The mechanism of thermal transpiration (= thermal osmosis)

Abstract

Floating-leaved aquatic plants have acquired a convective gas-throughflow system to supply oxygen from the atmosphere to their roots and rhizomes growing in anoxic sediments of shallow lakes and are dependent upon an efficient internal aeration. Tissue layers with minute pores inside the newly-emergent leaves, separating the aerenchyma of the leaves from the ambient atmosphere, create a pressurised ventilation by both the physical effects of hygrometric diffusion (humidity-induced diffusion) and thermal transpiration (thermal osmosis). In mature leaves, these pores are dilated to such an extent that free-flow of gas through this partition is possible. Therefore, the air, which moves into the young leaves (influx leaves) along humidity and temperature gradients, causing pressurisation in the aerenchyma, flows through the continuous intercellular space system and the mature leaves (efflux leaves) back to the atmosphere. Taking into account theoretical considerations and the physical background and anatomical requirements of the leaves, it is explained how the pressurised ventilation, based on thermal transpiration, operates in the aquatic plant during the growing season as soon as two floating leaves have been developed.