Comparative Anatomy and Phylogeny of the Casuarinaceae

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Abstract
An anatomical description of the secondary xylem and phloem of Casuarina is presented. Slides of old wood and young wood of 170 specimens of 29 spp. were examined. Some of the outstanding wood characters are both tracheids and fiber-tracheids are generally present, tracheids predominate; the fiber-tracheids often have very thick walls; spiral thickenings occur in the fibrous elements of 6 spp.; vessel distr. is universally solitary, diffuse porosity is characteristic; the majority of spp. have vessel elements which are thin-walled and circular in outline; the wood of 8 spp. has vessel elements with simple perforation plates, the remaining 6 spp. show scalariform and simple plates; most spp. have vessel elements with sloping end walls, transverse end walls are not uncommon; spiral thickenings occur in the vessel elements of 13 spp.; the vascular rays are heterogeneous IIB; aggregate rays occur in the old wood of 23 spp., but are absent in 4 spp.; metatracheal parenchyma occurs in all spp., diffuse in nearly all, and the paratracheal types in one. By careful interpretation of the wood anatomical data, and by comparison of this evidence with that from the phloem anatomy and that already offered by floral morphology, the author has concluded that the Casuarinaceae are a moderately specialized family of the Angiospermae and are not primitive. Floral morphology suggests that the Casuarinaceae have been derived from Hamamelidaceae-like ancestors, and the anatomical evidence plus additional data from other fields of botany are not inconsistent with this derivation.