EMBRYOPSIDA Pirani & Prado
Gametophyte dominant, independent, multicellular, thalloid, with single-celled apical meristem, showing gravitropism; rhizoids +, unicellular; flavonoids + [absorbtion of UV radiation]; chloroplasts lacking pyrenoids; protoplasm dessication tolerant [plant poikilohydric]; cuticle +; cell walls with (1->4)-ß-D-glucans [xyloglucans], lignin +; several chloroplasts per cell; glycolate metabolism in leaf peroxisomes [glyoxysomes]; centrioles in vegetative cells 0, metaphase spindle anastral, predictive preprophase band of microtubules, phragmoplast + [cell wall deposition spreading from around the spindle fibres], plasmodesmata +; antheridia and archegonia jacketed, stalked; spermatogenous cells monoplastidic, centrioles develop de novo, associated with basal bodies of flagellae, multilayered structure +, proximal end of basal bodies lacking symmetry, stellate pattern associated with doublet tubules of transition zone; spermatozoids with a left-handed coil; male gametes with 2 lateral flagellae; oogamy; sporophyte dependent on gametophyte, embryo initially surrounded by haploid gametophytic tissue, plane of first division horizontal [with respect to long axis of archegonium/embryo sac], suspensor/foot +, cell walls with nacreous thickenings; sporophyte multicellular, with at least transient apical cell [?level], sporangium +, single, dehiscence longitudinal; meiosis sporic, monoplastidic, microtubule organizing centre associated with plastid, cytokinesis simultaneous, preceding nuclear division, sporocytes 4-lobed, with a quadripolar microtubule system; spores in tetrads, sporopollenin in the spore wall, wall with several trilamellar layers [white-line centred layers, i.e. walls multilamellate]; close association between the trnLUAA and trnFGAA genes on the chloroplast genome.
Many of the bolded characters in the characterization above are apomorphies of subsets of streptophytes along the lineage leading to the embryophytes, not apomorphies of crown-group embryophytes per se.
All groups below are crown groups, nearly all are extant; characters mentioned are those of the common ancestor of the group.
Abscisic acid, ?D-methionine +; sporangium with seta, seta developing from basal meristem [between epibasal and hypobasal cells], sporangial columella + [developing from endothecial cells]; stomata +, anomocytic, cell lineage that produces them with symmetric divisions [perigenous]; underlying similarities in the development of conducting tissue and in rhizoids/root hairs; spores trilete; polar transport of auxins and class 1 KNOX genes expressed in the sporangium alone; MIKC, MI*K*C* and class 1 and 2 KNOX genes, post-transcriptional editing of chloroplast genes; gain of three group II mitochondrial introns.
[Anthocerophyta + Polysporangiophyta]: archegonia embedded/sunken in the gametophyte; sporophyte long-lived, chlorophyllous; sporophyte-gametophyte junction interdigitate, sporophyte cells showing rhizoid-like behaviour.
Sporophyte branched, branching apical, dichotomous; sporangia several; spore walls not multilamellate [?here].
EXTANT TRACHEOPHYTA / VASCULAR PLANTS
Photosynthetic red light response; water content of protoplasm relatively stable [plant homoiohydric]; control of leaf hydration passive; (condensed or nonhydrolyzable tannins/proanthocyanidins +); sporophyte soon independent, dominant, with basipetal polar auxin transport; vascular tissue +, sieve cells + [nucleus degenerating], tracheids +, in both protoxylem and metaxylem; endodermis +; root xylem exarch [development centripetal]; stem with an apical cell; branching dichotomous; leaves spirally arranged, blades with mean venation density 1.8 mm/mm2 [to 5 mm/mm2]; sporangia adaxial on the sporophyll, derived from periclinal divisions of several epidermal cells, wall multilayered [eusporangium]; columella 0; tapetum glandular; gametophytes exosporic, green, photosynthetic; stellate pattern split between doublet and triplet regions of transition zone; placenta with single layer of transfer cells in both sporophytic and gametophytic generations, embryonic axis not straight [root lateral with respect to the longitudinal axis; plant homorhizic].[MONILOPHYTA + LIGNOPHYTA]
Branching ± indeterminate; lateral roots +, endogenous, root apex multicellular, root cap +; tracheids with scalariform-bordered pits; leaves with apical/marginal growth, venation development basipetal, growth determinate; sporangia borne in pairs and grouped in terminal trusses, dehiscence longitudinal, a single slit; cells polyplastidic, microtubule organizing centres not associated with plastids, diffuse, perinuclear; male gametes multiflagellate, basal bodies staggered, blepharoplasts paired; chloroplast long single copy ca 30kb inversion [from psbM to ycf2].
Plant woody; lateral root origin from the pericycle; branching lateral, meristems axillary; cork cambium + [producing cork abaxially], vascular cambium bifacial [producing phloem abaxially and xylem adaxially].
