The Genesis of Form in Oleander (Nerium oleander L.)

Abstract

With the aid of three-dimensional scale drawings, the development of the shoot apex of Nerium oleander and its transition from decussate to whorled phyllotaxis are described. Normally there are whorls of three leaves, but vigorous seedlings can change from the decussate condition to whorls of four leaves in a single step. Axillary buds produce two leaves at their bases but switch immediately to whorls of three.

A study of the growth in volume of young primordia led to the conclusion that the bare apical surface was growing very slowly and was subject to considerable physical constraint by the whorls of massive leaf bases that are coplanar with it. Under circumstances as yet undefined the apex can become convex and the phyllotaxis change to a Fibonacci spiral system.

Massive glandular trichomes are produced in the axils of the leaves and of the cotyledons. They appear to obstruct the early growth of leaf primordia and may be a factor governing apical dominance in vegetative shoots of Nerium.