EMBRYOPSIDA Pirani & Prado
Gametophyte dominant, independent, multicellular, initially ±globular; showing gravitropism; acquisition of phenylalanine lysase [PAL], phenylpropanoid metabolism [lignans +, flavonoids + (absorbtion of UV radiation)], xyloglucans in primary cell wall with distinctive side chains; 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; glycolate metabolism in leaf peroxisomes [glyoxysomes]; centrioles/centrosomes in vegetative cells 0, microtubules with γ-tubulin along its length [?here], interphase microtubules form hoop-like system; metaphase spindle anastral, predictive preprophase band of microtubules [where cell plate will join parental cell wall], phragmoplast + [cell wall deposition spreading from around the spindle fibres], 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; oogamy; sporophyte multicellular, 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 [MTOC = microtubule organizing centre] associated with plastid, sporocytes 4-lobed, cytokinesis simultaneous, preceding nuclear division, quadripolar microtubule system +; wall development both centripetal and centrifugal, sporopollenin + laid down in association with trilamellar layers [white-line centred lamellae; tripartite lamellae], >1000 spores/sporangium; nuclear genome size <1.4 pg, main telomere sequence motif TTTAGGG, LEAFY and KNOX1 and KNOX2 genes present, ethylene involved in cell elongation; chloroplast genome with close association between trnLUAA and trnFGAA genes, precursor for starch synthesis in plastid.
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 immediate common ancestor of the group,  contains explanatory material, () features common in clade, exact status unclear.
Abscisic acid, L- and D-methionine distinguished metabolically; sporophyte with polar transport of auxins, class 1 KNOX genes expressed in sporangium alone; sporangium wall 4≤ cells across [≡ eusporangium], tapetum +, secreting sporopollenin, which obscures outer white-line centred lamellae, columella +, developing from endothecial cells; stomata +, on sporangium, anomocytic, cell lineage that produces them with symmetric divisions [perigenous]; underlying similarities in the development of conducting tissue and of rhizoids/root hairs; spores trilete; shoot meristem patterning gene families expressed; MIKC, MI*K*C* genes, post-transcriptional editing of chloroplast genes; gain of three group II mitochondrial introns, mitochondrial trnS(gcu) and trnN(guu) genes 0.
[Anthocerophyta + Polysporangiophyta]: xyloglucans in the primary cell wall with fucosylated subunits; gametophyte leafless; archegonia embedded/sunken [on;y neck protruding]; sporophyte long-lived, chlorophyllous; cell walls with xylans.
Sporophyte dominant, branched, branching apical, dichotomous, potentially indeterminate; vascular tissue +; stomata on stem; sporangia several, each opening independently; spore walls not multilamellate [?here].
EXTANT TRACHEOPHYTA / VASCULAR PLANTS
Sporophyte with photosynthetic red light response; (condensed or nonhydrolyzable tannins/proanthocyanidins +); plant homoiohydrous [water content of protoplasm relatively stable]; control of leaf hydration passive; plant endohydrous [physiologically important free water inside plant]; (condensed or nonhydrolyzable tannins/proanthocyanidins +); xylans in secondary walls of vascular and mechanical tissue; lignins +; stem apex multicellular, with cytohistochemical zonation, plasmodesmata formation based on cell lineage; tracheids +, in both protoxylem and metaxylem, G- and S-types; sieve cells + [nucleus degenerating]; endodermis +; leaves/sporophylls spirally arranged, blades with mean venation density ca 1.8 mm/mm2 [to 5 mm/mm2], all epidermal cells with chloroplasts; sporangia adaxial, columella 0; tapetum glandular; ?position of transfer cells; MTOCs not associated with plastids, basal body 350-550 nm long, stellate array in transition region initially joining microtubule triplets; 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 endomycorrhizal [with Glomeromycota]; growth ± monopodial, branching spiral; root cap +, protoxylem exarch, 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; chloroplast long single copy ca 30kb inversion [from psbM to ycf2]; LITTLE ZIPPER proteins.
