EMBRYOPSIDA Pirani & Prado
Gametophyte dominant, independent, multicellular, initially ±globular, not motile, branched; showing gravitropism; glycolate oxidase +, glycolate metabolism in leaf peroxisomes [glyoxysomes], acquisition of phenylalanine lysase* [PAL], flavonoid synthesis*, microbial terpene synthase-like genes +, triterpenoids produced by CYP716 enzymes, CYP73 and phenylpropanoid metabolism [development of phenolic network], xyloglucans in primary cell wall, side chains charged; plant poikilohydrous [protoplasm dessication tolerant], ectohydrous [free water outside plant physiologically important]; thalloid, leafy, with single-celled apical meristem, tissues little differentiated, rhizoids +, unicellular; chloroplasts several per cell, pyrenoids 0; centrioles/centrosomes in vegetative cells 0, microtubules with γ-tubulin along their lengths [?here], interphase microtubules form hoop-like system; metaphase spindle anastral, predictive preprophase band + [with microtubules and F-actin; where new cell wall will form], phragmoplast + [cell wall deposition centrifugal, from around the anaphase spindle], plasmodesmata +; antheridia and archegonia +, jacketed*, surficial; blepharoplast +, centrioles develop de novo, bicentriole pair coaxial, separate at midpoint, centrioles rotate, associated with basal bodies of cilia, multilayered structure + [4 layers: L1, L4, tubules; L2, L3, short vertical lamellae] (0), spline + [tubules from L1 encircling spermatid], basal body 200-250 nm long, associated with amorphous electron-dense material, microtubules in basal end lacking symmetry, stellate array of filaments in transition zone extended, axonemal cap 0 [microtubules disorganized at apex of cilium]; male gametes [spermatozoids] with a left-handed coil, cilia 2, lateral, asymmetrical; oogamy; sporophyte +*, multicellular, growth 3-dimensional*, cuticle +*, plane of first cell division transverse [with respect to long axis of archegonium/embryo sac], sporangium and upper part of seta developing from epibasal cell [towards the archegonial neck, exoscopic], with at least transient apical cell [?level], initially surrounded by and dependent on gametophyte, placental transfer cells +, in both sporophyte and gametophyte, wall ingrowths develop early; suspensor/foot +, cells at foot tip somewhat haustorial; sporangium +, single, terminal, dehiscence longitudinal; meiosis sporic, monoplastidic, MTOC [= MicroTubule Organizing Centre] associated with plastid, sporocytes 4-lobed, cytokinesis simultaneous, preceding nuclear division, quadripolar microtubule system +; wall development both centripetal and centrifugal, 1000 spores/sporangium, sporopollenin in the spore wall* laid down in association with trilamellar layers [white-line centred lamellae; tripartite lamellae]; plastid transmission maternal; nuclear genome [1C] <1.4 pg, main telomere sequence motif TTTAGGG, KNOX1 and KNOX2 [duplication] and LEAFY genes present, ethylene involved in cell elongation; chloroplast genome with close association between trnLUAA and trnFGAA genes [precursors for starch synthesis], tufA, minD, minE genes moved to nucleus; mitochondrial trnS(gcu) and trnN(guu) genes +.
Many of the bolded characters in the characterization above are apomorphies of more or less inclusive clades 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 immediate common ancestor of the group,  contains explanatory material, () features common in clade, exact status unclear.
Sporophyte well developed, branched, branching dichotomous, potentially indeterminate; hydroids +; stomata on stem; sporangia several, terminal; spore walls not multilamellate [?here].
