EMBRYOPSIDA Pirani & Prado (crown group)
Gametophyte dominant, independent, multicellular, thalloid, with single-celled apical meristem, showing gravitropism; flavonoids + [absorbtion of UV radiation]; protoplasm dessication tolerant [plant poikilohydric]; cuticle +; cell walls with (1->4)-ß-D-glucans [xyloglucans], lignin +; rhizoids unicellular; 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; diploid 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, sporangium +, single, with polar transport of auxin, 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]; spores trilete; close association between the trnLUAA and trnFGAA genes on the chloroplast genome.
Note that many of the bolded characters in the characterization above are apomorphies in the streptophyte clade along the lineage leading to the embryophytes rather than being apomorphies of the crown embryophytes.
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; 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.
[Hornworts + Polysporangiophyta]: archegonia embedded/sunken in the gametophyte; sporophyte long-lived, chlorophyllous, nutritionally largely independent of the gametophyte; sporophyte-gametophyte junction interdigitate, sporophyte cells showing rhizoid-like behaviour.
Sporophyte well developed, branched, free living, sporangia several; spore walls not multilamellate [?here]; apical meristem +.
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 +); 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, sporangia derived from periclinal divisions of several epidermal cells, wall multilayered [eusporangium]; columella 0; tapetum glandular; 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 ± monopodial; 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 with xylem and phloem originating on alternate radii, vascular tissue not medullated, 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; branches exogenous; 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; axillary buds +, (not associated with all leaves); 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; male gametophyte development first endo- then exosporic, 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.
EXTANT GYMNOSPERMS / PINOPHYTA / ACROGYMNOSPERMAE
Biflavonoids +; cuticle wax tubules with nonacosan-10-ol; ferulic acid ester-linked to primary unlignified cell walls; phloem sieve area with small pores generally less than 0.8 µm across that have cytoplasm and E.R., joining to form a median cavity in the region of the middle lamella, Strasburger/albuminous cells associated with sieve tubes, the two not derived from the same immediate mother cell, phloem fibres +, scattered; stomatal poles raised above pore, no outer stomatal ledges or vestibule, epidermis lignified; sclereids +, ± tracheidal transfusion tissue +; buds perulate/with cataphylls; lamina development marginal; plants dioecious; microsporangium with exothecium; pollen tectate, infratectum alveolate [esp. saccate pollen], endexine lamellate at maturity; ovule unitegmic, with pollen chamber formed by breakdown of nucellar cells, nucellus massive; ovules increasing considerably in size between pollination and fertilization, but aborting unless pollination occurs; pollination droplet +; pollen esp. intine with callose, germinates in two or more days, tube with wall of pectose + cellulose microfibrils, branched, growing away/towards ovule at up to 10(-20) µm/hour, haustorial, breaks down sporophytic cells; male gametophyte of two prothallial cells, a tube cell, and an antheridial cell producing a sterile cell and 2 gametes; fertilization 7 days to 12 months or more after pollination, gametes released by breakdown of pollen grain wall, with >1000 cilia; to ca 2 mm from receptive surface to egg, female gametophyte with radially-elongated cells [alveoli] that grow centripetally, the nucleus of the female gamete being on the open face and connected to adjacent nuclei by spindle fibres; seeds fleshy, "large" [ca 8 mm3], but not much bigger than ovule, with morphological dormancy; testa mainly of coloured sarcoexotesta and scleromesotesta, ± vascularized, and ± degenerating endotesta, ± vascularized; first zygotic nuclear division with chromosomes of male and female gametes lining up on separate but parallel spindles, embryogenesis initially nuclear; gametophyte persists in seed; 2C genome size 8-32(-76) pg [1 pg = 109 base pairs]; two copies of LEAFY gene [LEAFY, NEEDLY] and three of the PHY gene, [PHYP [PHYN + PHYO]], second intron in the mitochondrial rps3 gene [group II, rps3i2].
[GINKGOALES + PINALES]: tree branched; wood pycnoxylic; tracheid side wall pits with torus:margo construction, bordered; phloem with scattered fibres alone [Cycadales?]; axillary buds at at least some of the nodes; microsporangiophore/filament simple with terminal microsporangia; microsporangia abaxial, dehiscing by the action of the hypodermis [endothecium].
