Organogenesis in the shoot: later stages of leaf development

Abstract

The stages of leaf development discussed in the two previous chapters – the primordial stages, which culminate in a simple outgrowth at the margin of the shoot meristem, somewhat flattened on its adaxial face – give little indication of the diverse morphology of the mature leaves of various groups of vascular plants. The multipinnate frond of a fern, the needle leaves of many conifers, and the diverse simple and compound leaves of the angiosperms are remarkably similar in the period immediately following their inception. Thus, the diversity of leaf form can be interpreted best through an understanding of the later, as opposed to primordial, stages of development. On the other hand the evidence here cited indicates that in the primordial stages the leaf undergoes a determination that confers upon it a considerable degree of autonomy in its later development. Does this imply that all of the diverse morphology of leaves must be thought of as originating in a process of determination at a relatively undifferentiated stage? The answer to this question is not an easy one, and the issues involved may best be exposed by examining some of the events of later leaf development and some of the experiments that have sought to interpret them. DEVELOPMENT OF FERN LEAVES There is a substantial body of information about later stages of leaf development in ferns, much of it collected from species that have also been used for experimental analysis, so that descriptive and experimental data may be correlated.