EXTANT SEED PLANTS / SPERMATOPHYTA
Plant 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 [hence with p-hydroxyphenyl and guaiacyl lignin units, so no Maüle reaction]; root stele with xylem and phloem originating on alternate radii, not medullated [no pith], cork cambium deep seated; arbuscular mycorrhizae +; shoot apical meristem interface specific plasmodesmatal network; stem with vascular cylinder around central pith [eustele], phloem abaxial [ectophloic], endodermis 0, xylem endarch [development centrifugal]; wood homoxylous, tracheids and rays alone, tracheid/tracheid pits circular, bordered; mature sieve tube/cell lacking functioning nucleus, sieve tube plastids with starch grains; phloem fibres +; cork cambium superficial; leaves with single trace from vascular sympodium [nodes 1:1]; stomatal pore with active opening in response to leaf hydration, control by abscisic acid, metabolic regulation of water use efficiency, etc.; buds axillary (not associated with all leaves), exogenous; prophylls two, lateral; leaves with petiole and lamina, development basipetal, blade simple; plant heterosporous, sporangia borne on sporophylls; microsporophylls aggregated in indeterminate cones/strobili; grains monosulcate, aperture in ana- position [distal], exine and intine homogeneous; ovules unitegmic, parietal tissue 2+ cells across, megaspore tetrad linear, functional megaspore single, chalazal, lacking sporopollenin, megasporangium indehiscent; pollen grains land on ovule; gametophytes dependent on sporophyte; male gametophyte development initially endosporic, tube developing from distal end of grain, gametes two, developing after pollination, with cell walls; female gametophyte endosporic, initially syncytial, walls then surrounding individual nuclei; seeds "large" [ca 8 mm3], but not much bigger than ovule, with morphological dormancy; embryo cellular ab initio, endoscopic, plane of first cleavage of zygote transverse, suspensor +, short-minute, embryonic axis straight [shoot and root at opposite ends; plant allorhizic], white, cotyledons 2; plastid transmission maternal; ycf2 gene in inverted repeat, whole nuclear genome duplication [zeta duplication], 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.
ANGIOSPERMAE / MAGNOLIOPHYTA
Lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, 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, exodermis +; shoot apex with tunica-corpus construction, tunica 2-layered; reaction wood ?, associated gelatinous fibres [g-fibres] with innermost layer of secondary cell wall rich in cellulose and poor in lignin; 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 cell and sieve tube from same mother cell; sugar transport in phloem passive; nodes 1:?; stomata brachyparacytic [ends of subsidiary cells level with ends of pore], outer stomatal ledges producing vestibule, reduction in stomatal conductance to increasing CO2 concentration; lamina formed from the primordial leaf apex, margins toothed, development of venation acropetal, overall growth ± diffuse, venation hierarchical-reticulate, secondary veins pinnate, veins (1.7-)4.1(-5.7) mm/mm2, endings free; most/all leaves with axillary buds; flowers perfect, pedicellate, ± haplomorphic, parts spiral [esp. the A], free, numbers unstable, development in general centripetal; P +, members each with a single trace, outer members not sharply differentiated from the others, not enclosing the floral bud; 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, endothecium +, endothecial cells elongated at right angles to long axis of anther; (tapetum glandular), cells binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellate, endexine +, thin, compact, lamellate only in the apertural regions; nectary 0; carpels present, superior, free, several, ascidiate, with postgenital occlusion by secretion, stylulus short, hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, 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, functional megaspore, chalazal, lacking cuticle; female gametophyte four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; supra-stylar extra-gynoecial compitum +; ovule not increasing in size between pollination and fertilization; pollen grains land on stigma, bicellular at dispersal, mature male gametophyte tricellular, germinating in less than 3 hours, pollination siphonogamous, tube elongated, growing between cells, growth rate (20-)80-20,000 µm/hour, apex of pectins, wall with callose, lumen with callose plugs, penetration of ovules via micropyle [porogamous], whole process takes ca 18 hours, distance to first ovule 1.1-2.1 mm; male gametes lacking cell walls, flagellae 0, double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; seed exotestal, much larger than ovule at time of fertilization; endosperm diploid, cellular, heteropolar [micropylar and chalazal domains develop differently, 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; dark reversal Pfr → Pr; Arabidopsis-type telomeres [(TTTAGGG)n]; 2C genome size 1-8.2 pg [1 pg = 109 base pairs], whole nuclear genome duplication [epsilon duplication]; protoplasm dessication tolerant [plant poikilohydric]; 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]].
[NYMPHAEALES [AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]]: wood fibres +; axial parenchyma diffuse or diffuse-in-aggregates; pollen monosulcate [anasulcate], tectum reticulate-perforate [here?]; ?genome duplication; "DEAER" motif in AP3 and PI genes lost, gaps in these genes.
[AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]: vessel elements with scalariform perforation plates in primary xylem; essential oils in specialized cells [lamina and P ± pellucid-punctate]; tension wood +; tectum reticulate; anther wall with outer secondary parietal cell layer dividing; carpels plicate; nucellar cap + [character lost where in eudicots?]; 12BP [4 amino acids] deletion in P1 gene.
[[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]] / MESANGIOSPERMAE: benzylisoquinoline alkaloids +; sesquiterpene synthase subfamily a [TPS-a] [?level], polyacetate derived anthraquinones + [?level]; outer epidermal walls of root elongation zone with cellulose fibrils oriented transverse to root axis; P more or less whorled, 3-merous [possible position]; pollen tube growth intra-gynoecial; embryo sac bipolar, 8 nucleate, antipodal cells persisting; endosperm triploid.
[MONOCOTS [CERATOPHYLLALES + EUDICOTS]]: (extra-floral nectaries +); (veins in lamina often 7-17 mm/mm2 or more [mean for eudicots 8.0]); (stamens opposite [two whorls of] P); (pollen tube growth fast). Back to Main Tree
Age. This node has ben dated to 147-143 m.y.a. (Leebens-Mack et al. 2005). Chaw et al. (2004: 61 chloroplast genes, sampling poor) dated it to 150-140 m.y.a., Moore et al. (2010: 95% highest posterior density) estimated an age of (142-)135(-127) m.y., Clarke et al. (2011) an age of (161-)137(-124) m.y.; 138-134 m.y. was the estimate in Mennes et al. (2013), 145 or 142.3 m.y. in Naumann et al. (2013), and (158.5-)ca 143, 138.4(-130.5) m.y. in Xue et al. (2012). Ages of ca 213 m.y. in Z. Wu et al. (2014) and (394-)301(-208) m.y. in Zimmer et al. (2007) are rather higher, while the ages of around (126.8-)125.1(-124.1) m.y. in Iles et al. (2014) are the lowest.
