Structural Responses to the Practice of Topping Tobacco Plants: A Study of Cell Size, Cell Number, Leaf Size, and Veinage of Leaves at Different Levels on the Stalk

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Abstract
The removal of the inflorescence and axillary buds as rapidly as they appear (topping and suckering) causes an appreciable increase in the size of leaves on the upper 1/3 of the stalk. The size of epidermal and palisade parenchyma cells increases in direct proportion to the increase in the size of the leaf as a whole. The only increase in cell number occurs in the vascular tissue of the proximal end of the midrib, and the petiolar bundle, where cambial activity results in a 47% increase in the xylem. There is no appreciable increase in the number of phloem cells, despite cambial activity. This cessation of laying down of phloem is correlated with the absence of the developing inflorescence, and may be the result of discontinuance of translocation of material out of the leaves to developing flowers and seeds. The degree of development of the xylem and the amount of water loss show a high degree of correlation, as evidenced by the fact that veinage increases in almost constant proportion to the enlarging leaf blade.