Sporophyte woody; stem branching lateral, meristems axillary; lateral root origin from the pericycle; cork cambium + [producing cork abaxially], vascular cambium bifacial [producing phloem abaxially and xylem adaxially].
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].
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 particularly with guaiacyl and p-hydroxyphenyl [G + H] units [sinapyl units uncommon, no Maüle reaction]; root stele with xylem and phloem originating on alternate radii, cork cambium deep seated; 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.; 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; female gametophyte initially syncytial, walls then surrounding individual nuclei; embryo cellular ab initio, suspensor short-minute, embryonic axis straight [shoot and root at opposite ends; plant allorhizic], cotyledons 2; embryo ± dormant; ycf2 gene in inverted repeat, mitochondrial trans- nad2i542g2 and coxIIi3 introns present; whole nuclear genome duplication [ζ - zeta - duplication], two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], 5.8S and 5S rDNA in separate clusters.
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 apical meristem intermediate-open; stele di- to pentarch [oligarch], pith relatively inconspicuous, 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, 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, wood parenchyma +; sieve tubes enucleate, sieve plate 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; sugar transport in phloem passive; nodes 1:?; dark reversal Pfr → Pr; protoplasm dessication tolerant [plant poikilohydric]; stomata brachyparacytic [ends of subsidiary cells level with ends of pore], 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 +, ?insertion, 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], 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 lamellate only in the apertural regions, thin, compact, intine in apertural areas thick, pollenkitt +; nectary 0; carpels present, superior, free, several, ascidiate [postgenital occlusion by secretion], stylulus at most short [shorter than ovary], hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, dry, 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, not photosynthesising, four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; 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, pollen tube elongated, unbranched, 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, ciliae 0, siphonogamy; double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; mature seed much larger than fertilized ovule, small , dry [no sarcotesta], exotestal; endosperm +, 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 very small [1C = <1.4 pg, 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]]]]: 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]; tectum reticulate; anther wall with outer secondary parietal cell layer dividing; 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 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).
[CERATOPHYLLALES + EUDICOTS]: ethereal oils 0.
EUDICOTS: (Myricetin, delphinidin +), 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]; seed coat?
[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.
[BUXALES + CORE EUDICOTS]: ?
Age. This node dates to 121-117 m.y. (Anderson et al. 2005) or ca 123.9 m.y. (Tank et al. 2015: Table S2); Magallón and Castillo (2009) suggest an age of around ca 121.5 m.y. and ca 182 m.y. is the age in Z. Wu et al. (2014). Ages in Vekemans et al. (2012) were about (123-)122.4(-121.4) m.y., in Magallón et al. (2015) around 128.6 m.y., in Wikström et al. (2003) about (142-)137, 124(-119) m.y. - but c.f. sister groups there: [B [T + CE]], and Xue et al. (2012) ca 115.4 m.y..
BUXALES Reveal Main Tree.
Steroidal pregnan pseudoalkaloids +; true tracheids +; compression wood +; ?nodes; stomata cyclocytic; lamina margins entire; plant monoecious; flowers small [<7 mm across], imperfect; P members with a single trace; anthers ± dorsifixed; styluli stigmatic ± their entire length, strongly canaliculate; ovules 1-2/carpel; micropyle bistomal; seed testal, with several cell layers; endosperm development?, embryo size? - 1 family, 7 genera, 120 species.
Age. Wikström et al. (2001) thought that crown-group Buxaceae were (130-)124, 113(-109) m.y., Magallón and Castillo (2009) suggested an age of ca 111.5 m.y., Magallón et al. (2013) an age of about 118.4 m.y., and Bell et al. (2010) an age of (103-)99, 98(-97) m.y.. Anderson et al. (2005) suggested that this group was some 99 m.y. old, while the age in Magallón et al. (2015) was only about 85.1 m. years.
