Description from Flora of China
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.
Four genera and 97 species: mainly in Australia, extending to SE Asia, Malesia, Pacific Islands; commonly introduced and occasionally naturalized elsewhere; one genus and three species (all introduced) in China.
In addition, five species of the Australian endemic genus Allocasuarina L. A. S. Johnson are said to be occasionally introduced to China: A. fraseriana (Miquel) L. A. S. Johnson, A. huegeliana (Miquel) L. A. S. Johnson, A. nana (Sieber ex Sprengel) L. A. S. Johnson, A. verticillata (Lamarck) L. A. S. Johnson (formerly Casuarina stricta Aiton), and A. trichodon (Miquel) L. A. S. Johnson. Allocasuarina differs from Casuarina as follows: leaves 4-14 per whorl on branchlets; cone bracteoles thickly woody, mostly extending only slightly beyond cone body, mostly with a separate angular or divided abaxial protuberance; mature samaras red-brown to black, shiny. However, no specimens have been seen to confirm these occurrences; some may be misidentifications, e.g., C. equisetifolia has been misidentified at times as A. huegeliana in Taiwan.
Chen Te-chao. 1982. Casuarinaceae. In: Tseng Yung-chien, ed., Fl. Reipubl. Popularis Sin. 20(1): 1-5.
(Authors: Xia Nianhe (夏念和) ; Lawrence A. S. Johnson2, Karen L. Wilson)