i8 METABOLISM 



pressure on the cell-wall as the sugar solution would, viz. about two and a half 

 atmospheres. We may, however, employ other substances to bring about 

 plasmolysis, for so long as we know the molecular weights of these substances 

 and their isosmotic co-efficients, the estimation of osmotic pressure by the 

 plasmolytic method presents no difficulty. Potassium nitrate has often been 

 used in place of cane sugar because it has been found that protoplasm is 

 frequently quite impermeable to that salt. A i per cent, solution of potassium 

 nitrate is equivalent to a 5-13 per cent, solution of cane sugar. 



All vegetable cells are not equally suitable for plasmolytic research. Young 

 growing cells have their membranes stretched owing to osmotic pressure, and 

 such cells contract, as we have seen, when that pressure is withdrawn. This 

 contraction must also take place in plasmolysis, and so complications in 

 endeavouring to estimate osmotic pressure in such cells are introduced, which 

 had best be avoided. But neither are all mature cells suitable for the purpose ; 

 often the first beginnings of plasmolysis are not readily observable, and yet 

 that is the point of importance. For plasmolytic research in general, rather 

 than the determination of osmotic pressure in specific cells, it is preferable 

 to employ mature cells containing a coloured cell-sap, in which the separation 

 of the protoplasm from the wall may be especially well seen. De Vries, 

 for example, recommends the epidermal cells from the underside of the leaf 

 of Tradescantia discolor, and this material is frequently used for this purpose. 



This is not the place to quote detailed statistics as to the absolute amount 

 of osmotic pressure (compare Lecture XXXIII) ; it will be sufficient to note 

 that pressures of five to ten atmospheres are by no means infrequent in the 

 plant. Deviations from such average pressures are known to occur, both 

 above and b^low the mean. Osmotic pressure does not appear, however, to 

 sink under three atmospheres in starved cells — whilst it may reach fifteen to 

 twenty atmospheres in such plants as the beetroot and onion. 



Young cells which contain no vacuoles also exhibit a turgor pressure. 

 In this case the osmotically active substance must be dissolved in the protoplasm; 

 later on, it preponderates in the vacuole, where it accumulates proportionally 

 as the volume of the vacuole increases pari passu with the volume of the cell in 

 the p -ocess of growth. 



In the cells of the beet and the onion, which we have instanced as ex- 

 amples of plants showing especially high osmotic pressures, the turgor is mani- 

 festly due to the useless activity of the accumulated reserves. When these 

 substances, as happens frequently in other regions of storage, are condensed 

 into larger molecules, e. g. starch, and rendered insoluble, then the osmotic 

 pressure disappears. In many other cells also it is possible that the accumu- 

 lation of reserves may bring about a pressure capable of producing perhaps 

 a quite undesirable secondary effect ; but that is not of general occurrence. 

 Osmotic pressure has frequently one very definite function to fulfil. By its 

 means, as we have already seen, the cell membrane is kept tense, and so long 

 as the tension is maintained the cell is more rigid than in the plasmolysed 

 condition. Just as inflation of a caoutchouc bladder renders it rigid, owing 

 to the stretching of the membrane, so osmotic pressure induces a corresponding 

 rigidity in a plant cell. In thin- walled growing cells this rigidity is maintained 

 entirely by osmotic pressure, and a small loss of water at once relaxes the 

 tension of the cell-wall and annuls the rigidity of the cell. The condition of 

 tension is termed turgescence and we speak of it as caused by turgor pressure ; 

 turgor pressure is identical with osmotic pressure. (As to the part played by 

 turgor pressure in growth, see Lecture XXI.) 



The net result of our discussion, looked at from the point of view of the 

 absorption of nutritive material, is that we have found protoplasm to be easily 

 permeable to water, but quite impermeable to many of the substances soluble 

 in it. This result appears all the more surprising when we investigate how 



