Knudsen-transitional flow and gas pressurization in leaves of nelumbo.

Pressures in gas spaces of leaves of the lotus Nelumbo are higher than ambient pressure. The pressurization capacity of leaves was studied as a function of leaf temperature, and the composition of air entering evacuated leaves was used to calibrate the pore sizes which determine flow in these leaves. The adaxial side of the leaf of Nelumbo has two distinct regions in terms of gas exchange characteristics. There is a region of relatively high mean pore diameter in the center of the leaf opposite the point of petiole insertion. Gas exchange between the remainder of the leaf (>99% by area) and the atmosphere is restricted by "pores" with an effective mean diameter less than 0.03 micrometer. As a result, a flowthrough ventilation operates within each leaf. Air enters the leaf across the expanse of the lamina, and escapes back to the atmosphere through the highly porous region at the center of the lamina.