Studies on the Leaf of Populus Deltoides (Salicaceae): Quantitative Aspects, and Solute Concentrations of the Sieve‐tube Members

The vascular system of the leaf of Populus deltoides Bartr. ex Marsh, was examined quantitatively, and plasmolytic studies were carried out to determine the solute concentrations of sieve‐tube members at various locations in the leaf. Both the total number and total crosssectional area of each cell type decreases with decreasing vein size. Although the proportion of phloem occupied by sieve tubes varies considerably from location to location, a linear relationship exists between cross‐sectional area of the vascular bundles and both total and mean cross‐sectional area of sieve tubes. Collectively, the cross‐sectional area of all tertiary and minor veins feeding into a secondary exceeds the total cross‐sectional area of sieve tubes at the base of that secondary. Moreover, the total volume of sieve tubes in the “catchment area” of a secondary vein is much greater than the total sieve tube volume of the secondary itself. Both tracheary elements and sieve‐tube members undergo a reduction in both total and mean crosssectional area in the constricted zone at the base of the leaf. The plasmolytic studies revealed the presence of positive concentration gradients in sieve tubes of the lamina from the minor veins and tips of the secondaries to the bases of the secondaries and their associated subjacent midvein bundles and from the upper to lower portions of the median bundle of the midvein.