LIGNOPHYTA
True roots +; lateral meristems: cork cambium producing cork abaxially, vascular cambium producing phloem abaxially and xylem adaxially.
EXTANT SEED PLANTS/SPERMATOPHYTA
Plant woody, 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, thus containing p-hydroxyphenyl and guaiacyl lignin units, (lignins derived from p-coumaryl alcohol, i.e. S [syringyl] lignin units); true roots present, apex multicellular, xylem exarch, and branching endogenous; arbuscular mycorrhizae +; shoot apical meristem multicellular, interface specific plasmodesmatal network; stem with ectophloic eustele, endodermis 0, xylem endarch, branching exogenous; vascular tissue in t.s. discontinuous by interfascicular regions; vascular cambium + [xylem ("wood") differentiating internally, phloem externally]; wood homoxylous, tracheids and rays alone, tracheid/tracheid pits circular, bordered; mature sieve tube/cell lacking functioning nucleus, plastids with starch grains; phloem fibres +; stem cork cambium superficial, root cork cambium deep seated; leaves with single trace from sympodium ["nodes 1:1"]; stomata ?; leaf vascular bundles collateral; leaves megaphyllous [determinancy evolved first, then ad/abaxial symmetry], spiral, simple, lamina with vein density up to 5 mm/mm2 [mean for all non-angiosperms 1.8]; axillary buds associated with at most some leaves; prophylls [including bracteoles] two, lateral; plant heterosporous, sporangia eusporangiate, on sporophylls, sporophylls aggregated in indeterminate cones/strobili; true pollen [microspores, i.e. no distal pore for release of gametes] +, grains mono[ana]sulcate, exine and intine homogeneous; ovules unitegmic, crassinucellate, megaspore tetrad tetrahedral, only one megaspore develops, megasporangium indehiscent; male gametophyte development first endo- then exosporic, tube developing from distal end of grain, to ca 2 mm from receptive surface to egg, gametes two, developing after pollination, with cell walls, with many flagellae; female gametophyte endosporic, initially syncytial, walls then surrounding individual nuclei; seeds "large", first cell wall of zygote transverse, embryo straight, endoscopic [suspensor +], short-minute, with morphological dormancy, white, cotyledons 2; plastid transmission maternal; 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 with sieve and Strasburger cells, the 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; stomata perigenous, stomatal poles raised above pore, no outer stomatal ledges or vestibule; ± tracheidal transfusion tissue +; plants dioecious; microsporophylls and megasporophylls forming determinate strobili/cones; pollen tectate, infratectum alveolate [esp. saccate pollen], endexine lamellate at maturity; ovule unitegmic, with pollen chamber [developing by breakdown of nucellar cells], apex of nucellus massively thick; pollination droplet +, fertilisation 7 days to 4-6 months or more after pollination, pollen germinates in two or more days, tube, branched, haustorial, growing away from ovule at 1³-10(-20) µm/hour, breaks down sporophytic cells, wall of cellulose microfibrils, male gametophyte of two prothallial cells, a tube cell, and an antheridial cell producing a sterile cell and two multiflagellate gametes, zooidogamy, male gametes released by the breakdown of the pollen grain wall; female gametophyte monosporic, with radially-elongated cells [alveoli] that grow centripetally, the nucleus being on the open face and connected to adjacent nuclei by spindle fibres; seed fleshy, testa mainly of sarcotesta and sclerotesta, ± vascularized; chromosomes of male and female gametes line up on separate but parallel spindles, proembryo with many free-nuclear divisions; gametophyte persists in seed; genome size [1C value] intermediate, 3.5-14 pg; two copies of the LEAFY gene and three of the PHY gene, [PHYP [PHYN + PHYO]], second intron in the mitochondrial rps3 gene.
GINKGOALES + PINALES: wood pycnoxylic; bordered pits with margo-torus construction; phloem with scattered fibres alone [Cycadales?]; axillary buds +; sporangiophore/filament simple with terminal microsporangia.
PINALES: ovulate strobilus compound; pollen tube unbranched, growing towards the ovule, gametes non-motile, released from the distal end of the tube, siphonogamy; germination epigeal.
GNETALES Blume Main Tree, Synapomorphies.