II. TRACHEOPHYTA / VASCULAR PLANTS
Sporophyte long lived, cells polyplastidic, photosynthetic red light response, stomata open in response to blue light; plant homoiohydrous [water content of protoplasm relatively stable]; control of leaf hydration passive; plant endohydrous [physiologically important free water inside plant]; PIN[auxin efflux facilitators]-mediated polar auxin transport; (condensed or nonhydrolyzable tannins/proanthocyanidins +); borate cross-linked rhamnogalactan II, xyloglucans with side chains uncharged [?level], in secondary walls of vascular and mechanical tissue; lignins +; roots +, often ≤1 mm across, root hairs and root cap +; stem apex multicellular [several apical initials, no tunica], with cytohistochemical zonation, plasmodesmata formation based on cell lineage; vascular development acropetal, tracheids +, in both protoxylem and metaxylem, G- and S-types; sieve cells + [nucleus degenerating]; endodermis +; stomata numerous, involved in gas exchange; leaves +, vascularized, spirally arranged, blades with mean venation density ca 1.8 mm/mm2 [to 5 mm/mm2], all epidermal cells with chloroplasts; sporangia in strobili, sporangia adaxial, columella 0; tapetum glandular; sporophyte-gametophyte junction lacking dead gametophytic cells, mucilage, ?position of transfer cells; MTOCs not associated with plastids, basal body 350-550 nm long, stellate array in transition region initially joining microtubule triplets; archegonia embedded/sunken [only neck protruding]; embryo suspensor +, shoot apex developing away from micropyle/archegonial neck [from hypobasal cell, endoscopic], root lateral with respect to the longitudinal axis of the embryo [plant homorhizic].[MONILOPHYTA + LIGNOPHYTA]
Sporophyte growth ± monopodial, branching spiral; roots endomycorrhizal [with Glomeromycota], lateral roots +, endogenous; G-type tracheids +, with scalariform-bordered pits; leaves with apical/marginal growth, venation development basipetal, growth determinate; sporangium dehiscence by a single longitudinal slit; cells polyplastidic, MTOCs diffuse, perinuclear, migratory; blepharoplasts +, paired, with electron-dense material, centrioles on periphery, male gametes multiciliate; nuclear genome [1C] 7.6-10 pg [mode]; chloroplast long single copy ca 30kb inversion [from psbM to ycf2]; mitochondrion with loss of 4 genes, absence of numerous group II introns; LITTLE ZIPPER proteins.
Sporophyte woody; stem branching axillary, buds exogenous; lateral root origin from the pericycle; cork cambium + [producing cork abaxially], vascular cambium bifacial [producing phloem abaxially and xylem adaxially].
SEED PLANTS† / SPERMATOPHYTA†
Growth of plant bipolar [plumule/stem and radicle/root independent, roots positively geotropic]; plants heterosporous; megasporangium surrounded by cupule [i.e. = unitegmic ovule, cupule = integument]; pollen lands on ovule; megaspore germination endosporic, female gametophyte initially retained on the plant, free-nuclear/syncytial to start with, walls then coming to surround the individual nuclei, process proceeding centripetally.
EXTANT SEED PLANTS
Plant evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); microbial terpene synthase-like genes 0; primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignin chains started by monolignol dimerization [resinols common], particularly with guaiacyl and p-hydroxyphenyl [G + H] units [sinapyl units uncommon, no Maüle reaction]; roots often ≥1 mm across, stele diarch to pentarch, xylem and phloem originating on alternating radii, cork cambium deep seated, gravitropism response fast; stem apical meristem complex [with quiescent centre, etc.], plasmodesma density in SAM 1.6-6.2[mean]/μm2 [interface-specific plasmodesmatal network]; eustele +, protoxylem endarch, endodermis 0; 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; leaf nodes 1:1, a single trace leaving the vascular sympodium; leaf vascular bundles amphicribral; guard cells the only epidermal cells with chloroplasts, stomatal pore with active opening in response to leaf hydration, control by abscisic acid, metabolic regulation of water use efficiency, etc.; branching by axillary buds, exogenous; prophylls two, lateral; leaves with petiole and lamina, development basipetal, lamina simple; sporangia borne on sporophylls; spores not dormant; microsporophylls aggregated in indeterminate cones/strobili; grains monosulcate, aperture in ana- position [distal], primexine + [involved in exine pattern formation with deposition of sporopollenin from tapetum there], exine and intine homogeneous, exine alveolar/honeycomb; ovules with parietal tissue [= crassinucellate], megaspore tetrad linear, functional megaspore single, chalazal, sporopollenin 0; gametophyte ± wholly dependent on sporophyte, development initially endosporic [apical cell 0, rhizoids 0, etc.]; male gametophyte with tube developing from distal end of grain, male gametes two, developing after pollination, with cell walls; embryo cellular ab initio, suspensor short-minute, embryonic axis straight [shoot and root at opposite ends], primary root/radicle produces taproot [= allorhizic], cotyledons 2; embryo ± dormant; chloroplast ycf2 gene in inverted repeat, trans splicing of five mitochondrial group II introns, rpl6 gene absent; ??whole nuclear genome duplication [ζ/zeta duplication event], 2C genome size (0.71-)1.99(-5.49) pg, two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], 5.8S and 5S rDNA in separate clusters.
