Studies on the Expansion of the Leaf Surface: V. CELL DIVISION AND EXPANSION IN A DEVELOPING LEAF AS INFLUENCED BY LIGHT AND UPPER LEAVES

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Abstract
The development of the third leaf was followed from soon after initiation to the attainment of full area in cucumbers grown under two levels of radiation and, following leaf unfolding, with the shoot above the developing leaf present or absent. Cell numbers, proportion of cells in meta-, ana-, and telo-phases, leaf areas, and aspects of leaf structures were recorded. Cell numbers increased exponentially until near unfolding at both intensities—at a higher rate under high light and for a longer time under low light. Thereafter, the rate declined to zero. Final cell numbers were higher under high than low light. Leaf structure and cell dimensions parallel to the leaf surface were not affected by light intensity and therefore final areas depended solely on cell numbers. Apex removal resulted in much larger cells, and in consequence, correspondingly greater leaf areas. The proportions of cells in the mitotic stages recorded were similar at both intensities but fell steadily throughout the periods of exponential increase of cell number. Previous hypotheses that all cells are active in division until the leaf unfolds could be largely reconciled with mitotic behaviour under high light, but some decline in the proportion of dividing cells was indicated. Under low light, such a decline must be important. Greater cell numbers in leaves at high light arose, therefore, from a higher rate of division in dividing cells, combined with a larger proportion remaining active. Greater leaf areas following apex removal are consistent with hypotheses of competition between developing leaves but throw no new light on the specific factors involved.