Family Casuarinaceae

Casuarinaceae R. Br.

Habit and leaf form. Trees and shrubs (with ‘equisetiform’ shoots). Switch-plants; with the principal photosynthesizing function transferred to stems. Leaves much reduced. Leptocaul. Helophytic to xerophytic. Leaves minute; whorled; 4–12 per whorl; membranous; sessile; connate; aromatic (at least, the shoots so in Allocasuarina), or without marked odour; simple; exstipulate.

General anatomy. Accumulated starch exclusively ‘pteridophyte type’.

Leaf anatomy. Minor leaf veins without phloem transfer cells.

Stem anatomy. Cork cambium present. Nodes unilacunar. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with fibre tracheids; with vessels. Vessel end-walls scalariform, or scalariform and simple. Vessels without vestured pits. Sieve-tube plastids S-type.

Reproductive type, pollination. Plants monoecious, or dioecious. Pollination anemophilous.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in spikes, in heads, in glomerules, and in catkins. The fruiting inflorescences conelike. Inflorescences catkins, with males in simple or compound spikes, females in spherical or ovoid heads maturing into woody ‘cones’. Flowers bracteate (the bracts becoming woody in the female); bracteolate (the bracteoles becoming woody in the female); small. Hypogynous disk absent.

Perianth vestigial (male flowers), or absent (females); when present (male flowers), 1, or 2 (small).

Androecium 1. Androecial members unbranched (but tending to split); adnate. Androecium exclusively of fertile stamens. Stamens 1. Anthers basifixed; dehiscing via longitudinal slits. Endothecium developing fibrous thickenings. The initial microspore tetrads tetrahedral. Anther wall initially with more than one middle layer. Pollen grains aperturate; (2–)3(–5) aperturate; porate; 2-celled.

Gynoecium 2 carpelled. The pistil 1–2 celled. Gynoecium syncarpous; synstylovarious; superior. Ovary 2 locular (one of them abortive). Styles 1; apical. Stigmas 2. Ovules 2 per locule (in the fertile locule); collateral; non-arillate; orthotropous; bitegmic; crassinucellate. Outer integument not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing only after one has been fertilized, or fusing simultaneously with the male gamete (?). Antipodal cells formed; 3; not proliferating; ephemeral. Endosperm formation nuclear.

Fruit non-fleshy; indehiscent; a nut and a samara (single seeded, terminally winged). Gynoecia of adjoining flowers combining to form a multiple fruit (cones). Seeds non-endospermic. Seeds without starch (oil and protein only). Embryo well differentiated. Cotyledons 2 (oily). Embryo straight.

Seedling. Germination phanerocotylar.

Physiology, biochemistry. Nitrogen-fixing root nodules present (commonly), or absent. Not cyanogenic. Alkaloids absent (11 species). Iridoids not detected. Proanthocyanidins present; cyanidin, or delphinidin. Flavonols present, or absent; when present, kaempferol and quercetin. Ellagic acid present, or absent (2 species listed). Saponins/sapogenins absent (?). Aluminium accumulation not found. C3. C3 physiology recorded directly in Casuarina.

Geography, cytology. Temperate to tropical. Australia, Malaysia, New Caledonia, Fiji, Mascarene Is. X = 8–14. Supposed basic chromosome number of family: 9 (?).

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Rosiflorae; Casuarinales. Cronquist’s Subclass Hamamelidae; Casuarinales. APG 3 core angiosperms; core eudicot; Superorder Rosanae; fabid; Order Fagales.

Species 65. Genera 4; Allocasuarina, Casuarina, Ceuthostoma, Gymnostoma.

Economic uses, etc. Timber trees (‘she-oak’) where indigenous, and cultivated as ornamentals in warm regions elsewhere.

Illustrations.
• Technical details: Casuarina.
• Casuarina humilis, female (photo).
• Casuarina humilis, male (photo).