The Families of Gymnosperms
Vegetative. Pachycaul (a few being fern-like rosette plants, with short, tuberous subterranean or emergent stems (species of Macrozamia, Zamia, Encephalartos), but mostly of palm-like habit, from small to 15(-20) m high with leaves from 5 cm to 4 m or more long, the 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 with girdling leaf traces, pinnate (nearly always), or bipinnate (occasionally). The leaflets with numerous parallel or wavy, single or forked longitudinal veins but no prominent midrib.
Reproductive organization. Dioecious (via X and Y chromosomes). The ovules borne in female cones (these ranging from 2 cm long in Zamia pygmaea to 70 cm or more in some Macrozamia spp.); marginal or in notches, on discrete megasporophylls with sterile tips (the megasporophylls more or less peltate). The megasporophylls 2(–3) ovuled (the ovules on the adaxial margins of the megasporophylls, protected within the strobilus by their peltate or flattened and overlapping tips). The ovules orthotropus.
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(–3).
Wood anatomy. Growth rings indistict (absent, but a few species of Macrozamia and Encephalartos exhibit co-axial cylinders of secondary xylem and phloem, derived from successive cambia). Tracheids with opposite and multiseriate bordered pits, or with neither alternate nor opposite bordered pits (e.g., scalariform in Zamia).
Geography, cytology. Temperate to tropical; Australia (Chigua, Lepidozamia, Macrozamia), Neotropics (Georgia to Bolivia), Ceratozamia, Dioon, Microcycas, Zamia) and sub-Saharan Africa (Encephalartos). All species occupy subtropical or tropical habitats, with precipitation regimes ranging from the semiarid to swamps or tropical rainforests.
Basic chomosome number, n = 8 (Ceratozamia, Zamia), or 9 (mostly), or 13 (Microcycas).
Taxonomy. About 100 species; Ceratozamia, Chigua, Dioon, Encephalartos, Lepidozamia, Macrozamia, Microcycas, Zamia. Order Cycadales.
Comments. The young leaves sub-circinate in (e.g.) Ceratozamia and Zamia, but absolutely straight in Dioon, Macrozamia and Encephalartos.
Miscellaneous. • Macrozamia denisoni: habit and female cone. • Macrozamia denisoni and Zamia floridana: female cones. • Dioon spinulosum and Dioon edule: habit, male cone and male sporophylls. • Zamia portoricensis and Ceratozamia mexicana: male cones and male sporophylls. • Dioon, Ceratozamia, Macrozamia, Zamia: habit, strobili and sporophylls.
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’.