How do Plants Absorb Water?

How Roots Absorb and Move Water Inward

Roots absorb the great majority of a plant's water — exactly why watering is applied to the soil rather than the leaves — and the actual absorbing structures are root hairs: thin-walled, slender extensions of the root's epidermal cells, present in vast numbers near the root tips, which massively increase the surface area available for absorption. Water and dissolved mineral solutes enter root hairs simply by diffusion.

From the root hairs, water moving deeper into the root can take two distinct routes:

• Apoplast pathway — the continuous network of adjoining cell walls and intercellular spaces running throughout the plant, broken only at the casparian strips of the endodermis. Movement here never crosses a membrane, meets almost no resistance, and proceeds as mass flow, driven by tension that develops as water evaporates from the tissue.
• Symplast pathway — the network of interconnected cell interiors (protoplasts), linked cell-to-cell through cytoplasmic strands called plasmodesmata. Because water must cross a membrane to enter each cell, this route is slower, though it can be helped along by cytoplasmic streaming (visible, for instance, as circulating chloroplasts in Hydrilla leaf cells).

Most of the water moving through the loosely packed root cortex takes the easier apoplast route. But at the endodermis, the innermost layer of the cortex, a band of waterproofed (suberised) material called the casparian strip completely blocks apoplastic movement. Water cannot cross this band, so it is forced into the non-suberised part of the endodermal cell wall and must cross into the cell itself — making the endodermis the one place where water is forced onto the symplastic route, the only way water and dissolved solutes can enter the root's central vascular cylinder (the stele). Once past the endodermis, water moves freely again, and in young roots it enters directly into the xylem's vessels and tracheids — non-living conduits that are themselves considered part of the apoplast. Figures 11.6 and 11.7 trace this path across the root's tissue layers (epidermis → cortex → endodermis with its casparian strip and plasmodesmata → pericycle → xylem), colour-coding the symplastic and apoplastic legs and showing the apoplastic route being forced inward exactly at the casparian strip. …