Fossil-based estimates are somewhat younger, ca 100 m.y. (Crepet et al. 2004: monocots sister to magnoliids) or at least 110 m.y. (e.g. Friis et al. 2010: see below). However, the recent fossil findings of Sun et al. (2011) would imply a substantially greater age for the eudicot Ranunculales of some ca 152-140 m.y., so this node would be still older.
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), are slowly being clarified. For further information, see the mesangiosperm node. Chloranthales, eudicots, magnoliids, and Ceratophyllales are the other clades involved. There is, however, some evidence that Ceratophyllales are sister to eudicots.
MONOCOTYLEDONS / MONOCOTYLEDONEAE / LILIANAE Takhtajan Back to Main Tree
Plant herbaceous, perennial, rhizomatous, growth sympodial; non-hydrolyzable tannins [(ent-)epicatechin-4] +, neolignans, benzylisoquinoline alkaloids 0, hemicelluloses as xylans; root apical meristem?; root epidermis developed from outer layer of cortex; trichoblasts in atrichoblast [larger cell]/trichoblast cell pairs, the former further from apical meristem, in vertical files; endodermal cells with U-shaped thickenings; cork cambium in root [uncommon] superficial; stele oligo- to polyarch, < class="apo">medullated [with prominent pith], lateral roots arise opposite phloem poles; primary thickening meristem +; vascular bundles in stem scattered, (amphivasal), vascular cambium 0 [bundles closed]; tension wood 0; 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; prophyll single, adaxial; leaf blade linear, main venation parallel, the veins joining successively from the outside at the apex, transverse veinlets +, unbranched [leaf blade characters: ?level], vein/veinlet endings not free, margins entire, Vorläuferspitze +, base broad, ensheathing the stem, sheath open, petiole 0, colleters + ["intravaginal squamules"]; inflorescence terminal, racemose; flowers 3-merous [6-radiate to the pollinator], polysymmetric, pentacyclic; P = T, each with three traces, median T of outer whorl abaxial, aestivation open, members of whorls alternating, [pseudomonocyclic, each T member forming a sector of any tube]; 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, endothecium from outer secondary parietal cell layer, inner secondary parietal cell layer dividing; G , 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, apparently terminal, with a closed sheath, unifacial [hyperphyllar], both assimilating and haustorial, plumule apparently lateral; primary root unbranched, not very well developed, stem-borne roots numerous, hypocotyl short, (collar rhizoids +); no dark reversion Pfr → Pr; duplication producing monocot LOFSEP and FUL3 genes [latter duplication of AP1/FUL gene], PHYE gene lost. - 11 orders, ca 78 families, 60,100 species.
Age. The age of crown-group monocots has been variously estimated at ca 200±20 m.y. (Savard et al. 1994), ca 189 m.y. (Z. Wu et al. 2014), 160±16 m.y. (Goremykin et al. 1997), 135-131 m.y. (Leebens-Mack et al. 2005), or 133.8-124 m.y. (Moore et al. 2007), all using molecular data. Bremer (2000b) suggested that this node could be dated to (147-)134(-121) m.y.a, an age also used for dating monocot groups in general (Janssen & Bremer 2004); Magallón and Castillo (2009) suggest ca 177 m.y. or 127 m.y. for this split; Bell et al. (2010) estimate ages of (157-)146, 130(-109) m.y.; while Moore et al. (2010) offer an age of (129-)122(-117) m. years. Other suggestions range from (191-)164, 156(-139) m.y.a. (Smith et al. 2010: c.f. Table S3, slightly younger estimates) to 228.6-128.3 m.y. (Nauheimer et al. 2012: Table S4), although most estimates there are in the 150-139 m.y. range. A distinctly older age of (280, 252-)246, 209(-186) m.y. was proposed by Zeng et al. (2014). Zhang et al. (2012) suggested an age of (142-)124(-108) m.y. and a similar age (ca 125.1 or 121.5 m.y.) is suggested by Xue et al. (2012). Magallón et al. (2013) offer ages of around (154.4-)137.1, 134.1(-123.4) m.y., as little as as 106.7 m.y. is the age in Naumann et al. (2013) and (110.5-)104.2(-98) m.y. in Iles et al. (2014), while estimates in Schneider et al. (2004) pretty much cover all possibilities.
An early fossil-based estimate of the age of stem monocots was only ca 98 m.y. and that of crown monocots ca 90 m.y. (Crepet et al. 2004). Fossil evidence suggested to Jud and Wing (2012) that monocots and eudicots were present ca 125-119 m.y.a. by the Early Aptian; using pollen evidence alone, monocots will have to be at least as old as the tricolpate pollen that characterises eudicots.