Drinnan et al. (1991) described Spanomera from east North American deposits of 113-98 m.y. age (Albian); it is perhaps a member of stem group Buxaceae (see also Crepet et al. 2004; Doyle & Endress 2010; Doyle & Upchurch 2014). The carpellate flowers are dimerous, but there may be five stamens opposite five tepals, the pollen is tricolpate, and the fruit is follicular (Drinnan et al. 1991); relationships with Trochodendraceae have also been suggested (Doyle 1999). Lusistemon, also with striate, tricolpate pollen (but with different anther insertion and arrangement), from deposits in Portugal of about the same age, seems to be related (Pedersen et al. 2007). See also Friis et al. (2011) for fossils.
Note: Boldface denotes possible apomorphies, (....) denotes a feature common in the clade, exact status uncertain, [....] includes explanatory material. Note that the particular 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).
Phylogeny. For the phylogenetic position of Buxales, see the Ranuculales page.
Synonymy: Didymelales Takhtajan - Buxineae J. Presl - Buxanae Reveal & Doweld
BUXACEAE Dumortier, nom. cons. Back to Buxales
Evergreen, woody; cortex with sclereids/fibers; petiole with three arcuate traces; flowers imperfect.
7/120: four groups below. ± Temperate to ± tropical, very scattered, not in the Antipodes.
1. Didymeles Leandri
Cork ?; pericyclic fibres 0; pith chambered, nodes ?; petiole with the two lateral traces inverted; hairs small, peltate; leaves spiral; plant dioecious; staminate flowers: P 0; A 2, subsessile, connate basally, anthers extrorse; pollen colpi 2-orate, surface reticulate, with small spines; pistillode 0; carpellate flowers: ?P 1, scale-like; staminode 0; G 1; ovules 1(2)/carpel, median, epitropous, integuments ± elongated, protruding into the stylar canal, outer integument fringed apically; fruit a drupelet furrowed on one side; rudimentary aril +?; testa cell walls thickened, tegmen obliterated; endosperm 0, development?, embryo large; n = ?
1/2. E. Madagascar, the Comoro islands. [Photos - Inflorescence.]
Synonymy: Didymelaceae Leandri
[Stylocerateae, Haptanthus, Buxeae]: raffinose, stachyose + [phloem exudate], tannins 0; cork subepidermal or pericyclic; sieve tube plastids with protein crystalloids; nodes 3:3; secretory cells single or in branching rows; cuticle waxes as curled rodlets or irregular platelets; (stomata laterocytic); leaves opposite, lamina ptyxis flat to curved; inflorescence with a terminal carpellate flower, botyroid; staminate flowers: P (0-)2-4(5), not clearly separated from bracteoles, decussate; tapetal cells binucleate; pistillode +, nectariferous; carpellate flowers: P 6-20, at least some spiral; G , completely fused, with five bundles, a roof over the ovary, so styluli ± marginal, spreading, stigma dry or weakly secretory; ovules 2/carpel, apotropous, outer integument 5-8 cells across, inner integument 2(-3) cells across, parietal tissue 4-15+ cell layers across, (weak nucellar cap), suprachalazal nucellus massive [Buxus], ovular obturator +, hypostase +; fruit with persistent styles; (seed carunculate), exotesta lignified, palisade, hypodermis often lignified; endosperm copious, (perisperm +); x = ?7.
6/115. ± Temperate to ± tropical, very scattered, but not the Antipodes (map: see Meusel et al. 1978; Trop. Afr. Fl. Pl. Ecol. Distr. 5. 2010). [Photo - Inflorescence, Fruits.]
Age. Anderson et al. (2005) suggested that this node was some 111-63 m.y. old; ages in (N. Zhang et al. 2012) were ca 45 m.y.a. and in Wikström et al. (2001) (107-)100, 93(-86) m.y. ago.
