Water is first absorbed by the root hair cell from the soil due to a pressure gradient. As a result, the root hair cell becomes turgid and its DPD decreases. The adjacent cortical cells have a higher DPD as they have less water. As a result of this, water moves into the cortical cells increasing its TP. The next cortical cell draws water from this cell because it has less TP
In this way, through a series of concentration gradient water flows from one cortical cell to the other and finally reaches the endodermis. According to Sutcliffle (1969), the movement of the water through cortex is in response to water potential gradient.
There are two ways of radial movement of water- (i) Cell to cell movement across the root and (ii) movement of water along the water filled spaces in the cell wall forming a continuous system from soil water upto the endodermis. The flow of water meets with an impregnable barrier in endodermis as its cells have thick strips on the radial and tangential walls called casparian strips.
However there are certain thin walled cells in between the endodermal cells called passage cells. It is through these cells, water enters into the stele. The passage cells are usually positioned opposite the protoxylem elements.
ADVERTISEMENTS:
The passage cells allow the entry of water into the cells of the pericycle, which in turn become turgid. The protoxylem elements situated next to the pericycle always have a higher DPD and they absorb water from the cells of the pericycle. There are various views regarding the unidirectional flow of water into the xylem elements.
According to Lepeschkin, the parenchyma cell next to the xylem element has differential permeability at different sides. The side towards the xylem element is more permeable allowing the entry of water into it. According to Priestley, the xylem elements always have a higher DPD due to transpiration and hence draw water from the surrounding cells.