CASUARINA, a genus of trees containing about 30 species, chiefly Australian, but a few Indo-Malayan. The long whip-like green branches are longitudinally grooved, and bear at the nodes whorls of small scale-leaves, the shoots resembling those of Equisetum (horse-tail). The flowers are unisexual; the staminate are borne in spikes, each flower consisting of a central stamen which is surrounded by two scale-like perianth-leaves. The pistillate are borne in dense spherical heads; each flower stands in the axil of a bract and consists of two united carpels flanked by a pair of bracteoles; the long styles hang out beyond the bracts, and the one-chambered ovary contains two ovules. In the fruit the bracteoles form two woody valves between which is a nut; the aggregate of fruits resemble small cones. Pollen is transferred by the wind to the long styles. The pollen-tube does not penetrate the ovule through the micropyle but enters at the opposite end—the chalaza. This anomaly was discovered by Dr M. Treub (see Annal. Jardin Botan. Buitenzorg, x. 1891), and is associated with a peculiar development of the ovule, and an increased number and peculiar form of the embryo-sacs (nacrospores). Treub proposed to separate Casuarina as a distinct group of Angiosperms, and suggested the following arrangement:—
| Angiospermae | Porogamae | Dicotyledons. |
| Monocotyledons. | ||
| Chalazogamae (Casuarina). | ||
The names of the two subdivisions recall the manner of entrance of the pollen-tube. More recent investigations, chiefly by Nawaschin and Miss Benson, on members of the orders Betulaceae, Fagaceae, Juglans and Ulmus, showed a recurrence in a greater or less degree of the various anomalies previously observed in Casuarina, and suggest that the affinity of Casuarina is with these orders of Dicotyledons.
The wood is very hard, and several species are valuable timber trees. From a fancied resemblance of the wood to that of the oak these trees are known as “oaks,” and the same species has different names in different parts such as “she-oak,” “swamp-oak,” “shingle-oak,” “river-oak,” “iron-wood,” “beef-wood,” &c.
See J. H. Maiden, Useful Native Plants of Australia (London and Sydney, 1889).