EXTANT SEED PLANTS
Plant woody, evergreen; nicotinic acid metabolised to trigonelline; primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignins rich in guaiacyl units; true roots present, xylem exarch; shoot apical meristem complex; arbuscular mycorrhizae +; stem with ectophloic eustele, endodermis 0, xylem endarch; vascular tissue in t.s. discontinuous by interfascicular regions; vascular cambium + [xylem ("wood") differentiating internally, phloem externally]; wood homoxylous, tracheids +; tracheid/tracheid pits circular, bordered; sieve tube/cell plastids with starch grains; phloem fibers +; stem cork cambium superficial, root cork cambium deep seated; nodes ?; leaf vascular bundles collateral; leaves spiral, simple, axillary buds?, prophylls [including bracteoles] two, lateral; plant heterosporous, sporangia eusporangiate, on sporophylls, sporophylls aggregated in indeterminate cones/strobili; true pollen [microspores] +, mono[ana]sulcate, pollen exine and intine homogeneous, ovules unitegmic, crassinucellate, megaspore tetrad tetrahedral, only one megaspore develops, megasporangium indehiscent; male gametophyte development endo/exosporic, gametes two, with cell walls; 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 duplication, mitochondrial nad1 intron 2 and coxIIi3 intron present.
EXTANT GYMNOSPERMS/PINOPHYTA
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 pores joining to form median cavity in the region of the middle lamella; stomata haplocheilic; transfusion tissue +; microsporophylls and megasporophylls forming determinate strobili/cones; pollen tecate, infratectum alveolate [esp. saccate pollen], endexine lamellate at maturity; ovule unitegmic, with pollen chamber [developing by breakdown of nucellar cells]; pollination droplet +, fertilisation 4-6 months or more after pollination, pollen tube breaks down sporophytic cells and grows away from ovule, male gametophyte of two prothallial cells, tube cell, stalk/sterile cell, and two multiflagellate gametes, zooidogamy, male gametes released from the swollen proximal part of the tube; female gametophyte monosporic, with radially-elongated cells [alveoli]; testa mainly of sarcotesta and sclerotesta, ± vascularised; 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.
GINKGOALES + PINALES + GNETALES: wood pycnoxylic; bordered pits with margo-torus construction; phloem with scattered fibres alone [Cycadales?]; axillary buds +.
PINALES + GNETALES: 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 Luersson 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 fiber tracheids and tracheids +; mucilage cells +; stomata mesogenous; leaves opposite, joined at the base, with collateral buds; plant dioecious, strobili compound, micro- and megasporangium-bearing structures closely associated [one not fertile], bracts opposite; microsporangia in synangia, surrounded by a tubular "bract", dehiscing apically by the action of the epidermis [exothecium], pollen striate, with granular layer under the tectum [the infratectum, together making up the ektexine]; ovules terminal, erect, integument with much-elongated beak, not vascularised, surrounded by vascularised connate structure ["outer integument"]; sperm cells binucleate, both nuclei fuse with female gametes; seed with outer fleshy and inner sclerenchymatous layer derived from the outer integument; each nucleus of free-nuclear stage forms an embryo, secondary suspensor developing from upper embryonal tier, no primary suspensor; germination epigeal; plastid transmission maternal; loss of a PHY gene [i.e. only two copies overall], mitochondrial coxII.i3 intron 0.
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, Ginkgoales, flowering plants or Magnoliophyta , and Pinales; general discussion under seed plant evolution.
Crane (1996) summarised the fossil history of Gnetales. 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 >250 my, 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). Ovules of all three extant genera are visited by diptera (see Labandeira 2005 for references). Detailed studies of small Early Cretaceous seeds suggests that Erdmanithecales 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). 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 (Erdmanithecales) 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.
There are also nodal girdles of tissue very like transfusion tissue, at least in Ephedra (Beck et al. 1982). 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). A close association of the two occurs at least sometimes in micosporangiate plants in all three genera, although perhaps least in Ephedra; 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). The genome is small, C values being 1.4-3.5 picograms (Leitch et al. 2005). 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).
