WATER RELATIONS OF THE CELL As is shown above, under the heading of Cytology, the mature cell consists of a cell-wall lined inside with a thin layer of proto plasm, which itself encloses a central space, the vacuole, filled with cell sap. (See above, Cytology.) The sap contains various dissolved substances, sugars, nitrogenous materials, a variety of inorganic salts and other substances which are sometimes of col loidal nature. Curiously enough the majority of the substances are highly soluble in water and yet if a slice of beetroot is placed water the amount of material passing out from the sap of the living cells is comparatively small as long as the cells remain alive; the cells on the other hand are capable of taking up water. We have then a peculiar condition of affairs in which water can enter the cell but the passage out of soluble substances present inside the cell is hindered. This effect is due, not to the cell wall which appears on the whole to be dead and inert, but to the living layer of protoplasm which is often known as the plasma membrane. This exhibits when living a condition of semiperme ability since it lets water readily pass in and out, but hinders the passage of certain dissolved substances such as those in the sap. Water is attracted into the cell by the dissolved substances inside just as it would be into a sugar solution enclosed within a pig's bladder, which also roughly is semipermeable allowing water to enter freely but being more or less impermeable to the sugar. As a result of the entry of water the cell becomes swollen up and turgid and exerts what is known as osmotic pressure on the elastic cell wall or membrane which is thus stretched. Whole tissues of the plant are thus given a rigidity which is not due to their actual mechanical strength. The dependence of this rigidity on plentiful supplies of water is well seen in the case of leaves and the stems of herbaceous plants which often wilt, i.e., become soft and flaccid, on a hot summer's day. Directly so much water is lost that the wall is no longer expanded, the rigidity of the cell must completely disappear.
plant cells and those of the order of i so atmospheres have been observed. These pressures seem large but owing to the very small size of the cell the actual forces concerned are small.
Osmotic Pressure, Turgor Pressure and Suction Pressure. —When a cell is supplied with water it goes on absorbing and at the same time expanding with the result that the backward pres sure of the stretched and elastic cell wall becomes greater and greater, until finally it equals the osmotic pressure of the cell contents ; this backward pressure of the cell wall is known as turgor pressure. The cell is then fully expanded and fully imbibed and can absorb no more water. If on the other hand the cell is not fully imbibed the osmotic pressure is greater than the turgor pressure and the cell will tend to suck or draw water into the cell. When the cell is flaccid, as in the plasmolysed condition, there is no turgor pressure. It is clear that the difference between the osmotic pressure and the turgor pressure measures the pull with which the water is drawn into the cell; this difference is known as suction pressure. The term water absorbing power is sometimes employed but the first is by far the more satisfactory term since we are dealing with a pressure and not a power. If P= the osmotic pressure of the cell sap, T= the turgor pressure, and S the suction pressure, then P —T = S. We see then that the suction pressure is that fraction, if any, of the osmotic pressure left over from balancing the turgor pressure (backward pressure of the wall). When the cell is fully imbibed with water then P =T and so P—T=o and there is no suction pressure. When the cell is flaccid T=o and therefore P=S, i.e., the suction pressure is equal to the full osmotic pressure.
As the suction pressure is such an important quantity the question arises as to its measurement. As it represents the pres sure with which water is drawn into cell from outside it is evident that if we just balance this pressure inside the cell by an equal pressure outside, the cell will neither take in nor give out water, and therefore will neither expand nor contract, i.e., will remain unaltered in volume. This consideration is the basis of the method of measurement. The cell, the suction pressure of which is to be determined, is placed in purified paraffin oil under the micro scope and its volume measured ; this gives its normal volume since it does not absorb the oil nor does it lose water while immersed in the oil. The cell is then removed and placed in various strengths of cane sugar solution or some other plasmolysing agent until a concentration is found at which the volume is the same as meas ured in the oil. If the cell does not change in volume while lying in a watery solution it must be that its suction pressure is op posed by an equal pressure in the opposite direction. This counter pressure must be the osmotic pressure of the cane sugar solution which just keeps the cell at its original volume. The osmotic pressure of this solution thus gives the suction pressure of the cell in the state in which it was investigated; like the osmotic pressure it is usually expressed in atmospheres.