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 +; stem 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.; leaves with petiole and lamina, development basipetal, blade simple; branches axillary (buds not associated with all leaves), exogenous; prophylls two, lateral; plant heterosporous, sporangia borne on sporophylls; microsporophylls aggregated in indeterminate cones/strobili; true pollen +, grains mono[ana]sulcate, exine and intine homogeneous; ovules unitegmic, parietal tissue 2+ cells across, megaspore tetrad linear, functional megaspore single, chalazal, lacking sporopollenin, megasporangium indehiscent; pollen grains landing 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 landing 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).
[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; K/outer P members with three traces, ("C" +, with a single trace); A few, (polyandry widespread, initial primordia 5, 10, or ring, ± centrifugal), 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); 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 Wikström et al. (2003)e 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; inflorescence racemose; plant monoecious; flowers small [<7 mm across], imperfect; P members with a single trace; anthers ± dorsifixed; pollen also with endoapertures; styluli stigmatic their entire length, with two crests; ovules 1-2/carpel; micropyle bistomal; seed testal, with several cell layers; endosperm development?, embryo size? - 2 families, 7 genera, 120 species.
Age. Wikström et al. (2001) thought that crown-group Buxales 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.
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).
Chemistry, Morphology, etc. The stomata of Buxaceae, at least, may be described as being laterocytic (Baranova 1983).
Phylogeny. For discussion on the phylogenetic position of Buxales, see the Ranuculales page. Didymelaceae are sister to Buxaceae (representatives of both tribes 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. Including Didymelaceae in Buxaceae was an option in A.P.G. II (2003). 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 sister to Buxus was strongly suggested by analysis of data from the rbcL gene (Shipunov & Shipunova 2011).
Includes Buxaceae, Didymelaceae.
Synonymy: Didymelales Takhtajan - Buxanae Reveal & Doweld
DIDYMELACEAE Leandri Back to Buxales
Evergreen trees; cork ?; pericyclic fibres 0; pith chambered, nodes ?; fibres +; petiole with three traces, the two lateral inverted; hairs small, peltate; leaves spiral; plant dioecious; staminate flowers: P 0; A 2, connate basally, anthers extrorse; pollen tricolpate with 2-orate colpi; carpellate flowers: ?P 1, scale-like; 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; endosperm 0, type?, embryo large; n = ?
1[list]/2. E. Madagascar. [Photos - Inflorescence.]
Evolution. Divergence & Distribution. Fossil Didymeles is reported from New Zealand (Lee at al. 2001).
Chemistry, Morphology, etc. Iridoids are reported 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, and also cortical sclereids. Mabberley (1997) described the family as being glabrous.
Floral morphology has recently 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.
For a general summary, see Köhler (2006).
BUXACEAE Dumortier, nom. cons. Back to Buxales
Small evergreen trees to shrubs; 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; petiole bundles arcuate; cuticle waxes as curled rodlets or irregular platelets; (stomata laterocytic); leaves opposite (spiral), lamina ptyxis flat to curved; staminate flowers: P (0-)2-4(5), not clearly separated from bracteoles, decussate; (A 2-many); pistillode +, nectariferous; carpellate flowers: P 6-20, at least some spiral; G , completely fused, with five bundles, styluli ± marginal, (not - Styloceras), spreading, stigmatic their entire length, ± bicarinate, stigma dry or weakly secretory; ovules 2/carpel, apotropous, outer integument ca 5 cells across, inner integument 2(-3) cells across, parietal tissue 4-15+ cell layers across, (weak nucellar cap), ovular obturator +, hypostase +; fruit with persistent styles; (seed carunculate), exotesta lignified, palisade, hypodermis often lignified; endosperm +, (perisperm +), embryo short to long.
6[list]/115 - two tribes below. ± Temperate to ± tropical, although very scattered, 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 crown-group Buxaceae were 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..
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). The carpellate flowers are dimerous, but there may be five stamens opposite five tepals (Drinnan et al. 1991); relationships with Trochodendraceae have also been suggested (Doyle 1999). Lusistemon, also with striate 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).
1. Stylocerateae Pax
(Perennial herbs); (nodes 1:3 - Sarcococca); lamina margins toothed; no tanniniferous tissue in the flowers [?Didymelaceae]; staminate flowers: (A many - Styloceras), filaments conspicuous, (endothecium >biseriate); pollen pantoporate, with crotonoid exine processes, (pistilode 0); carpellate flowers: bracts many, all spiral; G 2, (loculi divided); micropyle bitegmic, outer integument 2-3 cells across; fruit a drupe (dry; ?dehiscent); 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]: G 3; outer integument ca 5 cells across.
2. 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, margins entire; inflorescences cymose, staminate flowers borne in two ranks along the branches, bracteoles 0; staminate flowers: P 0; A 2, anthers attached their length, endothecium >biseriate, filament broad; pollen tricolpate; pistillode 0; carpellate flowers: P ?4; staminodes 0; G stipitate, placentation parietal; ovules 8-15/carpel; fruit unknown; n = ?
1/1: Haptanthus hazlettii. Central America, Honduras, recently recollected.
Synonymy: Haptanthaceae C. Nelson
3. Buxeae Dumortier
(Cortical vascular bundles +); (venation dendritic [cladodromous] - ?level); staminate flowers: pollen 3-12 colporate, (colpus with 2-4 endoapertures), 12+ pantoporate; (pistillode 0); carpellate flowers: inflorescence with basal decussate "bracts"; median carpel adaxial, nectary between styles (0); 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 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 Tertiary.
"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) - species limits?
Pollination Biology & Seed Dispersal. Members of Buxeae are myrmecochorous (Lengyel et al. 2010).
Chemistry, Morphology, etc. Buxus often has fibre strands in stem. All genera (but Styloceras is unknown) have three vascular bundles in the upper part of the petiole.
The carpellate flowers have spirally inserted parts, but the basical floral construction of the family is perhaps dimerous. As von Balthazar and Endress (2002a) show, interpretation of inflorescence morphology is not easy, indeed, the nature of the paired "bracts" (?prophylls?) below the carpellate flowers of Buxus is particularly unclear. 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 Didymelaceae; 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).
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 - the former is the preferred interpretation, see Shipunov & Oskolski 2011); the endothecium is massive (Doust & Stevens 2005), as it is in Styloceras, the genus most similar to Haptanthus (Shipunov & Oskolski 2011). The staminate flowers are borne in a two-ranked arrangement along the two branches of the inflorescence, the carpellate flower being central; this inflorescence is unlike any others in angiosperms, although the first/central flowers in cymose inflorescences in Euphorbia-Euphorbiaceae, for example, may be the only carpellate flower in the inflorescence.
For additional information: ovules, see Wiger (1935), nectary types, see Vogel (1998b), for fossil pollen, Doyle (1999), for inflorescence morphology, von Balthazar and Endress (2001, 2002a), for floral morphology, von Balthazar and Endress (2001, esp. 2002b), and for a general summary, see van Tieghem (1897) and Köhler (2006).
Phylogeny. For relationships in the family, see von Balthazar et al. (2000), von Balthazar and Endress (2002b), and Shipunov and Shipunova (2011); the latter found that Haptanthus was sister to Buxeae in maximum likelihood but not maximum parsimony analyses.
Previous Relationships. In the past, Buxaceae were often associated with Euphorbiaceae s.l. (e.g. Cronquist 1981). When the very distinctive Haptanthus was first described, it was not even 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.