The Effect of Cell Elongation on Leaf Anatomy of Poa alpina

The hypothesis (Slade 1957) that the larger cells and intercellular spaces in the spongy mesophyll and the wider spacing of veinlets in leaves grown in deep shade might be due to differences in cell extension rather than in meristematic growth has been tested on leaves from Poa alpina plants in which cell extension has been stimulated by shade and/or gibberellin. Increase in leaf length in treated plants was always associated with increases in cell length and in distance between commissural veins and with greater longitudinal extension of the cells and intercellular spaces in the mesophyll. These changes occurred at the expense of increase in girth of both cells and intercellular spaces, the overall effect being reduction in thickness of the leaf blade. There were no significant differences in the number of cells separating successive commissural veins in treated and untreated leaves; so the greater spacing of these veins in shaded or gibberellin-treated leaves is due to greater extension of cells and intercellular spaces unaccompanied by any increase in cell division.