Water pathways in higher plants III. the transpiration stream within leaves

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

Water pathways in higher plants II. water pathways in boots

The technique for studying the pathways of water movement described, in the first paper of this series has been applied to the transpiration stream in the roots of higher plants. Free space is confined to the cell walls except in flooded tissue where intercellular spaces are also included. Errors involved in free-space estimates are discussed and free-space volumes of 5.4 per cent have been obtained for wheat root cortex and 4.1 per cent for carrot xylem parenchyma. The main water-absorbing regions of roots begin immediately behind the elongating zone, where the first xylem elements are fully differentiated, and end when the endodermis undergoes secondary wall development. In the cortex the transpiration stream is located mainly in the cell wall. Calculations indicate that the symplastic pathway is of only minor importance in transpiring plants. At the endodermis the free-space pathway is blocked by the Casparian strip and all water and solute entering the stele must pass through the lumen of the endodermal cells. The permeability of individual endodermal cells varies considerably both between cells of the same species and between those of different species. Once inside the endodermis, the transpiration stream returns to the cell-wall pathway until it reaches the xylem vessels where it enters the lumen of the mature xylem elements

Water pathways in higher plants - I. free space in wheat leaves

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 estimated to occupy 3 to 5 per cent of the volume of the tissue. <br/