IID. ANGIOSPERMAE / MAGNOLIOPHYTA
Lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, apigenin and/or luteolin scattered, [cyanogenesis in ANA grade?], lignin also with syringyl units common [G + S lignin, positive Maüle reaction - syringyl:guaiacyl ratio more than 2-2.5:1], hemicelluloses as xyloglucans; root cap meristem closed (open); pith relatively inconspicuous, lateral roots initiated immediately to the side of [when diarch] or opposite xylem poles; epidermis probably originating from inner layer of root cap, trichoblasts [differentiated root hair-forming cells] 0, hypodermis suberised and with Casparian strip [= exodermis]; shoot apex with tunica-corpus construction, tunica 2-layered; starch grains simple; primary cell wall mostly with pectic polysaccharides, poor in mannans; tracheid:tracheid [end wall] plates with scalariform pitting, multiseriate rays +, wood parenchyma +; sieve tubes enucleate, sieve plates with pores (0.1-)0.5-10< µm across, cytoplasm with P-proteins, not occluding pores of plate, companion cell and sieve tube from same mother cell; ?phloem loading/sugar transport; nodes 1:?; dark reversal Pfr → Pr; protoplasm dessication tolerant [plant poikilohydric]; stomata randomly oriented, brachyparacytic [ends of subsidiary cells ± level with ends of guard cells], outer stomatal ledges producing vestibule, reduction in stomatal conductance with increasing CO2 concentration; lamina formed from the primordial leaf apex, margins toothed, development of venation acropetal, overall growth ± diffuse, secondary veins pinnate, fine venation hierarchical-reticulate, (1.7-)4.1(-5.7) mm/mm2, vein endings free; flowers perfect, pedicellate, ± haplomorphic, protogynous; parts free, numbers variable, development centripetal; P = T, petal-like, 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], each theca dehiscing longitudinally by a common slit, ± embedded in the filament, walls with at least outer secondary parietal cells dividing, endothecium +, cells elongated at right angles to long axis of anther; tapetal cells binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellate, endexine restricted to the apertural regions, thin, compact, intine in apertural areas thick, orbicules +, pollenkitt +; nectary 0; carpels present, superior, free, several, spiral, ascidiate [postgenital occlusion by secretion], stylulus at most short [shorter than ovary], hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, dry; suprastylar extragynoecial compitum +; 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, nucellar cap?; megasporocyte single, hypodermal, functional megaspore lacking cuticle; female gametophyte lacking chlorophyll, four-celled [one module, egg and polar nuclei sisters]; ovule not increasing in size between pollination and fertilization; pollen grains bicellular at dispersal, germinating in less than 3 hours, siphonogamy, pollen tube unbranched, growing towards the ovule, between cells, growth rate (ca 10-)80-20,000 µm h-1, tube 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 gametophytes tricellular, gametes 2, lacking cell walls, ciliae 0, double fertilization +, ovules aborting unless fertilized; fruit indehiscent, P deciduous; mature seed much larger than fertilized ovule, small [<5 mm long], dry [no sarcotesta], exotestal; endosperm +, ?diploid [one polar nucleus + male gamete], cellular, development heteropolar [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 short [<¼ length of seed]; plastid and mitochondrial transmission maternal; Arabidopsis-type telomeres [(TTTAGGG)n]; nuclear genome [2C] (0.57-)1.45(-3.71) [1 pg = 109 base pairs], ??whole nuclear genome duplication [ε/epsilon event]; 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, palaeo AP3 and PI genes [paralogous B-class genes] +, with "DEAER" motif, SEP3/LOFSEP and three copies of the PHY gene, [PHYB [PHYA + PHYC]]; chloroplast chlB, -L, -N, trnP-GGG genes 0.
