Morphological and ontogenetic studies in Palms. II. Growth pattern of the leaves of Cocos nucifera and Borassus flabellifer after the initiation of plications

Abstract

Growth of the young leaf prirnordium into the adult leaf involves the operation of three types of tissue growth which may be designated as perpetuative growth, differentiative growth, and generative growth. In the first one, the tissue is perpetuated as such without histological differentiation into another, and the size of its cells is maintained constant so that growth in volume is due to increase in the number of cells. In the second, the tissue differentiates histologically into another, and growth is due to increase in the size of cells without increase in their number. The larger cell size thus attained enables the tissue to reduce the number of divisions necessary for subsequent growth of the first type. In the third, the size of the cells is reduced by rapid divisions and this is accompanied by the internal genesis of a new tissue within an already differentiated one.

In Cocos and Borassus, the leaf primordium, the lamina wing of the leaf primordium, and the plications in the submarginal portion of the lamina wing all have cells of the same size as those of the tunica and corpus of the shoot apex, at the time of their initiation. This is because they all arise by perpetuative growth before the onset of any differentiative growth, during the succession of their ontogeny from the shoot apex. By contrast, the rachis, the petiole, the marginal portion of the lamina wing, and the ridges of plications exhibit an increase in cell size due to the onset of differentiative growth.

The adaxial and the abaxial ridges that arise as a result of the development of plications in the lamina wing are not morphologically similar. In Cocos, the adaxial ridge joins the two halves of the same pinna whereas the abaxial ridge connects the adjacent halves of two neighbouring pinnae. The former becomes the hard midrib of the pinna and the latter is gradually weakened and finally lost by loosening of its cells to bring about the separation of the pinnae in the mature leaf. The terminal abaxial ridge, however, shows a different behaviour and becomes constricted off as a terminal whip between the two terminal pinnae.

In Borassus, the apex of the petiole resumes a limited amount of growth in length after an initial cessation of elongation, and produces a short intralaminar rachis and an abaxial crest. The basal and terminal halves of the abaxial ridges behave differently. While the former becomes thickened and provided with numerous vascular traces, the latter is provided with only a single trace and is finally constricted off as a filamentous structure to bring about the dissection of the palmate leaf.