Resin ducts/cells in phloem in vascular tissue [and elsewhere]; lignins lacking syringaldehyde [Mäule reaction negative]; cork cambium ± deep seated; bordered pits on tracheids round, opposite; compression wood +; nodes 1:1; leaves with single vein; plants monoecious; pollen exine thick [³2 µm thick]; ovulate strobilus compound, ovuliferous scales flattened, ± united with bract scales; ovules lacking pollen chamber; pollen tube unbranched, growing towards the ovule, wall with arabinogalactan proteins, gametes non-motile, lacking walls, released from the distal end of the tube, siphonogamy; seed coat dry, not vascularized; proembryo with 2 to 4 nuclear divisions, with upper tier or tiers of cells from which secondary suspensor develops, elongated primary suspensor cells and basal embryonal cells [or some variant]; germination phanerocotylar, epigeal, (seedlings green in the dark); plastid and mitochondrial transmission paternal, one duplication in the PHYP gene line, one copy of chloroplast inverted repeat missing. - 7 families, 68 genera, 545 species.
GNETALES Blume Main Tree.
<S [syringyl] lignin units common [positive Maüle reaction]; stem apex with tunica/corpus construction; roots diarch; bark with sclereids; gelatinous fibres [g-fibres] with innermost layer of secondary cell wall rich in cellulose and poor in lignin; protoxylem tracheids with large circular bordered pits, vessels + [from circular bordered pits], also in metaxylem, both fibre tracheids and tracheids +; phloem with fibre sclereids; nodes 1:2, vascular traces leaving stele one internode below exit; two primary veins in leaves (and cone bracts); resin canals 0, mucilage cells +; stomata paracytic [mesogenous]; leaves opposite, joined at the base, overall growth ± diffuse/marginal/from basal plate, axillary buds serial, collateral; strobili compound [micro- and megasporangium-bearing structures closely associated, one not fertile], megasporangia apical, bracts opposite; microsporangia in synangia, surrounded by a tubular "bract", dehiscing apically by the action of the epidermis [exothecium]; pollen not saccate, surface striate, tectate and with granular layer; ovulate cone scale 0, ovules terminal, surrounded by a vascularized connate structure ["outer integument"/seed envelope], papillae on the inner surface around the micropyle; ovule erect, integument single, with much-elongated beak, ca 2 cell layers across, not vascularized, micropylar tube with inner epidermis lignified, nucellar cap well developed; pollen reaches nucellus in less than 7 days, both sperm nuclei fuse with gametophytic cells ["double fertilization"]; first zygotic nuclear division with one spindle, tiered proembryo 0, free nuclear stage in which each nucleus forms an embryo, secondary suspensor developing from upper embryonal tier, no primary suspensor; germination epigeal, (seedlings green in the dark), cotyledons with connate bases; plastid transmission maternal; plastid ndh genes and rps16 gene lost, loss of PHY0 gene, mitochondrial coxII.i3 intron 0. - 3 families, 3 genera, 96 species.
Age. > Estimates of the age of crown-group Gnetales are (202-)155(-104) m.y. with eudicot calibration (Smith et al. 2010: see also Table S3), slightly older without. Magallón et al. (2013) suggested an age of 140-120 m.y., Won and Renner (2006) ages of (196-)159(-132) m.y., and Ickert-Bond et al. (2010: 95% h.p.d.) ages of (192.3-)166.6(-90.6) m.y..
Crane (1996) summarized the fossil history of Gnetales (see Won & Renner 2006; Rydin & Friis 2010 for additional references). For a probable Gnetalean fossil from the Permian, some 250-270 m.y.a., see Wang (2004). Both Ephedra and Welwitschia have polyplicate pollen that has a fossil record of ³250 m.y., being common from the Late Triassic onwards. Dilcher et al. (2005) suggest that Gnetalean-like (striate/ribbed) pollen was common in both N. and S. Hemispheres; in the former, records are from the Upper Triassic onwards, in the latter, especially in the early Cretaceous from the northern half of South America. The pollen found by Wang (2004) associated with his fossil, Palaeognetaleana auspicia, is of this general kind. That fossil was radiospermic and had two complete integuments, a possible third integument being represented by scales, and the arrangement of parts in the cone was spiral. Ephedra and Welwitschia had diverged by 110 m.y. ago or more, the welwitschioid seedling, Cratonia, being of this vintage (Rydin et al. 2003).