[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]]]]: phloem loading passive, via symplast, plasmodesmata numerous; vessel elements with scalariform perforation plates in primary xylem; essential oils in specialized cells [lamina and P ± pellucid-punctate]; tension wood + [reaction wood: with gelatinous fibres, G-fibres, on adaxial side of branch/stem junction]; anther wall with outer secondary parietal cell layer dividing; tectum reticulate; 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 [?here]; pollen tube growth intra-gynoecial; extragynoecial compitum 0; carpels plicate [?here]; embryo sac monosporic [spore chalazal], 8-celled, bipolar [Polygonum type], antipodal cells persisting; endosperm triploid.
[MONOCOTS [CERATOPHYLLALES + EUDICOTS]]: (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).
[CERATOPHYLLALES + EUDICOTS]: ethereal oils 0 [or next node up]; fruit dry [very labile].
EUDICOTS: (Myricetin +), asarone 0 [unknown in some groups, + in some asterids]; root epidermis derived from root cap [?Buxaceae, etc.]; (vessel elements with simple perforation plates in primary xylem); nodes 3:3; stomata anomocytic; flowers (dimerous), cyclic; protandry common; K/outer P members with three traces, ("C" +, with a single trace); A ?, filaments fairly slender, anthers basifixed; microsporogenesis simultaneous, pollen tricolpate, apertures in pairs at six points of the young tetrad [Fischer's rule], cleavage centripetal, wall with endexine; G with complete postgenital fusion, stylulus/style solid [?here], short [<2 x length of ovary]; seed coat?; palaeotetraploidy event.
[PROTEALES [TROCHODENDRALES [BUXALES + CORE EUDICOTS]]]: (axial/receptacular nectary +).
[TROCHODENDRALES [BUXALES + CORE EUDICOTS]]: benzylisoquinoline alkaloids 0; euAP3 + TM6 genes [duplication of paleoAP3 gene: B class], mitochondrial rps2 gene lost.
Age. Magallón and Castillo (2009) tentatively suggest an age of about 122.7 Ma for this node, Magallón et al. (2015) an age of about 129 Ma, Foster et al. (2016a: q.v. for details) an age of ca 137 Ma and Z. Wu et al. (2014) an age of around 188 Ma, far older, Vekemans et al. (2012: MRCA Buxales and Trochodendrales) an age of around (124.1-)122.6(-121.1) Ma or a little less; around 128.2/124.8 Ma are ages in Tank et al. (2015: Table S1, S2). The age for stem Trochodendrales in Wikström et al. (2003) is about (140-)135, 123(-118) Ma - but relationships are [B [T + CE]], and in Strijk et al. (2019) it is (125.8-)124.2(-121.8) Ma.
Evolution: Divergence & Distribution. Doyle (2013) is inclined to think that the evolution of 2-merous, wind pollinated flowers can be pegged to this node. M.-Y. Zhang et al. (2017) discuss pollen evolution.
Genes & Genomes. For the paleoAP3 duplication, etc., see Lamb and Irish (2003), S. Kim et al. (2004b, 2005a), Zahn et al. (2005a), and especially Kramer et al. (2006); Kramer and Zimmer (2006) noted that although the paleoAP3 gene has been found in Proteales and Sabiales, whether or not it occurs in Buxales and Trochodendrales was unclear. EuAP3/PI are involved in stamen identity very much as paleoAP3/PI, but the former may be involved in petal development in core eudicots. For the loss of the rps2 mitochondrial gene, see Adams et al. (2002b), and for the duplication of the RPB1 and RPB2 genes, which may have occurred in the immediate ancestor of Trochodendrales, see Oxelman et al. (2004) and Luo et al. (2007). This latter is a complicated problem, since Buxaceae have no duplication in either gene and there has also been widespread loss of both genes.
Indeed, Chanderbali et al. (2016b) suggested that about half the gene duplications associated with the palaeohexaploidy γ triplication event are to be placed here, the other half being later, being placed at the core eudicot node. However, Aköz and Nordberg (2019) have recently clarified the nature of this supposed triplication; for further details, see core eudicots.