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 relationships of Gnetales - in the late 1980s thought to be immediately related to angiosperms alone among extant organisms - see Cycadales page.
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, and references), for double ferilisation, see Friedman (1992), for pollen, see Osborn (2000: comparison with gymnospermous "anthophytes"), Yao et al. (2004: the pollen morphology of Gnetales is unlikely to be directly connected with that of Nymphaea colorata) and Rydin and Friis (2005), and for literature on pollination, see Kato and Inoue (1994). 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, Carmichael and Friedman (1996) and Friedman and Carmichael (1997, and references) discuss double fertilisation, Carlquist (1997) describes wood anatomy, Endress (1997) details of megasporangiate structures, and Hufford (1997a) microsporangium arrangement.
Includes - Ephedraceae, Gnetaceae, Welwitschiaceae. - 3 families, 3 genera, 96 species.
Synonymy: Ephedrales Dumortier, Welwitschiales Reveal - Ephedridae Reveal, Gnetidae Reveal, Welwitschiidae Reveal - Ephedropsida Reveal, Gnetopsida Thomé, Welwitschiopsida B. Boivin - Gnetophytina Reveal - Gnetophyta Bessey
EPHEDRACEAE Dumortier Back to Pinales
Xeromorphic small trees and shrubs (climbers); 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, exine shed on germination [microgametophyte naked]; micropyle blocked by mucilaginous secretion; 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: approximate only, see Gifford & Foster 1988). [Photo - Ripe seed, Megasporangia, Habit, Microsporangia, Dwarf plant]
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, may be much older, i.e. 127-110 million years before present, vs 32-8 million years before present (Rydin et al. 2004, cf. Huang & Price 2003). Indeed, fossils of Ephedra with "modern" morphology from about this time seem to be widespread, and the early Cretaceous E. paleorhytidosperma has distinctive seeds very like those of the extant E. rhytidosperma (Yang et al. 2005). 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 above).
Because the pollen exine of Ephedra is shed on germination, the male gametophyte is naked. Fertilisation occurs 10-15 hours after pollination.
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, for nodal anatomy, see Marsden and Steeves (1955).
Gnetaceae + Welwitschiaceae: pits lacking margo-torus construction; nodes multilacunar; branched sclereids +; stomata paracytic; male gametophyte with one prothallial cell but no sterile/stalk cell; micropyle blocked by tissue from expanded integument; megaspores tetrasporic, wall formation in female gametophyte enclosing groups of nuclei, no archegonia per se; embryo cellular, some cells of embryonal mass elongate [cleavage polyembryony can happen here], with lateral "feeder" [protrusion of the hypocotylar axis].
GNETACEAE Lindley Back to Pinales
Plant trees or lianoid, ectomycorrhizal; vessels with vestured pits; sieve tubes with companion cells [derived from different cells]; laticifers +; leaves with more than two orders of reticulate venation; (plant monoecious), ovules and microsporangiophores at same node in staminate plant; microsporangiophore with (1-)2(-4) sporangia, pollen not striate, surface spinose, ovule surrounded by additional connate "integument"; 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 Gifford & Foster 1988). [Photos - Collection]
Entomophily has been reported from Malesian species of Gnetum (Kato & Inoue 1994). For biogeographical relationships in the genus (post Eocene diversification, dispersal), see Won and Renner (2006). Horizontal gene transfer of the mitochondrial nad1 intron 2 from flowering plants (an asterid) to an Asian clade of Gnetum seems to have occured within the last 5 million years (Won & Renner 2003).
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) fibers 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?; fiber tracheids 0; successive cambia + [in root - derived from phelloderm]; sclereids 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; ovule also with pair of free bracts; megagametophyte with multinucleate cells, some grow upwards through nucellus forming female gametophytic tubes; outer integument forming membranouis 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.]
Welwitschia mirabilis grows in the Namib desert close to the ocean; plants may be some hundreds of years old, the two persistent leaves growing at the base and fraying at the apex. Pollination appears to be by diptera (Wetschnig & Depisch 1999).
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).
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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