The absorption of water by the root hair does not depend solely upon its osmotic concentration but also upon its osmotic potential which is equivalent to the difference between its osmotic potential and its pressure potential. If the root hair becomes fully turgid, in order that its osmotic pressure becomes zero, it will absorb no more water from the soil.
But if there is a steady movement of water out of the root hairs into the interior of the root, the absorption of water by the root hairs will continue. The lateral movement of water from the root hairs across the cortex of the root, through the endodermis and pericycle and into the xylem vessels takes place as follows (Fig. 7-5).
Ψπ = Ψπ – Ψp
As soon as the suction pressure of the root hair cell falls below that of the cortical cell B next to it, water passes from cell A to cell B. Cell B now becomes more turgid and its suction pressure falls below that of the next cortical cell C and water passes from the cell B to the cell C.
In this way, water passes from the cell C to the other cortical cells and from these into the endodermal cell E and the pericycle P, which is the innermost living cell. Soon the pericycle cell becomes turgid, its osmotic pressure falls to zero and it will not absorb water from the adjoining cells, if water from it fails to move into the xylem vessel.
As soon as its suction pressure falls below that of the sap of xylem vessels, with which it is in contact, water passes from it into the xylem vessel. This results in the maintenance of the absorptive capacity of the inner living cells.
The walls of the xylem vessels being non-elastic, its pressure potential are zero and its Ψ is equal to the whole of its osmotic pressure. This being higher than the reduced osmotic potential of the pericycle cell, water will be drawn into the xylem.
Thus, the movement of water from the soil to the xylem ducts via root hair and cortex is due to the presence of an increasing gradient of Ψ from the root hair to the xylem vessels. In the process one may regard all the layers of the living cells as constituting a single semipermeable membrane separating soil solution on the outside and the solution of xylem vessel at a higher concentration on the inside.
The water is drawn in as a result of difference between the osmotic potential of the soil solution and that of the xylem sap, irrespective of the magnitude of the osmotic potentials of the cells of cortex.
Once in the xylem of the root, water moves in the upward direction. The xylem tissue of the roots connects directly with the xylem tissue of the stem, thus allowing water to move out of the root and into the stem.
The xylem of the stem is divided and subs-divided many a times to form a network of water- conducting tissues, finally ending in the tiny veins of the leaf. Water moves from the leaf veins into the mesophyll cells, and is evaporated from their surface and finally moves as vapours through the stomata into the surrounding atmosphere (Fig. 7-4, 5).
It appears that most water absorption occurs through the mediation of osmotic mechanisms i.e., water is taken up passively. However, active, non-osmotic uptake of water also occurs and that metabolic energy is expended in the process.