2. Stylocerateae Pax
(Perennial rhizomatous herbs); (nodes 1:3 - Sarcococca); leaves spiral, lamina margins toothed, (venation triplinerved); no tanniniferous tissue in the flowers [?Didymeles]; (plant dioecious), (carpellate flowers basal in the inflorescence); staminate flowers: A (many, sessile - Styloceras), filaments conspicuous, (endothecium >2-latered); pollen pantoporate, exine crotonoid (reticulate, small spines - Styloceras), (pistillode 0); carpellate flowers: bracts many, all spiral; G , (loculi divided); ovules epitropous, micropyle bistomal, outer integument 2-3 cells across; fruit a drupe (dry; ?dehiscent, pulp surrounding blue seeds - Styloceras); testa multiplicative, endotegmen lignified [Sarcococca]; endosperm nuclear [Sarcococca], embryo with short radicle; n = 12-14.
3/22. E. Asia to W. Malesia, E. North America, N. South America.
Synonymy: Pachysandraceae J. Agardh, Stylocerataceae Reveal & Hoogland
[Haptanthus + Buxeae] (if this clade exists): G ; outer integument ca 5 cells across.
3. Haptanthus Goldberg & C. Nelson
Cork ?; fibres storied; vessel elements with scalariform perforation plates; nodes 1:1 [from bracts]; stomata ?; plant glabrous; lamina weakly pli-nerved at the base; inflorescences with staminate flowers borne decussately along the branches, (terminal staminate flower +), bracteoles 0; staminate flowers: P 1; A 2, completely adnate to P, dehiscence ?valvate, endothecium >2-layered; pollen tricolpate; pistillode 0; carpellate flowers: P 0; staminodes 0; G stipitate, symplicate, placentation parietal, no roof over the ovary, styles radiating; ovules 8-15/carpel, details?; fruit unknown, ?septicidal; n = ?
1/1: Haptanthus hazlettii. Central America, Honduras, recently recollected.
Synonymy: Haptanthaceae C. Nelson
4. Buxeae Dumortier
(Cortical vascular bundles in angles of stems); (venation dendritic [cladodromous] - ?level); staminate flowers: A 4-10, sessile; pollen 3-12 colporate, colpus 2-4 orate, 12+ pantoporate, surface reticulate, substriate, pilate-verrucate; (pistillode 0); carpellate flowers: inflorescence with basal decussate "bracts"; median carpel adaxial, nectary between styles (0); ovules apotropous, micropyle endostomal, outer integument 5-10 cells across, ?minute third integument; fruit an explosively-dehiscent capsule, loculicidal; embryo ± curved, radicle long; n = 14, 20.
2/95: Buxus (90). Scattered, inc. Madagasacar, nearly all Central America and West Indies in New World, not Australia nor much of Malesia.
Evolution. Divergence & Distribution. There are a number of distinctive tricolpate-striate pollen types perhaps assignable to plants of this relationship (?Buxeae?) that are widely distributed in early Cretaceous rocks (Pedersen et al. 2007 and references). Krutzsch (1989) discussed the later pollen record of the family, which is widespread in the northern hemisphere in the early Caenozoic. Fossil Didymeles is reported from New Zealand (Lee at al. 2001) and elsewhere in the southern hemisphere (Friis et al. 2011; c.f. Carpenter et al. 2010).
"Pistillode +, nectariferous" is provisionally placed as an apomorphy of the family; Köhler (2006) noted that in two of the three genera of the Sarcococceae there was a pistillode, and also that the nectary was central.
Ecology & Physiology. 17/34 of the Cuban species of Buxus are reported to accumulate nickel (Reeves et al. 1996; Brooks 1998) - species limits?
Pollination Biology & Seed Dispersal. Members of Buxeae are myrmecochorous (Lengyel et al. 2010).