Phylogeny. For discussion on the position of Trochodendrales, see the Ranunculales page.
TROCHODENDRALES Cronquist - Main Tree.
Just the one family, 2 genera, 2 species.
Note: In all node characterizations, boldface denotes a possible apomorphy, (....) denotes a feature the exact status of which in the clade is uncertain, [....] includes explanatory material; other text lists features found pretty much throughout the clade. Note that the precise node to which many characters, particularly the more cryptic ones, should be assigned is unclear. This is partly because homoplasy is very common, in addition, basic information for all too many characters 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 are the not-so-trivial issues of how character states are delimited and ancestral states are reconstructed (see above).
Synonymy: Trochodendrineae Engler - Trochodendranae Reveal
TROCHODENDRACEAE Eichler, nom. cons. - Back to Trochodendrales
Flavonols +; petiole bundle arcuate; cuticle waxes with nonacosan-10-ol a major component, stomata laterocytic; buds with scales; lamina with secondary veins proceeding to a clear, persistent cap, lateral veins also enter; flowers with cortical vascular system; anthers valvate, latrorse, filaments thin; pollen tectum reticulate-striate; G very slightly inferior, carpels laterally connate, nectariferous abaxially, 5 vascular bundles/carpel; stigma decurrent, unicellular-papillate, compitum 0; ovules apotropous, integuments long, micropyle bistomal, chalazal protrusion +; fruit an aggregate of follicles, styles becoming ± basal on the outer surface; seeds flattened, with marginal flange, chalazal hair-pin bundle +; endotestal cells with slightly thickened walls, exotegmic cells thick-walled, tracheidal, elongated; endosperm development?; chloroplast genome with ca 4 kb expansion of the inverted repeat.
2/2 [list]. Southeast Asia, scattered.
Age. Estimates for the divergence times of the two genera are (113-)106, 95(-88) Ma (Wikström et al. 2001), (65-)20, 19(-7) Ma (Bell et al. 2010) and 44-30 Ma (Sun et al. 2013) - but see below for the younger ages...
Fossils of Trochodendraceae are known from since the late Cretaceous as Nordenskioldia, more properly Nordenskioeldia, which is close to Trochodendron (see also Friis et al. 2011). They are widely distributed around the northern hemisphere in the Eocene, and are sometimes at very high latitudes (Crane et al. 1991; Pigg et al. 2001, 2007; Taylor et al. 2009; Harrington et al. 2011). However, at least some of these fossils lack a hairpin loop in the seed (Crane et al. 1991, but c.f. Doweld 1998c); postgenital fusion of the carpels may not occur. Fossils of both the extant genera are known from the Eocene onwards (Friis et al. 2011 and references).
1. Trochodendron Siebold & Zuccarini
Evergreen shrub to tree; myricetin +; plant glabrous; pits bordered; vessels 0; young stem with separate vascular bundles; rays ca 12-seriate; nodes 1:1-7:7; ± branched sclereids +; leaves spiral, lamina margin crenulate, venation subpinnate; plant androdioecious, inflorescence terminal, botyroid; P 0-5, minute, not vascularized; A many, ± spiral; G [(4-)6-11(-17)], with a secretion canal, placentation apical-axile, styluli erect, stigma decurrent in two crests; ovules many/carpel; (follicles also opening abaxially); seeds pendulous, endotesta sclerotic; n = 20.
1/1: Trochodendron aralioides. Japan to N. Taiwan (map: red, fossils blue [Japan - N.E. Honshu, not in the sea...], Pigg et al. 2001, 2007). [Photo - Collection, Inflorescences.]
Age. Flowers/fruits of Archaestella verticillata, from ca 89 Ma deposits in Japan, have been associated with Trochodendron (Takahashi et al. 2017). The flowers with their semi-inferior gynoecium are partly enclosed by rather massive tissue on the upper edge of which stamens and perianth are borne; these flowers are also described as having an hypanthium (Takahashi et al. 2017). Fossils of Trochodendron are known from the Eocene onwards (Friis et al. 2011 and references).
