لخّصلي

In order to simplify this dynamics, changes in the osmotic pressure of the cell sap resulting from volume changes into the cells, as shown in the mentioned diagram will be disregarded as usually there are not great enough to modify seriously any generalized picture of the water relations of plant cells. As the movement of water through plant cells was discussed on the basis of osmotic theory (or behavior) where the plant cell is regarded as an osmotic system. Accordingly water will move (by diffusion) from cell to celi according to the gradual increase in the OP of cell sap, in other words from cell with lower OP to that of
higher OP.
However, in the movement of water from cell to cell in plants, it is the diffusion-pressure deficits and not osmotic pressures which tend to equilibrate water movement, this is only a special aspect of the fundamental tendency of the diffusion-pressure of water to attain a uniform value throughout any system. It is by no means impossible, therefore, water to move from a cell so higher to one of lower osmotic pressure.
When the DPD of two adjacent cells are dissimilar (and this naturally occur), a DPD gradient exists between them. Movement of water from one cell to another can occur only when such a gradient excises. Other conditions being equal the steeper this gradient, i.e. the greater the difference in DPDs, the more rapidly one cell gains water from the other. The term "diffusion pressure deficit gradient" can also be applied to a chain of cells in which the DPD increases aerially from cell to cell.
The above discussion which is based on osmosis was supported by facts that different organs or tissues of the same plant exhibit a wide range of osmotic pressures. Even similar organs on the same plant leaves e.g, may vary considerably among themselves in the average osmotic pressure of their cells. Furthermore, the tissues within the same organ usually show a considerable variation in osmotic pressures. The mesophyll cells of most leaves, eg. show higher values than the epidermal cells.
Generally, it is clear therefore that the osmotic pressure of root hairs is higher than that of soil solution, and there is a gradual increase in OP from epidermal cells to that of the cortex, to the pericycle in root cells. Also, it is evident that there is a gradual increase in OP from the cells of the root to that of the leaves. More or less regular daily and seasonal variations occur in the osmotic pressures of plant cells.