SHOOT VASCULAR SYSTEM AND PHYLLOTAXIS OF CASUARINA (CASUARINACEAE)

In species of Casuarina with multileaved whorls, each stem vascular bundle divides radially into two at the site of a leaf trace separation, and the same two bundles rejoin acropetally to where the trace supplies a leaf. Such divisions are divisions of a single vascular bundle, and the rejoining of bundles forms a single bundle. Proposals that the extant primary vascular systems of dicotyledons may have been derived as in conifers are incorrect in so far as Casuarina is concerned, or the system has evolved beyond that so far proposed for dicotyledons. Reasons are offered, however, for considering that fernlike leaf gaps are not present. Leaf traces supply leaves at the first nodes distal to their origins. The ways by which an increase or decrease of stem bundles occur are described. Phyllotactic patterns range from helical (rare) to whorled. In the embryo, where leaves occur decussately, of certain species with multileaved whorls, and in the shoot apices of species with tetramerous whorls, slight differences in the levels of leaf attachments and the bending of leaf traces indicate the probable evolution of extant whorled phyllotaxies from one or more helical arrangements. Stages in the evolution are suggested. The leaves in most species with multileaved whorls are in true whorls. The original periderm of branchlets lies internally to the internodal traces and chlorenchyma, but is otherwise external to the vascular system. It is concluded that each leaf originates at its level of separation from the axis despite several structural features suggesting that the leaf bases have become congenitally adnate to the stem.