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Trees or shrubs evergreen, monoecious or dioecious. Ultimate branchlets jointed, with several short, basal articles and 1-numerous elongated articles, slender, ridged, often pubescent at least when young. Leaves small, toothlike, in whorls of 4 to ca. 20 (equal to number of ridges on branchlets); stipules absent. Inflorescences spikelike or headlike, with alternating whorls of toothlike bracts; within each bract a single flower, with 2 lateral scalelike bracteoles, persistent (rarely deciduous in male Allocasuarina); parts sometimes pubescent when young, mostly glabrous at maturity. Flowers unisexual, without pedicels. Male inflorescences spikes, terminal or lateral, terete, slender. Female inflorescences headlike, globose or ellipsoid, usually terminal on short, lateral branches. Male flowers: tepals 1 or 2, scalelike, deciduous, hooded; stamen 1; anthers 2-loculed, longitudinally dehiscent, basifixed. Female flowers: perianth none; carpels 2, fused; only anterior carpel fertile, posterior carpel usually ± reduced or obsolete; ovules 2 (rarely 4), parietal, paired at base of carpel, chalazogamous; style short; stigmas 2, red, linear. Infructescences ± woody, conelike, with dense whorls of fruit, with 2 bracteoles of each flower enlarged as valves, persistent. Samaras flat, (usually) winged at apex, initially enclosed by 2 bracteoles, which separate at maturity to release samara. Seed 1; cotyledons large; endosperm absent; embryo straight, often more than 1.
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.
Dioecious or monoecious trees or shrubs. Branchlets articulate, slender, wiry, with several short basal articles and 1–numerous elongated articles; articles with as many ridges (phyllichnia) as there are teeth (reduced leaves), the phyllichnia separated by furrows (shallow and open in Gymnostoma, deep and closed in other genera) containing the stomates. Leaves reduced to teeth in a whorl of 4–20 at apex of each article of assimilatory branchlets. Inflorescences of alternating whorls of tooth-like bracts; within each bract 2 lateral scale-like bracteoles, persistent, occasionally deciduous in male Allocasuarina, and a single flower. Male inflorescence a short to elongated spike (Wilson & Johnson 1989: 101, figs 45J, K) Male flowers: tepals 1 or 2, hooded, scale-like, deciduous at anthesis; stamen 1; anther 2-locular, basifixed. Female inflorescence a globular or ovoid head (Wilson & Johnson 1989: fig. 45 I). Female flowers perianth absent
Leaves on both kinds of branches reduced to whorls of small triangular scale leaves united at the base (tab. 36, fig. 3), becoming free on the persistent branches; midribs decurrent to the node below giving a ribbed or grooved appearance to the internode; leaf whorls, and therefore also the ribs, alternating at consecutive nodes
Male inflorescence spicate (tab. 36, fig. 2), solitary and terminal on deciduous branchlets (rarely also axillary on woody branches), cylindrical but tapering to a sterile basal region; flowers sessile and solitary in the axil of each bract of a whorl, a pair of lateral scarious bracteoles enclosing each flower
Infructescence cone-like, globular, ovoid or cylindrical; ± woody due to the enlargement and thickening of accrescent bracts and bracteoles of individual flowers, the latter much the larger (often with a dorsal protuberance) and forming pairs of valves enclosing the true fruit and opening when ripe
Female inflorescence globose or ovoid (tab. 36, fig. 2), shortly stalked or subsessile, axillary toward the ends of woody branches; flowers sessile in the axil of each bract of a whorl; bracteoles as in male flowers
Fruit a dark brown to black and shiny or pale grey-fawn and rather dull samara, laterally compressed, bearing a large ± translucent wing with a single longitudinal nerve excurrent at the apex
Male flower: a single stamen enclosed in bud by 1 or 2 (anterior and posterior) concave or cucullate, membranous perianth segments which fall as the stamen develops; mature anther exserted
Branches of 2 kinds: normal persistent woody branches and deciduous green branchlets, the latter thin, flexible and little branched
Flowers grouped into unisexual inflorescences with closely spaced, alternating whorls of bracts similar to the scale leaves
Female flower: perianth 0; ovary 1-locular with a short terminal style; stigmas 2, long slender, well-exserted at maturity
Trees (shrubs), dioecious less often monoecious
