Path of Water Across Root Cells

Once water has been absorbed at the root hair, it must travel radially inward, crossing successive tissue layers - the cortex, the endodermis and the pericycle - before it finally reaches the xylem elements, at which point it moves on osmotically. Three distinct routes are available for this inward journey. The apoplast pathway uses only non-living tissue - the continuous network of cell walls, extracellular spaces, and the interiors of already-dead cells such as vessel elements and tracheids - so water moving this way never crosses a single living membrane, which makes it a comparatively fast, low-resistance route. The symplast pathway instead uses the interconnected mass of living cytoplasm of the plant's cells, linked from cell to cell by cytoplasmic channels called plasmodesmata; water on this route crosses a membrane only once, on entering the outermost root cell, and then travels onward through the cytoplasm of successive cells around their vacuoles without needing to cross any further membrane. The transmembrane route is the most membrane-intensive of the three: water here must cross at least two membranes fo …