Liliacidites pollen, boat-shaped, monosulcate, and with reticulate sculpture that becomes finer at the ends of the grain, and often found dispersed can be assigned to monocots, as can leaves of Acaciaephyllum; both are well known in the fossil record (Doyle et al. 2008). Distinctive pollen assigned to Pothooideae-Monstereae has been found in Early Cretaceous deposits of the late Barremian-early Aptian of some 110-120 m.y. old in Portugal (Friis et al. 2004; see also Hesse & Zetter 2007). Although the identity of some of these grains has been questioned (Hoffmann & Zetter 2010), macrofossils apparently of Araceae-Aroideae (a decidedly non-basal clade) have recently been discovered in deposits of a similar age in Portugal (Friis et al. 2010). For these and other fossil monocots, c.f. Gandolfo et al. (2000), Friis et al. (2006b, 2011) and Doyle et al. (2008).
Note: Possible apomorphies are in bold. However, the actual level at which many of these features, particularly the more cryptic ones, should be assigned is unclear. This is partly because many characters show considerable homoplasy, in addition, basic information for all too many is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there is the not-so-trivial issue of how ancestral states are reconstructed (see above).
Evolution. Divergence & Distribution. Even if monocots were sister to the aquatic Ceratophyllales and/or their origin can be linked to the adoption of some kind of marshy or aquatic habitat (see below), it does not help much in our understanding of how the distinctive monocot features evolved. Monocots appear to be so different from other angiosperms that relating their morphology, anatomy and development to that of broad-leaved angiosperms has been difficult (e.g. Zimmermann & Tomlinson 1972; Tomlinson 1995). Thus it has been suggested that vessels in monocots and those in other angiosperms evolved independently (Cheadle 1943a, 1953; c.f. Carlquist 2012a). The aquatic Ceratophyllales may be close, but they are even more remarkable in both their vegetative and floral morphology. Nymphaeales are aquatics that were also once believed to be close to monocots, and they now even include the ex-monocot Hydatellaceae. Similarities are likely to be convergences, and the common ancestors of all these clades with other angiosperms are likely to have been plants with broad, petiolate leaves and a woody stem with conventional lateral thickening meristems, cork and vascular cambia (e.g. Doyle 2013; see also early angiosperm evolution).
If Ceratophyllaceae were indeed 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). Note that over half the putative synapomorphies for monocots in Table 4.1 of Soltis et al. (2005b) may be best assigned elsewhere. The nature of the anther-filament junction has not been optimised in this part of the tree. For pollen and tapetum diversification, see Furness (2013), and for the evolution of syncarpy and of septal nectaries, see Sokoloff et al. (2013: various trees, various definitions).
Ecology & Physiology. Monocot vegetative morphology, their ecology, and their physiology are all closely linked. It has long been noted that many of their distinctive features 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 and flexible leaves, absence of secondary thickening, clusters of adventitious roots rather than a single, branched taproot (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; Schutten et al. 2005 and references for the biomechanics of living in water), and Carlquist (2012a) discussed variation in xylem anatomy in context of a more or less aquatic origin of the clade. 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. Indeed, aquatic herbs, unlike terrestrial herbs, often entirely lose the capacity to produce cambium, and reacquisition of a "normal" bifacial cambium in such plants is unknown (Groover 2005; Feild & Arens 2007).
In their analysis of major functional traits in vascular plants, Cornwell et al. (2014) noted that plants above the first node of the monocot branch were notably small, although Acoraceae do not differ from other monocots in this feature; palms and to a lesser extent bambusoid grasses show a marked increase in plant size. Most monocots, like Acoraceae themselves, are perennial, sympodially growing plants (Holttum 1955; see also Levichev 2013) that form tufts of leaves in part of each growth cycle and/or are geophytes, so internode elongation is slight. The stem apex is under or at the surface of the ground except at flowering time. So-called "adventitious" roots develop from the growing stem, the older part of the stem decaying along with any primary root system that may have formed initially.
Since there is usually no secondary thickening, the hydraulic systems of root and stem are not in direct contact (Carlquist 2009). Distinctive monocot-type secondary thickening (Rudall 1995b for records) is very infrequent, although somewhat less so in Asparagales. Interestingly, the major monocot woody clades, Arecaceae and Poaceae-Bambusoideae, have no monocot-type secondary thickening (but c.f. Botánico & Angyalossy 2013) and so by implication the xylem and phloem tissue in their vascular tissue must be very old, but in both groups - perhaps the latter in particular - root pressures are extremely high, which may at least help in embolism repair after xylem cavitation develops (Davis 1961; Cao et al. 2012). It is unclear if high root pressures, perhaps associated with tolerance to cavitation, occur throughout the monocots (Cao et al. 2012), however, root pressures listed by Fisher et al. (1997) for vines and woody species show no differences between monocots and other angiosperms.
Many monocots are plants of quite considerable size, some being giant herbs or large trees, and there is a period, often designated as establishment growth (e.g. Tomlinson & Esler 1972; Bell & Bryan 2008), between germination and the mature (flower-producing) stage. It usually occurs before stem elongation, and is accompanied by changes, particularly in the size of the apical meristem and often in leaf morphology. 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 (the "dropper") with the plumule and radicle/root area at the bottom grows out of the seed and so carries both away from the seed, and establishment growth proceeds while the meristem in underground. However, the stem may thicken in other ways, as in some palms where ground tissues in both stem and root remains undifferentiated for some time, with limited mitosis and/or cell expansion and/or formation of schizogenous lacunae occurring and the trunk markedly thickening and lengthening - sustained primary growth (Waterhouse & Quinn 1978, see also Arecaceae).
A number of monocots have erect stems with elongated internodes, at least for part of the year, and the stem's response to gravity in the absence of secondary thickening is interesting. In some Poales (Cyperaceae, Juncaceae, Poaceae), at least, adjustments are made by an intercalary meristem at the base of in the internodal zone. For the role of the leaf sheath in supporting the stem under such conditions, see Kempe et al. (2013 and references).
