Development, Cell Shape, Suberization of Internal Surface, and Abscission in the Leaf of the Valencia Orange, Citrus sinensis

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Abstract
The development of the leaf in the Valencia orange is considered in 4 stages (a) resting bud and meristematic primordia, (b) expanding leaves, (c) maturing leaves, (d) leaf fall. Stages a.-c parallel similar phases in stem differentiation. The differentiation of vascular and other tissues is described. Cell shape is considered in detail in spongy and palisade parenchyma. Two principal cell types are common to both, the 8-armed or 8-lobed cells, and the 6 armed or 6-lobed. Plasmodesmata may be regarded as relatively fixed points on the cell surface and therefore are of importance in controlling cell shape and wall area. Cell shape depends also on the time of cell division in relation to the growth of the organ as a whole. Plasmodesmata are also directly concerned in the orientiation of new cell walls. In all cells examined new walls are anchored to the wall of the mother cell in plasmodesmatal areas. Leaf abscission occurs between lamina and petiole, and also at the leaf base. The mechanism is the same in both. Papillate cells produced by distal elongation of cells of the absciss zones expand outwards against heavily suberized tissue at the bases of lamina and petiole. Tissue break occurs at the junction of the thin-walled papillate cells and the opposing suberized elements. Suberin deposition occurs not only in absciss regions but throughout the mesophyll of the leaf. In half-grown leaves it appears in the intercellular spaces and also within the cell as a film at the limit of visibility. In mature and old leaves suberin lines the internal surface and impregnates the middle lamella to a greater or lesser extent and also forms a tertiary lamella on the inner surfaces of all cells.