436 MORPHOLOGY OF THE ANGIOSPERMS 



uous, photosynthetic twigs — has been responsible for a great range 

 in interpretations of the phyletic position of the family. 



The family is unigeneric. The only genus, CasiiarUm, consists of about 

 fifty species of trees and shrubs of xerophytic habitats in Australasia, 

 often on sea beaches at tidal limits. It is not well understood morpho- 

 logically or taxonomically; species apparently undescribed are in culti- 

 vation, and little attention has been given to the more primitive species, 

 in which the pistillate inflorescences are less specialized and are borne 

 on vegetative short-shoots, like the staminate. 



The simple, perianthless flowers — the staminate consisting of a single 

 "terminal" stamen and the pistillate of two carpels, one vestigial — and 

 their apparent aggregation in a unisexual catkin or conelike inflorescence 

 long kept the family in the Amentiferae. Support for this classification 

 was found in resemblances to conifers in the ovulate inflorescences, in 

 anemophily, and needlelike, deciduous twigs. On the basis of similarities 

 in whorled, scalelike leaves and "jointed" stems, even more distant 

 relationships have been suggested, such as to the horsetails and calamites 

 and to Ephedra. But critical morphological comparison of the Casuarina- 

 ceae with angiosperms in general demonstrates that this family is un- 

 doubtedly angiospermous. Casuarina is still insufficiently known, tax- 

 onomically and morphologically. Both its vegetative and reproductive 

 structures represent adaptation accompanied by strong reduction. (The 

 resemblance to Ephedra and Equisetiim is in the jointed stems, with 

 photosynthetic cortex, and the whorled, scalelike leaves.) But stelar 

 and nodal structure are those of specialized angiosperms — the node uni- 

 lacunar, with one trace. Histologically, the vascular tissue is angio- 

 spermous. The phloem has companion cells (not present in the Gnetales 

 or lower taxa). The wood is typically angiospermous; its scalariform 

 vessels are of the angiosperm type — not of the Ephedra (Gnetalian) 

 type — with round perforations. 



The flowers are extremely simple; the simplicity is clearly the result 

 of reduction, as shown by anatomical evidence and from comparison 

 with similar simple flowers in highly specialized families. The staminate 

 flower consists of a single, "terminal" stamen, like that of Euphorbia, 

 which is generally recognized as reduced, on evidence of many kinds. 

 The stamens are advanced in form and structiu-e, with filaments terete 

 and slender and anthers sharply set apart. The gynoecium is also 

 highly specialized; it consists of two completely connate carpels, one 

 vestigial. The ovule is solitary. The stigma is threadlike, the specialized 

 form of wind-pollinated plants. The structure of the ovule is insuffi- 

 ciently known, especially that of the more primitive species, but it is 

 angiospermous. It has two integuments and a typical 8-nucleate embryo 

 sac. There are several archesporial cells (Fig. Ill), and sometimes an 



