Trees [or shrubs], evergreen. Photosynthetic branchlets slender, wiry, with several very short, basal segments and 1-numerous elongate segments; segments terete [quadrangular], with as many longitudinal ridges as leaves; ridges separated by furrows containing stomates. Leaves reduced to small teeth in whorls of [4-]6-17 at apex of each segment of photosynthetic branchlets. Inflorescences of alternating whorls of flowers, each flower subtended by toothlike bract and 2 bracteoles, bracteoles usually persistent, lateral, scalelike; staminate inflorescences catkinlike spikes, short to elongate; pistillate inflorescences heads, globular to ovoid. Flowers unisexual, staminate and pistillate on same or different plants. Staminate flowers : sepals deciduous at anthesis, 1-2, hooded, scalelike; stamen 1; anthers basifixed, 2-locular. Pistillate flowers : perianth absent; pistil 1, compound, 2-carpellate, 1 fertile, the other usually reduced or absent; ovules 2, an additional 2 abortive ovules in reduced carpel; styles 2-branched, reddish. Infructescences ± woody, cylindric, conelike; floral bracteoles 2, enlarged as lateral valves. Fruits compressed, winged nuts (samaras). Seeds 1 in each samara. Casuarinaceae are occasionally referred to as the beefwood family or the Australian-pine family. Species have been cultivated in the warmest parts of the flora as ornamentals and shelterbelts, and for sand binding. Their suitability for such uses is partly because of the presence in root nodules of actinomycetes ( Frankia ); such actinomycetes are effective in fixation of atmospheric nitrogen. Vesicular-arbuscular, endotrophic mycorrhizae have also been reported. In addition to the species described here, the following have all been recorded as cultivated in the flora, but they are not known to be naturalized: Allocasuarina decussata (Bentham) L. A. S. Johnson, A . helmsii (Ewart & M. Gordon) L. A. S. Johnson, A . littoralis (Salisbury) L. A. S. Johnson ( Casuarina suberosa Otto & Dietrich), A . torulosa (Aiton) L. A. S. Johnson, A . verticillata (Lamarck) L. A. S. Johnson ( C . stricta Aiton), C . cristata Miquel ( C . lepidophloia F. Mueller) and Gymnostoma sumatranum (Junghuhn ex de Vriese) L. A. S. Johnson ( C . sumatrana Junghuhn ex de Vriese). See K. L. Wilson and L. A. S. Johnson (1989) for distinguishing features of the Australian species.
Dried specimens differ significantly from fresh material. When fresh, fruiting bracteoles of the infructescence are nearly always appressed to each other, enclosing the samara; when the infructescence dries out, the bracteoles separate. Measurements given here for infructescence body diameter do not include any portion of the bracteoles extending beyond the main body of the infructescence. The softer tissues of branchlets contract when dried, so that features such as angularity or convexity of longitudinal ridges are emphasized in dried specimens. The key and descriptions are generally based on dried specimens.