Water Pathways in Higher Plants

A technique has been developed for the study of pathways of water movement in the xylem and free space of wheat leaves. Plants were treated with Lead-EDTA chelate through either the roots or the leaves; after treatment the lead was precipitated in situ as lead sulphide with hydrogen sulphide gas and its location determined by light and electron microscopy. Bulk water movement was in the lumen of the xylem, where there was always a heavy deposit of lead sulphide after root treatments. Outside the xylem the deposits were confined to the cell walls and were most dense in the middle lamella. Deposits were not found in the cells themselves. The main zones of water loss, marked by heavy deposits of lead sulphide, were associated with the stomata, the junctions of the periclinal walls of the epidermal cells, and the cuticle, leaf hairs, and specialized epidermal cells with pitted walls associated with the vascular bundles. Entry of lead chelate into the leaves was adequately described by a diffusion model. The free space seemed to be located mainly in the water of hydration of the pectin middle lamella and was esmated to occupy 3 to 5 per cent of the volume of the tissue.