The mechanism of plication inception in palm leaves: problem and developmental morphology

Abstract

As the first part of a general reinvestigation of the mechanism of plication inception in palms, the present paper reviews the past literature and describes the overall morphogenesis of leaves of Chrysalidocarpus lutescens, Chamaedorea seifrizii, and Rhapis excelsa as observed principally with the scanning electron microscope. Plication inception in the pinnate leaf of Chrysalidocarpus begins when the leaf is 700–800 μm in length and continues until it is approximately 5–6 mm in length or after 40–42 pleats have been initiated. Initiation is bidirectional with the majority of folds being added basipetally. Plications initiated in the apical region of the leaf are vertically oriented but become reoriented to a more horizontal plane as a result of greater elongation in the abaxial part of the rachis. The adaxial portion of the plications is covered to varying degrees by a marked adaxial outgrowth of the rachis axis. But as the lamina continues to grow in surface, the plications grow beyond this rachis plug and no longer have their adaxial surfaces concealed. At a leaf length of 18–20 mm secondary furrows develop through the abaxial plication ridges cleaving the folds into individual leaflets. Although similarly pinnate in morphology and direction of plication initiation, the leaf of Chamaedorea initiates fewer leaflets (15–20) at a more accelerated relative rate and shows an earlier process of leaflet cleavage (at a length of 3–4 mm). Because growth in plication surface occurs at an earlier stage than in Chrysalidocarpus, the folds in Chamaedorea are never covered by the rachis plug even though one is present. By contrast with the pinnate species, plication orientation in the palmate leaf of Rhapis is vertical and proceeds centrifugally up to a length of 4–5 mm. Early in development the leaf of Rhapis appears more distinctly arched abaxially and both the adaxial and abaxial folds are covered to varying degrees by the dorsal and ventral hastulae. With subsequent elongation on the adaxial side and reduction in hastula growth, the plications elongate beyond the hastula and become evident externally. Plication cleavage in Rhapis occurs in the intercostal regions rather than at the costae and the cells that will undergo this abscission are already defined by a leaf length of 1.5 mm. From the comparison of morphogenesis of pinnate and palmate leaves it is concluded that leaves of each type of morphology have features of growth that are distinctive from inception and hence do not exhibit the kind of common developmental pathway hypothesized previously. Furthermore, analysis of both longitudinal and transverse spatial relationships of palm leaves in the bud has shown that while successive leaves can be sufficiently tightly packed to leave an imprint on the surface of the surrounding older leaf, it does not mean that leaf compression in the bud is responsible for the inception of leaf plication. It is more likely that packing causes the flattened appearance of the abaxial ridges of Chrysalidocarpus and the images of its furrow lines as incision-like slits. While SEM observations have not, by themselves, discriminated between folding and splitting as mechanisms of plication origin, they have provided a fundamental appreciation of the three-dimensional context of plication development and have been critical for subsequent histogenetic studies.