2. Tetracentron Oliver
Deciduous trees; chalcones or dihydrochalcones +; secretory cells +; leaves two-ranked, lamina vernation supervolute, margin serrate, secondary veins palmate, leaf base broad, thin latero-basal flange enveloping axillary bud, lamina with palmate venation; inflorescence axillary, spicate, pendulous; flowers sessile, small, 4-merous; P 4, vascular bundle rudimentary; A equal and opposite P; G , alternate with P, styluli short, spreading, placentation axile; ovules 5-6/carpel, micropyle bistomal, outer integument 3-4 cells across, inner integument ca 2 cells across, parietal tissue 2-3 cells across; testa multiplicative, meso- and endotesta lignified, tegmen two layered; endosperm nuclear/coenocytic, with starch; n = 24.
1/1: Tetracentron sinense. China, Nepal (map: from Hara & Kanai 1964; fossils green, from Grímsson et al. 2008).
Age. Fossils of Tettracentron are known from the Eocene onwards (Friis et al. 2011 and references).
Synonymy: Tetracentraceae A. C. Smith, nom. cons.
Evolution: Divergence & Distribution. Both Trochodendron and Tetracentron were widely distributed in the northern hemisphere is the Caenozoic (Grímsson et al. 2008: Tetracentron; Friis et al. 2011; Manchester et al. 2018b: v. diverse in NW North America ca 50 Ma). Note that in some fossils assigned to Trochodendron there are paired auricles or foliaceous stipels at the base of the lamina (Pigg et al. 2007).
Genes & Genomes. For the chloroplast genome of the two species, see Sun et al. (2013).
Chemistry, Morphology, etc.. Vessel elements with scalariform to scalariform-reticulate perforation plates have recently been reported from Tetracentron and Trochodendron, although both genera had long been considered to lack vessels (Hacke et al. 2007; Ren et al. 2007a; H.-F. Li et al. 2011; c.f. Bailey & Thompson 1918).
In Tetracentron the petiole becomes round towards the base, and there is a marginal flange that tightly and totally envelops the axillary bud; the base of the petiole is broad and the scar encircles much of the stem. In Trochodendron, on the other hand, not only is nodal anatomy variable and the leaves on the adult plant have a narrow petiole and often lack axillary buds, in the young plant the leaves are more similar to those of Tetracentron (Bailey & Nast 1945; Nast & Bailey 1955). Although Baranova (1983) described both genera as having laterocytic stomata, Metcalfe and Chalk (1950) described and drew the stomata of Trochodendron as being laterocylic, while the stomata are of Trochodendron are cyclocytic, according to Carlquist (1982)...
For inflorescence development in Trochodendron, see Hsu et al. (2016). The micropyle of Trochodendron is described as being endostomal in Johri et al. (1992) and Endress and Igersheim (1999), but it looks bistomal in Takhtajan (1991). In both genera the adaxial side of the carpel develops greatly as the fruits ripen and so the style becomes basal on the abaxial side of the follicle.
See Endress (1993) for general information, Aiso et al. (2016) for reaction (tension) wood, Nast and Bailey (1945) for flowers, fruits and summary, Endress (1986b) for floral morphology, Wu et al. (2007) for perianth evolution, Erbar (2014) for nectaries, and Chen et al. (2007) for floral morphology and Pan et al. (1993) for embryology of Tetracentron; Rix and Crane (2007) provide general information about Tetracentron.
Previous Relationships. The relationships of the two genera included in Trochodendraceae have been somewhat obscure, although their apparent lack of vessels had long typed them as being primitive angiosperms. Endress (1986b) compared the sclereids of Trochodendron and the secretory cells of Tetracentron with the cells secreting ethereal oils in magnoliids, etc., and linked the two genera with Cercidiphyllaceae (here Saxifragales) and Eupteleaceae (Ranunculales) in an expanded Trochodendrales (c.f. also Takhtajan 1997 and Thorne 2007 in part). Kai-yu et al. (1993) suggested that Tetracentron should be put in its own order.
Classification. Including Tetracentraceae in Trochodendraceae was an option in A.P.G. II (2003). The two families do have quite a lot in common, as is clear from the fairly lengthy ordinal description, and since both are monotypic, combination is in order (see A.P.G. III 2009).