Lignins with syringaldehyde [Mäule reaction positive, syringyl:guaiacyl ratio under 2-2.5:1]; stem apex with tunica/corpus construction; roots diarch; vessels + [from circular bordered pits], both fibre tracheids and tracheids +; vascular traces leaving stele one internode belopw exit; two primary veins in leaves (and cone bracts); resin canals 0, mucilage cells +; stomata mesogenous; leaves opposite, joined at the base, axillary buds collateral; plant dioecious; strobili compound, micro- and megasporangium-bearing structures closely associated [one not fertile], latter apical, bracts opposite; microsporangia in synangia, surrounded by a tubular "bract", dehiscing apically by the action of the epidermis [exothecium]; pollen not saccate, striate, tectate and with granular layer; ovulate cone scale 0, ovules terminal, erect, surrounded by a vascularized connate structure ["outer integument"/seed envelope], integument single, with much-elongated beak ca 2 cell layers across, not vascularized, micropylar tube with inner epidermis lignified; pollen germinates in 1-2 hours, reaches nucellus in 10-16 hours [Ephedra], both sperm nuclei fuse with female gametes; seed with outer fleshy and inner sclerenchymatous layer derived from the outer integument; tiered proembryo 0, but free nuclear stage in which each nucleus forms an embryo, secondary suspensor developing from upper embryonal tier, no primary suspensor; germination epigeal; plastid transmission maternal; plastid ndh genes and rps16 gene lost, loss of PHY0 gene, mitochondrial coxII.i3 intron 0.
Evolution. Divergence & Distribution. Molecule-based estimates of the age of crown Gnetales (as Gnetophyta) range widely from (202-)155(-104) million years old (with eudicot calibration) to (208-)158(-110) million years (without: Smith et al. 2010, see also Table S3).
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 million years before present, see Wang (2004). Both Ephedra and Welwitschia have polyplicate pollen of a kind that has a fossil record of ³250 million years, being common from the Late Triassic onwards. Dilcher et al. (2005) suggest that Gnetalean-like (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 may have diverged by 110 million years before present or more, given the welwitschioid seedling, Cratonia, that is of this vintage (Rydin et al. 2003); Ickert-Bond et al. (2010: 95% highest posterior density) suggest ages of (192.3-)166.6(-90.6) million years for this split.
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; cf. 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). Gnetales s.l., i.e., stem-group Gnetales and including these two wholly fossil groups show considerably more variation than perhaps might have been expected.
Pollination & 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.
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 (what about other seed plants growing in similar environments?); note that some Pinaceae have also lost a number of the chloroplast genes that are missing in Gnetales (Wu et al. 2009). 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).
Morphology, Anatomy, etc. There are also 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).
Note that there are substantially different interpretations of the parts of both the microsporangium- and megasporangium-bearing structures (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).
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: but the pollen morphology of Gnetales is unlikely to be directly connected with that of Nymphaea colorata), Rydin and Friis (2005: pollen germination) and Tekleva ans Krassilov (2009: pollen morphology, inc. fossils). Martens (1971) provides an extensive treatment of the whole group, Gifford and Foster (1989) a clear summary of what was then known; Crane (1996) reviews the fossil history, Friedman (1992), Carmichael and Friedman (1996) and Friedman and Carmichael (1997, and references) discuss double fertilisation, Carlquist (1997) 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 five major seed-plant clades, direct links are provided to the four others from here: Cycadales, Gnetales, flowering plants or Magnoliophyta , and Pinales; general discussion under seed plant evolution. On balance, however, the evidence is pointing to a [Pinaceae + Gnetales] clade - so Gnetales will disappear.
Within Gnetales relationships are clearly [Ephedra [Gnetum + Welwitschia]] (e.g. Price 1996). The binucleate sperm cells, basic proembryo structure, development of polyembryony, etc., of Ephedra agree with Pinales in general and perhaps Pinaceae in particular. Furthermore, strobili with both micro- and megasporangia are common as abnormalities in Pinales (Chamberlain 1935), while some Pinus species have mesogenous stomata (Gifford & Foster 1989).
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. - 3 families, 3 genera, 96 species.
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 reduced, or at least without a lamina; microsporangiophores with 2-8 synangia, each with 2(-4) sporangia, dehiscence porose; pollen inaperturate; micropyle blocked by mucilaginous secretion, nucellar cap well developed; pollen exine shed during microgametophyte germination [microgametophyte naked]; archegonia exposed at base of deep pollen chamber; seed with papillae on the inner side of the outer covering, (bracts below ovule become fleshy); n = 7.
1/65. North (warm) temperate, W. South America; drier habitats (map: see Caveney et al. 2001). [Photo - Ripe seed, Megasporangia, Habit, Microsporangia, Dwarf plant]
Evolution. Divergence & Distribution. The distinctive pollen of Ephedra has been found inside fossil seeds that are morphologically also Ephedra in deposits that date from the late Aptian to Early Albian (early Cretaceous) from Portugal, suggesting that diversification in the genus, previously thought to be recent, 32-8 million years before present, may be much older, i.e. 127-110 million years before present (Rydin et al. 2004, cf. Huang & Price 2003). Indeed, fossils of Ephedra with "modern" morphology from the early Cretaceous seem to be widespread, E. paleorhytidosperma having distinctive seeds very like those of the extant E. rhytidosperma (Yang et al. 2005). However, Ickert-Bond et al. (2010: 95% highest posterior density interval; see also Rydin & Ickert-Bond 2010; Rydin et al. 2010; Ickert-Bond & Rydin 2011) estimate that divergence with the genus occurred quite recently (73.5-)30.4(-20.55) million years ago, with Ephedra moving from the Old to the New World in the Oligocene (41.5-)29.6(-8.8) million years ago and to South America in the Miocene. Indeed, Ephedra went into severe decline at the end of the Cretaceous, extant taxa showing little genetic divergence and most relationships within the genus having little support (Rydin et al. 2010). There has been parallel evolution in micromorphological details of the seed envelope (Ickert-Bond & Rydin 2011).
