely because the broad-leaved angiosperm outgroups have similar features. Morphological cladistic analyses of the net-veined taxa by themselves (Conran 1989) also suggested relationships which now seem rather unsatisfactory.

Synonymy: Acoranae Reveal, Alismatanae Takhtajan, Aranae Reveal, Arecanae Takhtajan, Bromelianae Reveal, Butomanae Reveal, Commelinanae Takhtajan, Cyclanthanae Reveal, Dioscoreanae Reveal & Doweld, Iridanae Doweld, JuncanaeMelanthianae Doweld, Myrtanae Takhtajan, Najadanae Reveal, Orchidanae Doweld, Pandananae Reveal, Petrosavianae Doweld, Poanae Doweld & Reveal, Pontederianae Reveal, Rapateanae Doweld, Triuridanae Reveal, Typhanae Reveal, Zingiberanae Reveal, Zosteranae Doweld

ACORALES Martius, see Main Tree, Synapomorphies.

Inflorescence a spadix [dense spike], with spathe; flowers weakly monosymmetric [abaxial member of outer T whorl precocious and large]; tapetal cells 2-4-nucleate; tectum continuous; carpels ascidiate-plicate, syncarpy congenital; ovules straight; endosperm copious, perisperm +, derived from nucellar epidermis, not starchy; collar rhizoids +. - 1 family, 1 genus, 2-4 species.

Chemistry, Morphology, etc. It is unclear if the plant has true vessels (Carlquist 2009 for references).

Includes Acoraceae.

ACORACEAE Martinov   Back to Acorales

Vessels also in rhizomes, perforations long-scalariform; stem with endodermis; leaves two-ranked, equitant and isobifacial [oriented edge on to the stem]; peduncle with two separate vascular systems; bracts and bracteoles 0 [but see below]; T ± hooded; anther thecae confluent apically on dehiscence, endothecial thickenings stellate; pollen sulcus lacking ectexine, endexine lamellate; intra-ovarian trichomes +, placentae apical, pendulous, style broad, massive, stylar canal with exudate; ovules several/carpel, outer integument 3-5 cells across, integuments with hairs, hypostase massive, with central column and radiating cells, postament +; antipodal cells ± persistent, (dividing); fruit a berry; tegmen cells thickened?; P persistent; n = 9, 11, 12; first leaf terete, unifacial.

1[list]/2-4. E. America to East and South East Asia (map: from Hultén 1962; Fl. N. Am. 22: 2000, perhaps naturalised in Europe and America, see Mayo et al. 1997). [Photo - Habit.]

• Acoraceae are recognisable by their sweetly-smelling, two-ranked, isobifacial leaves and their densely spicate inflorescence overtopped by - and appearing lateral to - the leaf-like spathe. The flowers are small, perfect and pentacyclic.

Evolution. There has been a great increase in the rate of synonymous substitutions in the mitochondrial genome, but not in that of the chloroplast genome (Mower et al. 2007; see also G. Petersen et al. 2006b).

Chemistry, Morphology, etc. The root stele is pentarch. The abaxial tepal is large, bract-like, and encloses the young flower; it looks as if it has "merged" with the bract (Buzgo 2001), being depicted as an organ of "hybrid" nature (Bateman et al. 2006b); Ronse deCraene (2010) interprets it as a bract, one tepal being missing. There are non-secreting slits in the ovary septae; if these are considered to be septal nectaries, this feature becomes a synapomorphy (subsequently lost many, many times) of monocots as a whole. The ovules are encased in mucilage secreted by the intra-ovarian trichomes.

Some information is taken from Grayum (1987), Bogner and Mayo (1998), and Bogner (2011), all general, Buell (1938: ovule), Kaplan (1970a: leaf development), Tillich (1985: seedling), Carlquist and Schneider (1997: anatomy), Buzgo and Endress (2000) and Buzgo (2001: both floral morphology), Floyd and Friedman (2000: endosperm development), Keating (2003a: anatomy), Soukup et al. (2005: root development, intermediate) and Stockey (2006: evaluation of fossil remains). For a checklist and bibliography, see Govaerts and Frodin (2002) and World Checklist of Monocots.