Chemistry, Morphology, etc. Iridoids are reported from Didymeles by Sutton (1989); this should be confirmed. There is a broad pith and at best late-developing cork (Leandri 1937); the axial parenchyma is replaced by lignified cells (Takhtajan 1997). Harms (1893) described sclereids in the pericyclic position in Didymeles, and also cortical sclereids. Buxus often has fibre strands in stem. All genera (Styloceras unknown; Hapt.) have three vascular bundles in the upper part of the petiole. The stomata of some Buxaceae may be described as being laterocytic (Baranova 1983).
The floral morphology of Didymeles has been described by von Balthazar and Endress (2002a) and Von Balthazar et al. (2003). In carpellate flowers there is a little scale between the ovule and the bract; its morphological nature is unclear; is it a third integument (Endress 2011b)? The flowers may be borne in pairs with a small hump between them, perhaps a modified cymose unit, the hump being a reduced terminal flower.
Haptanthus has parietal placentation and odd staminate flowers that were originally described as consisting of a single stamen (Goldberg & Nelson S. 1989). However, there are two stamens each with two thecae attached their length to an elliptic structure with a single arcuate vascular bundle (?very broad filament; ?prophyll, ?perianth member - the former interpretation is preferred interpretation by Shipunov & Oskolski 2011, the latter by Oskolski et al. 2015); the endothecium is massive (Doust & Stevens 2005), as it is in Styloceras, the genus perhaps most similar to Haptanthus (Shipunov & Oskolski 2011). The inflorescence is distinctive, and is described as a botryoid with a central terminal flower (Oskolski et al. 2015).
The carpellate flowers of other taxa have spirally inserted parts, but the basical floral construction of the family is perhaps dimerous (see also Oskolski et al. 2015). Interpretation of inflorescence morphology is not easy, indeed, the nature of the paired "bracts" (?prophylls?) below the carpellate flowers of Buxus is particularly unclear (von Balthazar & Endress 2001, 2002a). The perianth parts have a single vascular bundle (von Balthazar and Endress 2002a). Some species of Buxus have distinctive pollen with two or more orae per colpus that is rather like that of Didymeles; pollen in the family is very variable (Köhler 2006 for references). Both Styloceras and Pachysandra have subdivided carpels; the relative positions of the carpels and perianth is unclear (Wiger 1935).
For additional information: ovules, see Wiger (1935), nectary types, see Vogel (1998b), for pollen of Buxus, very variable, see Köhler (1994 and references), for fossil pollen, Doyle (1999), for floral morphology, von Balthazar and Endress (2001, esp. 2002b), and for a general summary, see van Tieghem (1897) and Köhler (2006: as Buxaceae and Didymelaceae).
Phylogeny. Didymeles is sister to Buxaceae s. str. (representatives of Buxeae and Sarcococceae were included) in the three-gene analysis of Worberg et al. (2007), although Hilu et al. (2003: matK [incomplete sequence] alone) found Didymeles to be embedded in a paraphyletic Buxaceae. The poorly-known Haptanthaceae were tentatively included in Buxales in pre-March 2011 versions of the site based on the anatomy of the staminate flower (Doust & Stevens 2005), but a position within Buxaceae s.l. was strongly suggested by analysis of data from the rbcL gene, although the exact position differed in maximum likelihood and maximum parsimony analyses (Shipunov & Shipunova 2011). For relationships in the family, see also von Balthazar et al. (2000) and von Balthazar and Endress (2002b); relationships within Stylocerateae are [Sarcococca [Styloceras + Pachysandra]].
Classification. Including Didymelaceae in Buxaceae was an option in A.P.G. II (2003);the two were combined in A.P.G. III (2009).
Previous Relationships. Buxaceae have often been associated with Euphorbiaceae s.l. (e.g. Cronquist 1981). When the very distinctive Haptanthus was first described, it was not assigned to a family. Shipunov (2003) placed Haptanthaceae in his Violales (= Malpighiales here), close to Salicaceae and Lacistemataceae (see also Goldberg & Alden 2005), probably because of its parietal placentation.