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" (sedges and grasses) litter decomposed more slowly than that of forbs, and "monocot" lignin more slowly than that of other angiosperms, at about the same rate as that of gymnosperms. The rate of lignin composition is also affected by its nitrogen content, but more work on lignin decomposition is in order.
Scattered in monocots are taxa with broad leaf blades that have reticulate venation (see Cameron & Dickison 1998, also below) 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) suggested that reticulate venation has arisen at least 26 times in monocots (and fleshy fruits 21 times), and have sometimes subsequently been lost. Both features showed very strong signs of tending to be gained (or lost) together, a phenomenon described as "concerted convergence" (Givnish et al. 2005, 2006b).
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 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 the hispine beeteles Hispinae-Cassidinae being derived. However, Gómez-Zurita et al. (2007) suggested that the two main clades of monocot-eating chrysomelid beetles he included in his study were unrelated, and neither was close to the galerucines, and also that the chrysomelids diversified 86-63 m.y.a., well after the origin of monocots. García-Robledo and Staines (2008) discuss problems when ascribing herbivory to particular groups when using fossil material.
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). Most monocots 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).
Vegetative Variation. Monocots show great variation in their basic leaf construction (e.g. Kaplan 1973). However, the 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 (Stewart & Dermen 1979; Jouannic 2011 and references). The outer tunica layer can proliferate at the leaf margin as can be seen in some variegated leaves (Zonneveld 2007).
The relation between the blade of a monocot leaf and that of a broad-leaved angiosperm is of the greatest interest. Leaves in general can be divided into a hyperphyll and hypophyll (e.g. ). In broad-leaved angiosperms the former gives rise to the blade, which has a marginal meristem and 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, a rather late-developing part of the leaf, 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 are cut off both distally towards the apex and proximally towards the base (Ichihashi et al. 2011). In many monocots, most of the leaf is developed from the hypophyll alone and maturation of the blade proceeds basipetally as cell files are cut off from a transverse basal plate. Thus tissues in the apex of the blade of grass or palm leaf emerging from the sheath are mature while cells at the base are actively dividing and elongating. There is often a "Vorläuferspitze", a usually small abaxial unifacial conical or cylindrical protrusion at the apex of the mature leaf; this may represent the entire hyperphyll (e.g. Knoll 1948; Troll 1955; Bharathan 1996). In the "typical" monocot leaf the blade develops from the equivalent of the broad-leaved angiosperm leaf base. In leaves in general, localization of PIN-FORMED1 auxin transport proteins is associated with provascular strand development, whether at the marginal meristem or the sheathing leaf base (Johnston et al. 2014).
One commonly thinks of monocots as having broad, sheathing bases and leaf blades with parallel venation that are about the same width, but this is perhaps because of the ubiquity of grasses (for the sheathing base of Zea, see Johnston et al. 2014) and the fact that many commonly-cultivated bulbs have such leaves. (Although Gifford and Foster [1988: Fig. 19-13] prefer to think of such venation as being striate, emphasizing how the main veins join sequentially at the apex, the different leaf venations they show are variations on this parallel theme.) However, in Acorus (Kaplan 1970a) and most, but not all, Alismatales studied (e.g. Bloedel & Hirsch 1979) a bifacial blade may develop from the upper part of the leaf primordium. Such leaves are quite similar in development to those of broad-leaved angiosperms (Doyle 2013). Quite commonly there is still a small Vorläuferspitze, but this is visible only in cataphylls and/or early developed leaves in Cyclanthaceae and Alismataceae (Wilder 1986; Bloedel & Hirsch 1979). How much hyperphyll a Vorläuferspitze represents varies, and Bharathan (1996) noted that a Vorläuferspitze was to be found in some monocot leaves whose blade develops from the hyperphyll.
Many of the features that make up a "typical" monocot leaf seem to vary independently: Leaf base surrounding the stem/not; blade developed from the hypophyll/hyperphyll; Vorläuferspitze present/absent; venation parallel/reticulate; blade develops basipetally/acropetally; petiole and blade develop simultaneously/petiole tends to develop somewhat later (monocot states first: see e.g. Kaplan 1973; Bharathan 1996). Variation seems to be especially great in Alismatales and Acorus; although relating this variation to the early branches of monocot phylogeny is difficult, "typical" monocot leaves are unlikely to be an apomorphy of monocots (see also Doyle 2013). Leaf development in monocots in general needs more comparative study.
Terete, unifacial blades with stomata all over the surface are scattered in monocots. These may result from the elaboration of the unifacial Vorläuferspitze (e.g. Arber 1925; Troll 1955; Troll & Meyer 1955; Kaplan 1973, 1975: comparison with Oxypolis, Acacia; Townsley & Sinha 2012). Monocots may also have laterally flattened and isobifacial leaves that are edge on to the stem (e.g. Linder & Caddick 2001: seedlings of Restionaceae showing all sorts of leaf morphologies). Although these leaves look like a bifacial dorsiventral blade that has folded and become connate adaxially, they may represent the adaxial elaboration of a midrib/costal region (Kaplan 1970a: Acorus). However, Rudall and Buzgo (2002) suggest that the isobifacial leaf of Acorus originates from an intermediate zone between hyperphyll and hypophyll. Developmentally both isobifacial and terete unifacial leaves may represent the genetic abaxialization of the leaf, the genes normally expressed abaxially being the only genes expressed, at least at the leaf surface (Yamaguchi & Tsukaya 2010; Nakayama et al. 2013). Yamaguchi et al. (2010) show how in Juncus isobifacial leaves differ from terete leaves by the activity of the DL gene that elsewhere in monocots is involved in midrib development.
