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, branching endogenous; arbuscular mycorrhizae +; shoot apical meristem complex; 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 +; tracheid/tracheid pits circular, bordered; sieve tube/cell plastids with starch grains; phloem fibers +; stem cork cambium superficial, root cork cambium deep seated; nodes ?; stomata ?; leaf vascular bundles collateral; leaves spiral, simple, axillary buds?, prophylls [including bracteoles] two, lateral, veins -5(-8) mm/mm²; plant heterosporous, sporangia eusporangiate, on sporophylls, sporophylls aggregated in indeterminate cones/strobili; true pollen [microspores] +, 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, 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 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 perigenous; 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 7 days to 4-6 months or more after pollination, pollen germinates in two or more days, tube grows at 1³-10(-20) µm/hour, breaks down sporophytic cells and grows away from ovule, wall of cellulose microfibrils, 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] that grow centripetally, the nucleus being on the open face and connected to adjacent nuclei by spindle fibers; 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]: wood pycnoxylic; bordered pits with margo-torus construction; phloem with scattered fibres alone [Cycadales?]; axillary buds +.

GINKGOALES Gorozh.  Main Tree, Synapomorphies.

VAM present; biflavones, non-hydrolysable tannins +, lignins lacking syringaldehyde [Mäule reaction negative]; compression wood +; nodes 1:2, venation dichotomising, open; plant dioecious; 2 pendulous microsporangia/microsporophyll, dehiscing by the action of the hypodermis [endothecium], exine thin [2³ µm thick]; 2(-4) terminal erect ovules/podium, with basal collar; spermatogenous cells delimited by circular anticlinal wall, pollen tube branched, penetrating between sporophytic cells, growth non-destructive; seed with inner fleshy layer alone vascularised; germination cryptocotylar; one duplication in the PHYO clade. - 1 family, 1 genus, 1 species [all rather redundant].

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.

The leaf is innervated by two leaf traces that originate from independent sympodia; there are a very few anastomoses between the veins in the blade.

Includes: Ginkgoaceae. - 1 family/1 genus/1 species [rather redundant].

Synonymy: Ginkgooidae Reveal - Ginkgoopsida Engler - Ginkgoophytina Reveal - Ginkgoophyta Reveal

GINKGOACEAE Engler Back to Ginkgoales

Plant resinous, mucilage +; cork cambium subhypodermal; short shoots + [bearing sporophores/strobili], wood there manoxylic; sclereids +; leaves deciduous; (cotyledons 3); n = 12.

1/1: Ginkgo biloba. E. China, but perhaps now only in cultivation. [Photo - Microsporangia] [Photo - Ovules] [Photo - Seeds]

• Ginkgoaceae are deciduous trees with stout and conspicuous short shoots; the leaves have dichotomous venation.

Ginkgoales were almost world-wide in distribution and included several genera in the Mesozoic. They have a possible origin from Palaezoic pteridosperms, perhaps in the Upper Carboniferous (Thomas & Spicer 1987; Zhou 1997). The morphology of these early Ginkgo-like plants is uncertain, but the ovules may have been more numerous, very differently arranged and some at least were inverted (and/or platyspermic). Ginkgo-like leaves are known from the Permian onwards (see Zhou & Zhang 2003).

It is reported that dioecy is associated with chromosomal differentiation (female xx, male xy), but cf. Hizume (1997). The integument is initiated in two places, but soon becomes confluent. The chalazal cell of the linear tetrad develops to form the female gametophyte. Note that the nuclei of the female gametophyte have the DNA content of diploid cells (Friedman & Gifford 1997).

For additional information, see Friedman (1987: male gametophyte development), Soma (1997: female gametophyte and embryogeny), the papers in Hori et al. (1997) in general, Douglas et al. (2007: ovule), and the Gymnosperm Database (general).