The mechanism of plication inception in palm leaves: histogenetic observations on the pinnate leaf of Chrysalidocarpus lutescens

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Abstract
The reduplicate, pinnately compound leaf of Chrysalidocarpus lutescens begins development as a hood-shaped primordium with a smooth lamina. By the time the leaf is 5.5 mm in length, 40–42 pleats consisting of alternating ridges and furrows have been formed in a submarginal position on each half of the lamina. Serial sections of plastic-embedded leaf primordia were studied to distinguish between two alternative mechanisms of plication inception proposed in the literature: differential growth or tissue separation. Counts of the number of cell layers in the lamina during the period of plication initiation support the concept of differential growth, i.e., an increase in the number of tissue layers is observed at the site of furrow formation rather than a decrease in cell layers which would be expected if tissue separation were occurring. Ridges appear first on the adaxial side of the lamina, accompanied by periclinal and oblique divisions in the middle lamina layers. Further growth is associated with zones of anticlinal divisions alternating with the ridges. This plate meristem growth increases the surface area of a portion of the lamina bounded by the leaf apex, the unplicate margin, the petiole, and the rachis, and therefore tends to be accommodated by a buckling of the lamina between adaxial ridges. Intercalary growth of the intercostal sector causes the adaxial and abaxial ridges to be displaced from each other, deepening the plications. Examination of early stages of leaf development with the transmission electron microscope corroborates these results; there is no indication of schizogenous separation of cells within the ground tissue of the lamina or between protodermal cells. The protoderm and its cuticle are continuous at all stages of plication initiation, supporting the concept of differential growth.