A number of monocots have broad leaf blades, petioles, the venation is reticulate, with some free vein endings, and the stomata are unoriented (see Cameron & Dickison 1998). Indeed, net venation may have arisen at least 26 times in monocots (see above: Givnish et al. 2005, 2006b: see above). The plants are often vines/lianes (Smilax, [Liliales], Dioscorea [Dioscoreales]) and/or plants which live in shady habits for at least parts of their lives (Trillium [Liliales]). Thus the leaf blades of Hosta and Orontium are similar only in a functional sense (Troll 1955). Some kind of midrib/central vein is common, or there may be a few strong veins diverging from the base (Doyle et al. 2008, which see for further details of the venation of monocot leaves, etc.). Members of Zingiberales typically have a well-developed midrib from which numerous closely parallel veins leave, either proceeding straight to the margin, as in Musa, or taking a more arcuate path. Transverse veins joining the parallel veins are ubiquitous in monocots, and those in the broadly cordate blades of Stemona (Pandanales-Stemonaceae) are particularly conspicuous.
Truly compound leaves are rare in monocots - Zamioculcas, Anthurium and a few species of Dioscorea are examples - 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 leaf blade to become dissected and appear compound (Nowak et al. 2007, 2008).
A ligule may mark the two parts of the leaf, and Zamioculcas has a ligule very near the base of the petiole, suggesting that the rest of the leaf is equivalent to the hyperphyll, i.e. it is like the leaf of a broad leaved angiosperm. Ligules 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. Ligules may be paired. Smilax has paired tendrils near the base of the petiole, but 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 are not 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). There are also developmental similarities between stipules and leaf sheaths (the latter as in Poaceae), and abaxializing factors may interfere with their development (Townsley & Sinha 2012). 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)!
Genes & Genomes. For the evolution of the IR/LSC junction in monocots, see R.-J. Wang et al. (2008). Lee et al. (2011: c.f. 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) suggested that the common ancestor of monocots had five protochromosomes; there may be a genome duplication common to the monocots as a whole (Jiao et al. 2014). There is relatively little colinearity and synteny when monocots and rosids are compared, although these are extensive within each group (Tang et al. 2008).
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 do not know how common it is, but at best it seems to be rare. For the primary thickening meristem in the stem, see e.g. Esau (1943), Rudall (1991a, a summary), de Menezes et al. (2005) and Pizzolato (2009). This is quite variable in details of its origin and the tissues to which it gives rise, and endodermal initials in at least some cases produce radially-arranged cortical cells centrifugally, while derivatives of the pericycle (itself the very outside of the phloic tissue - see Esau 1943), initially produce the vascular system centripetally (de Menezes et al. 2011; Cury et al. 2012). Indeed, de Menezes et al. (2011) suggested that there was no distinct primary thickening meristem in monocots, but some of the argument here seems to be more definitional than anything else.
Cambial tissue in the stem, which gives rise to monocot-type of secondary thickening, as in a number of Asparagales (see above: Rudall 1995b for a summary), may represent a continuation of the activity of the primary thickening meristem (Carlquist 2012a) or whatever this tissue is. Vascular bundles in a number of monocots may have a sort of cambial layer, but its products never amount to much (Arber 1919). The roots of Dracaena produce secondary bundles (Carlquist 2012a).
Cheadle (e.g. 1942, 1943a, 1943b, 1944) 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 re-evaluated. Cheadle (e.g. 1944) noted that there could be substantial variation in vessel morphology between closely related (congeneric) species, and even between different organs on the same plant (see also Carlquist 2009). Such variation was used to establish his evolutionary trends (Cheadle 1955, 1964, 1969a, b, 1970; Cheadle & Tucker 1961; Cheadle & Kosakai 1971). However, Carlquist (2012a) found criteria for recognising vessels as distinct from tracheids to be difficult to formulate and questioned a number of these earlier reports of vessels, which makes life a bit difficult. Tomlinson and Fisher (2000) noted a correlation in climbing monocots between the 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 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). See Botha (2005) for distinctive thick-walled late-formed sieve tubes in some monocots.
Many monocots, although not the old Helobieae (here in Alismatales), have thin-walled bulliform cells in the adaxial epidermis 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 variations in how they develop may characterise major clades, although there is also much variation within them (see e.g. [Poales [Commelinales + Zingiberales]]: c.f. Tomlinson 1974a, q.v. for data, also Paliwal 1969; Pant & Kidwai 1965); more observations are still needed (Rudall 2000).
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).
Floral orientation 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). Stuetzel and Marx (2005) also note the variability in bracteole position; they think that this may be because what appear to be axillary flowers are reduced racemes. Be that as it may, when the prophyll/bracteole is lateral, the floral orientation can be quite variable (although less so with respect to the bracteole - Remizowa et al. 2013b). Monosymmetric flowers are very frequently presented with the median sepal adaxial, i.e., the flowers are inverted; in taxa with a labellum, the labellum is the median tepal of the inner whorl. This may be because this tepal acts as a landing platform and is partly supported by the two adjacent tepals of the outermost perianth whorl when the flower is inverted; if the landing platform were a member of the outer whorl, there would not be the same support. In those Commelinaceae where the abaxial tepal is very small, the well developed inflorescence bract may serve the same purpose. Connected with this inverted monosymmetry is the suppression of at least the adaxial median stamen (Pattern I zygomorphy: Rudall & Bateman 2004); after inversion the flowers effectively have Pattern II zygomorphy, in which abaxial stamens are sterilized. 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). For inflorescence morphology, see Remizowa et al. (2011a) and Remizowa and Lock (2012), for bracts in early divergent monocots, see Remizowa et al. (2013a), for floral evolution, see Vogel (1981a) although now somewhat outdated.
