Water Pathways in Higher Plants

Water in the transpiration stream is distributed throughout the leaves in the vascular bundles. In wheat, water appears to be confined to the main veins by the mestome sheath and to enter the mesophyll through the walls of the smaller veins. Within the mesophyll the water in the transpiration stream moves in the free space of the cell walls to the evaporating surfaces of the leaf. The lead chelate, which is used to trace the transpiration stream, accumulates at the final points of evaporation at the margin of the leaf. Lead chelate accumulates beneath and on the surface of the cuticle, being partly associated with the anticlinal walls of the epidermal cells, the walls of the stomatal guard cells and specialized epidermal cells. Chelate does not accumulate at the base of substomatal cavities, indicating that the cuticle of the epidermis is the main evaporating surface of the leaf. The behaviour in broad bean, laurel, and plantain is essentially the same. The rate of peristomatal and cuticular transpiration is closely related to the size of the stomatal aperture. Conditions which control stomatal aperture also cause changes in the dimensions of the epidermal cells.