The mechanism of plication inception in palm leaves: histogenetic observations on the palmate leaf of Rhapis excelsa

Plications in the leaf of Rhapis excelsa are initiated at the lateral, basal margin of the palmate lamina and are oriented vertically from inception. When first initiated the plications appear in transection as distinct, sinusoidal folds. However, with the addition of more folds laterally, the earlier pleats become more markedly appressed and constrained to grow more symmetrically in a radial plane. As a result of asymmetry of the entire leaf the cathodic side of the lamina which abuts against the next oldest leaf shows an earlier folding and more radial, symmetrical growth than the anodic, free margin. However, there is no evidence of a significant developmental difference among plications in different regions of the blade. Changes in shape from the unplicated apex of the fold to its markedly plicated base parallel precisely the changes in shape exhibited by a single plication position followed through time. Quantitative observations on plication growth demonstrate that initially there is more extensive upgrowth in the adaxial plication ridges than their abaxial counterparts. But adaxial ridge formation is correlated with cell division whereas abaxial protuberance is the result almost exclusively of cell enlargement. Once a pattern of alternating adaxial and abaxial ridges is established, plication growth shifts from the ridges to the intercostal zones between the ridges and occurs by cell division with no net cell enlargement during the early phases studied. These quantitative observations indicate that increases in plication furrow depth are a result of ridge upgrowth rather than cell separation, and histogenetic observations give no sign of protodermal disruption and redifferentiation. Plication inception in Rhapis is therefore a result of meristem folding and not tissue cleavage. Because quantitative measurements of plication origin and growth in Rhapis are virtually identical to those described in the pinnate palm Chrysalidocarpus lutescens, we conclude that morphogenetic processes are the same and that differing plication shape between them is merely a consequence of physical features within the leaf itself and does not indicate different morphogenetic mechanisms as suggested in the past.