Other fossils apparently assignable to Ephedraceae are known from perhaps a little earlier in 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 perhaps unlikely to be assignable to crown group Gnetales.
Pollination & Seed Dispersal. Pollination may sometimes be by wind. Because the pollen exine of Ephedra is shed on germination, the male gametophyte is naked. Fertilisation 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 be dry and form a wing, or 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).
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 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-2 compartment analysis of extant species of Ephedra, indeed, there is notably little molecular divergence within the genus (Rydin & Korall 2009).
[Gnetaceae + Welwitschiaceae]: cyclopropenoid fatty acids in seed oil, polysaccharide gums +; successive cambia +; pits lacking margo-torus construction; nodes multilacunar, three primary veins [or more] proceeding to the leaves; branched sclereids +; stomata paracytic [mesogenous]; leaves with second order venation; male gametophyte with one ephemeral prothallial cell, sterile cell absent; micropyle blocked by tissue from expanded integument; ovules with additional pair of bracts; megaspores tetrasporic, wall formation in female gametophyte enclosing groups of nuclei that later fuse, no alveoli or archegonia per se; embryo cellular, some cells of embryonal mass elongate [cleavage polyembryony can happen here], embryo with lateral "feeder" [protrusion of the hypocotylar axis].
Evolution. Divergence & Distribution. Ickert-Bond et al. (2010: 95% highest posterior density) suggest ages of (127-)111.3(-87.2) million years for divergence within this clade. Siphonospermum, 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).
GNETACEAE Blume Back to Pinales
Plant trees or lianoid, ectomycorrhizal; vessel elements with vestured pits; sieve tubes with companion cells [derived from different cells]; laticifers +; leaves with more than two orders of reticulate venation, 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 not striate, surface spinose; additional bracts connate [another "integument"]; micropylar tube closed by radially-expanding integumentary cells [obturator], nucellar cap well developed; embryo with elongated suspensor tubes initially formed, nucleus at end divides forming a embryonal mass; n = 11; one copy of the LEAFY gene.
1/30. Tropical, rather disjunct (map: see Renner 2005b). [Photos - Collection]
Evolution. Divergence & Distribution. For biogeographical relationships in the genus (post Eocene diversification and dispersal), see Renner (2005b) and Won and Renner (2006).
Pollination & Seed Dispersal. Entomophily has been reported from Malesian species of Gnetum (Kato & Inoue 1994), and the genus is ectomycorrhizal (Brundrett 2009).
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 million years (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 outer two coverings of the ovule, but vascular bundles barely enter the base of the inner integument.
For reproductive morphology and development, see Sanwal (1962), and for general information, see the Gymnosperm Database.
Synonymy: Thoaceae Kuntze
WELWITSCHIACEAE Caruel Back to Pinales
Stem apex lacking tunica-corpus construction?; fibre tracheids 0; [successive cambia are in root - derived from phelloderm]; leaf traces in cortex?; sclereids with crystals in wall; leaves amphistomatic; three pairs of leaves only, the second pair persisting for the life of the plant and elongating from the base, venation parallel; ovules and microsporangiophores in intimate association, microsporangiophores 6, basally connate, with synangia of three sporangia, dehiscence radial; additional bracts free; pollen exine not shed during microgametophyte germination; megagametophyte with multinucleate cells, some grow upwards through nucellus forming female gametophytic tubes; outer integument forming membranous wing; fertilisation in apical bulge [both gametes involved?], proembryo pushed back down tube by elongating embryonal suspensor; n = 24; 2nd intron in nad1 lost.
1/1: Welwitschia mirabilis. S.W. Africa, desert close to the 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 million years before present, and other fossils of welwitschiaceous affinity have been found in the same area (Dilcher et al. 2005).
Ecology & Physiology. Welwitschia mirabilis grows in the Namib desert close to the ocean; although there is little rain, fogs are frequent. Plants may be some hundreds of years old, the two persistent leaves growing at the base and fraying at the apex.
Pollination & 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 an inverted repeat (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). Martens (1971) describes the vascularisation of the bracts of the megasporangia and the complex organisation of the axis of the megaporangiate strobilus.
For general information, see the Gymnosperm Database.
Synonymy: Tumboaceae Wettstein
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