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 is quite widespread near the base of the angiosperm tree 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 similar and so are called tepals, however, there is usually a slight difference between the members of the two whorls. The stamens are individually opposite members of each whorl, stamen-tepal primordia being common; the individual perianth whorls do not completely encircle the floral apex (look at the base of a tulip, lily, or iris flower, for example). As a result, the tepals of each whorl, particularly the outer, may have open aestivation, although in Smilax, for example, the outer tepal members seem to completely surround the apex and are valvate. Such trimerous monocot flowers are rather highly stereotyped and usually pentacyclic; functionally, they are often six-merous. They are at best extremely uncommon in broad-leaved angiosperms and are here considered to be an apomorphy for monocots (c.f. Soltis et al. 2005b; Bateman et al. 2006b).
There is some variation in the vasculature even of the stereotypical monocot flower, as Gatin (1920) described in her extensive survey of the flowers of Old Style Liliaceae. She found that a number of genera had only a single vascular bundle entering each tepal member; sometimes successively-dividing traces supplied different parts of the flowers, or each floral part was supplied by a separate trace, the gynoecium was variously vascularized, and so on. T-A primordia may develop quite quickly while the appearance of G primordia is delayed (Endress 1995b). In some Alismatales, the outer whorl of stamens may come to lie outside the inner perianth whorl, as in Juncaginaceae (see e.g. Dahlgren et al. 1985; Endress 1995b; Remizowa et al. 2010b and references).
In commelinids the two perianth whorls are quite often differentiated into a clearly smaller, more or less green outer calyx, and a larger, coloured inner corolla. These whorls individually surround the floral apex and the androecium is inside the inner whorl.
Gene expression in floral development varies somewhat from that in core eudicots like Arabidopsis. Thus B-class genes, usually expressed in petals in core eudicots, are also expressed in the outer petal-like tepal whorl of Tulipa (Kanno et al. 2003) and Dendrobium (Y. zu et al. 2006) (Tzeng & Yang 2001). Furthermore, even if expressed in both outer whorls, whether or not the B-function genes form obligate heterodimers varied (Zu et al. 2006). Moreover, Ochiai et al. (2004) found that DEF, a B-class gene, was not expressed in the sepals of two Commelinaceae they examined, and also not in the outer more or less petal-like tepal whorls of Asparagus or Lilium (Park et al. 2003, 2004; Tzeng & Yang 2001), while Almeida et al. (2013) found that in Canna ABC-type floral genes had very broad expression patterns across the various floral organs.
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.
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)? Although having a terminal cotyledon was initially suggested to be a potential synapomorphy of the monocots, Kaplan (1997: 1 ch. 4) noted that this terminal position is because the single cotyledon pushes the erstwhile terminal meristem to one side, so evicting it. Conversely, in Poaceae the whole embryo is well developed, primordia of foliage leaves being visible, so it is perhaps not surprising to find that the cotyledon is more obviously lateral there, while the cotyledon of broad-leaved angiosperms that have only a single cotyledon is more or less terminal. The relationship of the radicle to the suspensor seems to vary, and its point of origin is distinctly to one side in several Alismatales, at least (Yamashita 1976).
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 classifications by Kubitzki (see especially 1998a, c), also contain a great deal of information. For the morphology of sieve tube plastids, see Behnke (1981a, 2000, 2001, esp. 2003), for dimorphism of the cells of the root epidermis and hypodermis, see Kauff et al. (2000), for rhizosheaths, known from Poaceae and other Poales, rare in broad-leaved angiosperms?), see McCulley (1995), for androecial variation, see Ronse Decraene and Smets (1995a), for endosperm development see above, 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 gynoecial morphology and evolution, see Remizowa et al. (2010b), for a summary of embryology, see Danilova et al. (1990a) and Rudall (1997), for antipodal cells, see Holloway and Friedman (2008), for nuclear DNA content, see Bharathan et al. (1994), for seed and fruit morphology and anatomy, Takhtajan et al. (1985), for the evolution of seeds, see Danilova et al. (1990b: now somewhat outdated), for seedling morphology, see Takhtajan et al. (1985: compilation), Kaplan (1997: 1 ch. 5) and Tillich (2007), and for a discussion on the evolution of the berry, see Rasmussen et al. (2006).
Phylogeny. For the immediate relatives of monocots, see the discussion at the mesangiosperm node. Chloranthales, magnoliids, and Ceratophyllales are the other clades immediately basal to the eudicots whose position is still rather uncertain.
Both molecular and much morphological data strongly support monocot monophyly. General 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), Soltis et al. (2011), Barrett et al. (2012b: 83 chloroplast genes, focus on commelinids), Iles et al. (2013) and Ruhfel et al. (2014: chloroplast genomes) are followed here; further comments may be found at various nodes within the monocot tree. Davis et al. (2013) tabulate support for various clades afforded by individual chloroplast genes.
However, monophyly is not always found in morphological analyses (see below). Also, in an analysis of fifteen chloroplast genomes, the five monocots included were not always monophyletic... (Goremykin et al. 2005); Duvall et al. (2006) discuss other studies in which monocots appear not to be monophyletic - the 18S gene is implicated in producing this topology.
