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

1. The development of the leaf in the Valencia orange is considered in four stages: (a) resting bud and meristematic primordia; (b) expanding leaves; (c) maturing leaves; and (d) leaf fall. Stages a-c parallel similar phases in stem differentiation. The differentiation of vascular and other tissues is described. 2. Cell shape is considered in detail in spongy and in palisade parenchyma. Two principal cell types are common to both, the eight-armed or eight-lobed cell and the six-armed or six-lobed. 3. In the spongy mesophyll, eight-armed cells occur at vein level, in midareole, the region of largest intercellular spaces in which cell division first ceases. The eight-armed cell may be derived from a tetrakaidekahedral meristematic cell in plasmodesmatal connection with eight others. 4. In the lower mesophyll and along the margins of the areoles, cell division continues later than in mid-mesophyll and here six-armed cells differentiate. The six-armed cell may be derived from a tetrakaidekahedral element which has undergone comparatively late cell division. 5. In the palisade parenchyma the counterparts of the eight- and six-armed cells are eight- and six-contact cells, in which the lobes through which the plasmodesmata pass are reduced to a minimum. 6. The above cell types occur in all dicotyledonous leaves so far examined by the authors. 7. 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. 8. Plasmodesmata are directly concerned in the orientation of new cell walls. In all cells examined, new walls are anchored to the wall of the mother cell in plasmodesmatal areas. 9. The position of the sheath of calcium oxalate crystals is, as in the idioblasts of Beloperone and Ficus, controlled by plasmodesmatal connections. Cytoplasmic strands in crystal sheath formation resemble, in their function of wall deposition, cytoplasmic plates-the phragmosomes of normal cell division. 10. 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 abscission zones expand outward against heavily suberized tissue at the bases of lamina and of petiole. Tissue break occurs at the junction of the thin-walled papillate cells and the opposing suberized elements. 11. Suberin deposition occurs not only in abscission 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 walls of the inner surfaces of all cells. 12. Progressive suberization appears to be a general phenomenon in the aging of leaves, since suberin deposition occurs in squash, castor bean, avocado, and sycamore leaves. In sycamore (Platanus racemosa) suberization is sufficiently heavy to prevent the usual cell-wall swelling and tissue disintegration which result from treatment of leaf sections with IKI-H2SO4. The suberized mesophyll persists as intact network. 13. Suberization of the internal surface presumably facilitates the distribution by capillarity of spray oil which may enter the leaf. 14. The development and structure of guard cells and of oil glands is similar to that seen in the fruit rind. 15. The wax canals in the outer wall of the epidermal cells resemble those of the fruit rind, and through them the surface wax and the material of the stomatal plugs is secreted.