The Families of Gymnosperms
Vegetative. Pachycaul (of palm-like habit, from small to 12 metres high and leaves to 3 m or more long, the subterranean to emergent stem to 40 cm wide, with relatively little wood, clothed in persistent leaf bases); evergreen; trees, or shrubs; with apogeotropic roots bearing cyanobacteria, born in coralloid masses just above ground level. Not resinous (secreting mucilage). Mature leaves pinnate (with girdling leaf traces). The leaflets with a prominent midrib and no laterals (the basal ones sometimes reduced to spines).
Reproductive organization. Dioecious (via X and Y chromosomes). The ovules not in cones (the leaf-like sporophylls alernating with vegetative fronds in a loose terminal rosette, through which the stem subsequently continues to grow); marginal or in notches, on discrete megasporophylls with sterile tips (the tomentose sporophylls flat and elongate, with dilated, acuminate and toothed or pinnatifid apices). The megasporophylls (1–)3–8 ovuled (borne marginally in notches in the lower part of the megasporophyll). The ovules orthotropus.
Pollen-sacs numerous, borne abaxially on the cuneate microsporophylls. Pollen without air bladders. Pollination probably always anemophilous; involving a liquid drop mechanism. Fertilization involving spirally flagellate, motile spermatozoids.
Seeds and seedlings. Seeds large, with a fleshy investment; the fleshy investment developed from the integument (from the outer layer of the testa, which surrounds the woody inner one); wingless. Cotyledons 2.
Wood anatomy. Growth rings indistict (absent, but a few species exhibit successive cambia giving rise to co-axial cylinders of xylem and phloem). Tracheids with opposite and multiseriate bordered pits.
Geography, cytology. Temperate to tropical; Southeast Asia, southern China, Malaysia, tropical Australia, Oceania, Japan, Africa, and Madagascar.
Basic chomosome number, n = 11.
Taxonomy. About 90 species; Cycas. Order Cycadales.
Comments. Leaves and leaflets usually with more or less circinate vernation.
Miscellaneous. • Cycas revoluta and Cycas circinalis: habit and female sporophylls. • Cycas circinalis: habit, megasporophyll and seed (Le Maout and Decaisne). • Cycas spp., habit and sporophylls (Sporne).
To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2008 onwards. The families of gymnosperms. Version: 6th November 2009. http://delta-intkey.com’.