MOVEMENTS DUE TO TURGOR AND GROWTH 419 



Speaking generally, we may say then that in ordinary plant-cells the osmotic 

 pressure is equivalent to from five to ten atmospheres, but that variations 

 both above and below this average are not infrequent. The pressure in ordinary 

 parenchymatous cells, if they be in an extremely starved condition (Stance, 

 1892, 391), does not fall below 3-5 atmospheres, and even in the cells of leaves 

 which have fallen off and are becoming yellow a quite obvious osmotic pressure 

 may still always be recognized. Whether or not a far less pressure occurs in 

 the tubers of artichoke than in other cells, as H. Fischer (1898) and Copeland 

 (1896) affirm, requires further confirmation. Examples of great osmotic pres- 

 sure, in addition to those quoted at the end of the table given above, are to 

 be found in the onion and beet, which possess large reserves of grape and cane 

 sugar, and in which pressures of fifteen to twenty-one atmospheres have been 

 registered, but the maximum pressure has been recorded in the nodal cells of 

 grasses, where Pfeffer has observed (1893,399) an osmotic pressure amounting 

 to as much as forty atmospheres. Greater pressures even than these, which 

 occur only under certain conditions, will be referred to afterwards. 



It has already been pointed out that the osmotic pressure in a cell never 

 remains constant ; continual variations or adaptations are for ever taking place 

 in it. When the cell grows the absorption of water leads to a reduction in the 

 concentration of the cell-sap, and consequently to a reduction in osmotic pres- 

 sure, but if such a reduced pressure does not make its appearance, or appearing 

 does not continue, it may be assumed that a re-formation of osmotic substance 

 has taken place which rapidly leads to the re-establishment of the pressure 

 previously existing. Far more remarkable adaptations are obtainable by cultivat- 

 ing the cells in concentrated media, for we have already seen that the pressure 

 rapidly rises and may reach the enormous pressure of 150 atmospheres. An 

 internal pressure as great as that is, of course, possible only if the external liquid 

 be capable of exerting a strong osmotic pressure ; for instance, if we put a cell 

 which has been lying in a highly concentrated sugar solution into water the 

 internal pressure may operate unilaterally and be sufficiently great to burst the 

 cell. Such ruptures of cells, as we shall presently see, occur normally in some 

 cases during development, but, generally speaking, osmotic pressure is regulated 

 in such a way that the cell-wall is stretched only up to its limits of elasticity. 



Let us now glance at the significance of osmotic pressure. In many cases, 

 e. g. in the beet and onion, great osmotic pressure is to be regarded as a secon- 

 dary and undesirable result of the accumulation of large quantities of reserves, 

 so much so that in most reserve stores an effort is made, by changing these 

 bodies into others which are insoluble and which have large molecules (e. g. 

 starch), to reduce the osmotic activity of the cell-sap. In other cases it is quite 

 likely that such high osmotic pressures are of some service to the plant. 

 Apart altogether from the fact that, generally speaking, osmotic pressure would 

 appear to be favourable to growth, one important function must be ascribed to 

 it, viz. that young cells attain by its means alone the necessary degree of rigidity. 

 In general, the rigidity of the elements which specially subserve mechanical ends 

 is attained by employing firm cell-walls, but Correns (1891) has shown that in 

 specific mechanical tissues osmotic pressure may play an important part, as in 

 the hairs of Aristolochia, whose articulating cells are thin- walled and maintain 

 the necessary rigidity only by marked turgor pressure (twenty-two atmospheres) . 



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