Detailed studies of small Early Cretaceous seeds suggests that Erdtmanithecales and Bennettitales have seeds very similar to those of Gnetum and Welwitschia in particular, the latter order agreeing in details of micropylar closure, and all have paracytic stomata (Friis et al. 2007, 2009; Mendes et al. 2008; c.f. Rothwell et al. 2009). A further link with Ephedra is in the granular infratectum of the pollen that all share (Friis et al. 2007), although the pollen of Eucommiidites (Erdtmanithecales) is psilate and has two equatorial colpi as well (Pedersen et al. 1989).
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...
Evolution. Divergence & Distribution. Gnetales s.l., i.e., stem-group Gnetales and including the fossil groups above, show considerably more variation than perhaps might have been expected given the small size of the clade. Several genera of Gnetalean affinity have recent been described from the Brazilian Crato formation, some 115-112 m.y.o. (Löwe et al. 2013 and references). Polyplicate (ephedroid) pollen was notably common 125-85 m.y.a. in middle latitudes (northern Gondwana), angiosperms and Gnetales perhaps growing togther then (Crane & Lidgard 1980).
Pollination Biology & Seed Dispersal. Ovules of all three extant genera are visited by diptera (see Kato & Inoue 1994 and Labandeira 2005 for references); sweetish droplets exude from the micropyle. For details of the time from pollination to fertilization, short for a gymnosperm, see Williams (2008 and references).
Genes & Genomes. The nuclear genome is small, C values being 1.4-3.5 picograms (Leitch et al. 2001, 2005). All three genera also have very small chloroplast genomes, Welwitschia rather less so than the others, and it has been suggested that this is because they grow in resource-poor environments, but genome size in Pinus, for example, may not be much bigger (see also C.-S. Wu et al. 2007, 2009 and references: other seed plants growing in similar environments?). Up to 18 genes have been lost from the chloroplast (McCoy et al. 2008; C.-S. Wu et al. 2009; Jansen & Ruhlman 2012 and references).
Morphology, Anatomy, etc. Although vessels in Gnetum, for example, are commonly described as being derived from circular pits, this has been questioned (e.g. Rodin 1969; Muhammad & Sattler 1982). Rodin (1969) suggested that Gnetales lack pits with a margo-torus construction, but they are clearly shown for Ephedra, but not Gnetum, by Eicke (1957). For gelatinous fibres (g-fibres), see Montes et al. (2012); in Ephedra, at least, their presence had nothing to do with bending and they are not associated with wood tissues, so they are not reaction wood (c.f. angiosperms). There are nodal girdles of tissue very like transfusion tissue, at least in Ephedra (Beck et al. 1982). For the numbers of veins entering the leaves, see Rydin and Friis (2010). Boyce and Knoll (2002), Nardmann and Werr (2013), and others discuss leaf development; the scale leaves of Ephedra are reductions/
Interpretations of the parts of both the microsporangium- and megasporangium-bearing structures differ substantially (e.g. Gifford & Foster 1989; Hufford 1997a; Mundry & Stützel 2004). In microsporangiate plants of all three extant genera both stamens and non-functional ovules (although pollination droplets may still be produced) are closely associated, although this perhaps least marked in Ephedra (see also Flores-Rentería et al. 2011), and the microsporangiate cones can be interpreted as being compound (Mundry & Stützel 2004), rather like the megasporangiate cones of Pinales. The plants themselves are functionally dioecious. Gnetum ula is reported as having two sperm cells (Singh 1978). Plastid transmission appears to be maternal, at least in Ephedra distachya (Moussel 1978). The megaspore membrane is thin, but is definitely present (Doyle 2006). Variation in the nad1 intron 2 needs clarification; it is absent in Welwitschia, present in Gnetum, and what is going on in Ephedra is not entirely clear (Gugerli et al. 2001).
For the morphology of Gnetales in the context of that of fossil gymnosperms, see e.g. Doyle and Donoghue (1986a, b) and especially Doyle (2006, 2008b, and references), for mycorrhizae, see Jacobson et al. (1993), and for pollen, see Osborn (2000: comparison with gymnospermous "anthophytes"), Yao et al. (2004: pollen of Gnetales compared with that of Nymphaea colorata), Rydin and Friis (2005: pollen germination) and Tekleva and Krassilov (2009: pollen morphology, inc. fossils). Martens (1971) provides an extensive treatment of the whole group (see also Gifford & Foster 1989), Friedman (1992), Carmichael and Friedman (1996) and Friedman and Carmichael (1997, and references) discuss double fertilization, Carlquist (1997, 2012b) describes wood anatomy, Takaso (1985 and references) integument morphology, Endress (1997) details of megasporangiate structures, and Hufford (1997a) microsporangium arrangement.