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]; a grouping [Liliales [Pandanales + Dioscoreales]] also appeared - and had moderate support - in MP, but not in ML analyses of plastid genomes in Barrett et al. (2013: sampling).
However, Stevenson et al. (2000) suggest a rather different set of relationships - [Acoraceae + most of Alismatales] [Araceae + all other monocots]]. Davis et al. (2001) found a clade [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). Mitochondrial genes showed a much higher rate of change in Acoraceae and many Alismatales, but not Tofieldiaceae and Araceae; Acoraceae linked with the fast-evolving group (G. Petersen 2006c). Some characters of floral development are also 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, although in the latter it is not found in Cymodoceaceae or Tofieldiaceae (Davis et al. 2004). 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 trees based on analyses of mitochondrial data in general to be rather incongruent with those based on 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 how the mitochondrial genome evolves seem more plausible. 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. Analyses using complete chloroplast genomes sometimes yielded the clade [Liliales [Pandanales + Dioscoreales]], especially when fewer genes were included in the analyses (Liu et al. 2012; Ruhfel et al. 2014: chloroplast genomes, only one species of each included), indeed, a variety of relationships turned up in the various analyses carried out in the first study, including Alismatales embedded in the commelinids. For other suggestions of relationships in this whole area, see Fiz-Palacios et al. (2011).
Note that monocots were not monophyletic in some morphological studies such as those by Hay and Mabberley (1991); Araceae were independently derived from broad-leaved angiosperms, perhaps from Nymphaeales. 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.); the broad-leaved angiosperm outgroups have similar foliar features. This harks back to some ideas of Lindley (1853), who thought that the monocots that had leaves with reticulate venation, which he called dictyogens, were intermediate between the exogens (dicots) and the endogens (other monocots). Morphological cladistic analyses of the net-veined taxa by themselves (Conran 1989) also suggested relationships which now seem rather unsatisfactory. The analysis of morphological characters alone in monocots has tended 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). Focussing on the single character of apocarpous gynoecium, Endress (1995b) suggested that Triuris (Triuridaceae) might be a rather basal monocot.
Thanks. I am grateful to Claudia Henriquez for discussion on leaf development in monocots.
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 - Alismatidae Takhtajan, Arecidae Takhtajan, Aridae Takhtajan, Bromeliidae C. Y. Wu, Burmanniidae Heintze, Commelinidae Takhtajan, Juncidae Doweld, Liliidae J. H. Schnaffner, Orchididae Heintze, Triuridae Reveal, Zingiberidae Cronquist - Aropsida Bartling, Bromeliopsida Brongniart, Crinopsida Horaninov, Hydrocharitopsida Bartling, Juncopsida Bartling, Liliopsida Batsch, Liriopsida Brongniart, Najadiopsida Hoffmannsegg & Link, Orchidopsida Bartling, Pandanopsida Brongniart, Phoenicopsida Brongniart
ACORALES Martius, see Main Tree.
Vessels 0; inflorescence scapose, densely spicate [spadix], with large associated inflorescence bract [spathe]; flowers sessile, 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.
Note: Possible apomorphies are in bold. However, the actual level at which many of these features, particularly the more cryptic ones, should be assigned is unclear. This is partly because many characters show considerable homoplasy, in addition, basic information for all too many is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there is the not-so-trivial issue of how ancestral states are reconstructed (see above).
ACORACEAE Martinov Back to Acorales
Stem with endodermis; leaves two-ranked, isobifacial [oriented edge on to the stem]; peduncle with two separate vascular systems; bracts 0, bracteoles 0; floral vasculature forms a basal complex; T ± hooded; anthers introrse [?level], thecae confluent apically on dehiscence, endothecial thickenings stellate; tapetum secretory, cells 2-4-nucleate; pollen sulcus lacking ectexine, endexine lamellate; dorsal carpellary bundle 0, ovary loculi with secretory 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. Northern Hemisphere (map: from Hultén 1962; Fl. N. Am. 22: 2000), but perhaps naturalised in Europe and America (Mayo et al. 1997). [Photo - Habit.]
Age. Crown-group Acoraceae were dated to 19 ± 5.7 m.y. by Merckx et al. (2008a) and 52-4 m.y. by Mennes et al. (2013).
Evolution. Divergence & Distribution. See Stockey (2006) for an evaluation of fossil remains or Acoraceae.
Genes & Genomes. 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. Does Acorus have vessels? It seems to depend on one's definition, and Carlquist (2012a, see also 2009) calls the plant functionally vesselless, the tracheids being "pre-vessel" in morphology - although derived.
The abaxial tepal is large, bract-like, and encloses the young flower, indeed, it seems to have "merged" with the bract (Buzgo 2001), being depicted as an organ of "hybrid" nature (Bateman et al. 2006b); Ronse de Craene (2010) interprets it as a bract, one tepal being missing (if this is the correct interpretation, some apomorphies above will have to be changhed). 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.
Additional information is taken from Grayum (1987), Bogner and Mayo (1998), and Bogner (2011), all general, Kaplan (1970a: leaf development), Carlquist and Schneider (1997) and Keating (2003a), both anatomy, Soukup et al. (2005: root development, intermediate), Buzgo and Endress (2000: floral morphology), Eyde et al. (1967: floral anatomy), Buell (1938: ovule), Floyd and Friedman (2000: endosperm development), and Tillich (1985: seedling). For a checklist and bibliography, see Govaerts and Frodin (2002) and World Checklist of Monocots.