Morphological and ontogenetic studies in palms. V. Early ontogeny and vascular architecture of the leaf of Rhapis flabelliformis

Abstract

The leaf primordium of Rhapis flabelliformis arises as a crescent-shaped structure on one side of the shoot apex. Apical growth of the primordium ceases after the initiation of a transversely oriented lamina wing on the adaxial face of its apex. Plications are initiated in the submarginal region of the lamina wing by a characteristic growth modification of its meristem, during which no splitting of any sort is involved. The tissues adaxial and abaxial to the place of initiation of the lamina wing develop respectively into prominent adaxial and abaxial crests.

Very early in its development, the palmate lamina becomes segmented into five or fewer strips by means of constrictions extending longitudinally. These constrictions have rather precise locations, arising in the tissue between the ridges rather than in that along them. The segments remain connected by a very reduced and slender rein until the leaf emerges.

The dorsal median strand running to the first-formed abaxial ridge of the plications is the first to differentiate. Three series of strands then arise at different transverse levels -middle, outer, and inner -in the petiole as well as the sheathing base. Those situated in the petiole and petiolar part of the sheathing base supply the lamina. The inner strands of both the petiolar and sacking parts of the sheath give rise to a unique vascular skeleton of the ventral sacking. In addition to interweaving of the strands as warps and wefts along the median ventral line, the vasculature of the sacking is reinforced by a very regularly arranged inner series of strands.