Phylogeny. Given the uncertainty in our knowledge of the relationships between the major seed-plant clades, direct links are provided to the others from here: Cycadales, Ginkgoales, flowering plants, and Pinales; there is general discussion under seed plant evolution. If the evidence continues to point to a [Pinaceae + Gnetales] clade, Gnetales will disappear.
Within Gnetales relationships are [Ephedra [Gnetum + Welwitschia]] (e.g. Price 1996).
For additional details of the relationships of Gnetales - in the late 1980s thought to be immediately related to angiosperms alone among extant organisms - see Cycadales page.
Includes - Ephedraceae, Gnetaceae, Welwitschiaceae
Synonymy: Ephedrales Dumortier, Tumboales Wttstein, Welwitschiales Reveal - Ephedridae Reveal, Gnetidae Pax, Welwitschiidae Reveal - Ephedropsida Reveal, Gnetopsida Thomé, Welwitschiopsida B. Boivin - Gnetophytina Reveal - Gnetophyta Bessey
EPHEDRACEAE Dumortier Back to Pinales
Xeromorphic small trees and shrubs (climbers); cyclopropyl amino acids +; nodes 1:2; leaves much reduced (not); microsporangiophores with 2-8 synangia, each with 2(-4) sporangia, dehiscence porose; pollen inaperturate; micropyle blocked by mucilaginous secretion; pollen exine shed during microgametophyte germination [microgametophyte naked]; pollen germinates in 1-2 hours, reaches nucellus in 10-16 hours; archegonia exposed at base of deep pollen chamber; one gamete fuses with nucleus of ventral canal cell; (bracts below ovule become fleshy); seed with papillae on the inner side of the outer covering; n = 7, polyploids frequent; 2C genome size 16.2-76 pg; loss of two more group II mitochondrial introns.
1/65. North (warm) temperate, W. South America; drier habitats (map: see Frankenberg & Klaus 1980; Caveney et al. 2001). [Photos - Ripe seed, Megasporangia, Habit, Microsporangia, Dwarf plant.]
Age. Ickert-Bond et al. (2009; see also Rydin & Ickert-Bond 2010; Rydin et al. 2010) estimate that divergence with the genus occurred quite recently, only (73.5-)30.4(-20.55) m.y.a. (see also Huang & Price 2003: 32-8 m.y.), c.f. the fossil record below.
Fossils apparently assignable to Ephedraceae are known from the lower Cretaceous in China (Zhou et al. 2003) and seeds clearly of Ephedraceae are similar to the fossil Erdmanithecales (Rydin et al. 2006: see seed plant evolution). Rothwell and Stockey (2009) report a fossil from the Lower Cretaceous that has purportedly ancestral characters for Ephedra - two ovules together, and absence of a tubular micropyle and of a structure surrounding the ovule (seed envelope above), but this is unlikely to be assignable to crown group Gnetales. The distinctive pollen of Ephedra has been found inside fossil seeds that are morphologically also Ephedra in late Aptian to Early Albian (early Cretaceous) deposits from Portugal, suggesting that diversification in the genus occurred some 127-110 m.y.a. (Rydin et al. 2004). Indeed, Early Cretaceous fossils of Ephedra have a "modern" morphology, E. paleorhytidosperma having distinctive seeds very like those of the extant E. rhytidosperma (Yang et al. 2005).
Evolution. Divergence & Distribution. Ephedra went into severe decline at the end of the Cretaceous, and extant taxa show little genetic divergence and most relationships have little support (Rydin et al. 2010). It moved from the Old to the New World in the Oligocene (41.5-)29.6(-8.8) m.y.a. and to South America in the Miocene (Ickert-Bond et al. 2009). A shift from entomophily to anemophily may perhaps be connected with Tertiary duversification in the clade (Bolinder et al. 2012). There has been parallel evolution in micromorphological details of the seed envelope (Ickert-Bond & Rydin 2011).Pollination Biology & Seed Dispersal. Pollination may sometimes be by wind. Because the pollen exine of Ephedra is shed on germination, the male gametophyte is naked. Fertilization occurs 10-15 hours after pollination.
As the seeds ripen, the "outer integument" surrounding the ovule may become fleshy and brightly coloured, or it may dry and become a wing, or it may be faintly nondescript, the seeds then being dispersed by scatter-hoarding rodents (Hollander & Vander Wall 2009).
