Water Pathways in Higher Plants

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.