Genes & Genomes. There has been a great increase in the rate of synonymous substitutions in the mitochondrial genome and chloroplast and nuclear sequences are also divergent compared with those of other seed plants (Mower et al. 2007 and references). The nuclear genome is very variable in size, and can be huge (Ickert-Bond et al. 2014).
Chemistry, Morphology, etc. Species of Ephedra are pharmacologically very active and contain a number of distinctive secondary metabolites (Caveney et al. 2001). Biswas and Johri (1997) mention the "deep origin of the periderm", a position that should be confirmed. For leaf and nodal anatomy of species of Ephedra with well developed leaves, i.e. long and linear, see Dörken (2014) and Deshpande and Keswani (1963).
For some general information, see Rydin et al. (2004) and the Gymnosperm Database, and for nodal anatomy, see Marsden and Steeves (1955) and Singh and Maheshwari (1962).
Phylogeny. There is little strong phylogenetic structure along the backbone of a 7 plastid gene-2 compartment analysis of extant species of Ephedra, indeed, there is notably little molecular divergence within the genus (Rydin & Korall 2009; Rydin et al. 2010: see also above). Ephedra foeminea may be sister to the rest of the genus.
Clasification. For a classification of Ephedraceae, including fossil members, see Yang (2014).
[Gnetaceae + Welwitschiaceae]: cyclopropenoid fatty acids in seed oil, polysaccharide gums +; (successive cambia in roots); pits lacking margo-torus construction; nodes multilacunar, three [or more] primary veins proceeding to the leaves; branched sclereids +; cataphylls 0; leaves with second order venation; microsporangiate strobili with abortive apical ovules; male gametophyte with one ephemeral prothallial cell, sterile cell absent; micropyle blocked by tissue from expanded integument [by periclinal cell divisions, also radial cell expansion]; archegonia not obvious; ovules with additional pair of bracts; megaspores tetrasporic, wall formation in female gametophyte enclosing groups of nuclei that later fuse, no alveoli [?level], no archegonia per se; both male gametes fuse with egg nuclei; embryo cellular, some cells of embryonal mass elongate, (cleavage polyembryony +), embryo with lateral protrusion of the hypocotylar axis ["feeder"]; germination hypogeal.
Age. Ickert-Bond et al. (2010: 95% highest posterior density) suggest ages of (127-)111.3(-87.2) m.y. for divergence within this clade, Won and Renner (2006) ages of (175-)138(-112) m.y., and Magallón et al. (2013) an age of around 81.9 m.y., but c.f. below for fossils placed in Welwitschiaceae.
Siphonospermum, a fossil from the Lower Cretaceous from Northeast China, may be assignable to this part of the tree (Rydin & Friis 2010).
Chemistry, Morphology, etc. For cyclopropenoid acids, similar to those in Malvales, see Aitzetmüller and Vosmann (1998). Rodin (1968) suggested that the reticulate venation of Gnetum, at least, was a modified dichotomizing system.
GNETACEAE Blume Back to Pinales
Plant trees or lianoid, ectomycorrhizae +; (successive cambia in shoots - climbers); vessel elements with vestured pits; sieve tubes with companion cells [derived from different cells]; laticifers +; leaves petiolate, lamina hierarchical-reticulate venation [more than two orders], development dispersed, veins (4.4-)5.7(-7.4) mm/mm2; (plant monoecious), ovules and microsporangiophores at same node in staminate plant; microsporangiophore with (1-)2(-4) sporangia; pollen surface spinose; additional ovule envelope formed by connate bracts ["integument"]; pollen reaches nucellus in up to 7 days; outer ovule envelope becomes fleshy; embryo with elongated suspensor tubes initially formed, nucleus at end divides forming a embryonal mass; n = 11; 2C genome size 4.5-8 pg, one copy of the LEAFY gene.
1/30. Tropical, rather disjunct (map: see Renner 2005b). [Photos - Collection]
Age. Crown-group Gnetaceae are (77-)44-26(-13) m.y.o., or using a strict clock, as little as 6 m.y. (Won and Renner 2006).
Evolution. Divergence & Distribution. For biogeographical relationships in the genus, basically a story of post-Eocene diversification and dispersal, see Renner (2005b) and Won and Renner (2006).
Pollination Biology & Seed Dispersal. Entomophily has been reported from Malesian species of Gnetum (Kato & Inoue 1994).
Bacterial/Fungal Associations. Brundrett (2008, seen viii.2012) summarizes information on the mycorrhizal status of members of Gnetales as a whole.
Genes & Genomes. Horizontal gene transfer of the mitochondrial nad1 intron 2 from flowering plants (an asterid) to an Asian clade of Gnetum seems to have occurred within the last 5 m.y. (Won & Renner 2003).
Chemistry, Morphology, etc. Not surprisingly, the wood of the lianoid taxa is distinctive, with serial cambia being formed. The reaction wood in Gnetum consists of gelatinous extra-xylary (reaction) fibres in the adaxial position (Tomlinson 2001b, 2003; see also Höster & Liese 1966); it is not typical tension wood. See Martens (1971) for the vascularization of the leaves; pairs of vascular bundles leave the central stele in close proximity.
There is vascular tissue in the two outer coverings of the ovule, but vascular bundles barely enter the base of the inner integument. The outer covering in definitely bilobed early in development, the lobes alternating with bracts, but the middle covering is only weakly bilobed (Takaso & Bouman 1986). Although some gametophyte development occurs after fertilization, the ovule increases appreciably in size between pollination and fertilization (Leslie & Boyce 2012).
For reproductive morphology and development, see Sanwal (1962), for mycorrhizae, see Ongene and and for general information, see the Gymnosperm Database.
Phylogeny. Relationships within the genus are strongly correlated with geography, i.e. [South America [Africa + S.E. Asia-Malesia]] (Won & Renner 2006).
Synonymy: Thoaceae Kuntze
WELWITSCHIACEAE Caruel Back to Pinales
Stem apex lacking tunica-corpus construction?; ?bark; fibre tracheids 0; successive cambia in root, derived from phelloderm; no typical vascular cambium in stem; leaf traces in cortex?; sclereids with crystals in wall; leaves amphistomatic; stem apex aborts, plant three pairs of leaves, second pair of leaves persisting for the life of the plant, leaf development from a basal cambium, venation parallel; ovules and microsporangiophores in intimate association; microsporangiophores 6, basally connate, with synangia of three sporangia, dehiscence radial; additional bracts free; nucellar cap?; megagametophyte with multinucleate cells, some grow upwards through nucellus forming female gametophytic tubes; only one sperm nucleus functional, fertilization in gametophytic tube; seed with seed envelope forming membranous wing; proembryo pushed back down gametophytic tube by elongating embryonal suspensor; seedling with collar; n = 24; 2C genome size ca 13.1 pg, 2nd intron in nad1 lost.
1/1: Welwitschia mirabilis. S.W. Africa, desert close to the Atlantic ocean. [Photos - Collection.]
Evolution. Divergence & Distribution. Cratonia cotyledon is a fossil seedling with distinctive cotyledon vasculature very like that of the leaves of Welwitschia, the secondary veins leaving from the primary veins fuse to form an inverted "Y" (Rydin et al. 2003). Cratonia was found in N.E. Brazil and is late Aptian or early Albian in age, perhaps 114-112 m.y. old; other fossils of welwitschiaceous or more generally gnetalean affinity have been found in the same area (Dilcher et al. 2005; Löwe et al. 2013).
Ecology & Physiology. Welwitschia mirabilis grows in the Namib desert close to the ocean; although there is little rain, fogs are frequent - but not where Welwitschia grows (von Willert 1985). Plants may be some hundreds of years old, the two persistent leaves growing at the base and fraying at the apex.
Pollination Biology & Seed Dispersal. Pollination appears to be by diptera (Wetschnig & Depisch 1999).
Genes & Genomes. The chloroplast genome of Welwitschia mirabilis is the smallest plastid genome of all non-parasitic land plants that still have inverted repeats (McCoy et al. 2008).
Chemistry, Morphology, etc. Because of the abundant, branched sclereids in the plant, "One might as well try to cut sections of a thick Scotch plaid blanket as to try and cut a stem of Welwitschia without imbedding." (Chamberlain 1935: pp. 388-389).
Kaplan (1997, vol. 1:6) described the seedling as having a haustorial collet (collar). Serial axillary buds are added throughout the course of the long life of the plant, the youngest buds being in the centre of the axil - a branch-like organization? See Martens (1971) for the vascularisation of the bracts of the megasporangia and the complex organisation of the axis of the megasporangiate strobilus.
For fertilization, see Friedman (2014), for general information, see the Gymnosperm Database.
Synonymy: Tumboaceae Wettstein