420 TRANSFORMATION OF ENERGY 



It is easily understood how far osmotic pressure has an effect on the 

 rigidity of the cell-wall. The pressure stretches the delicate cell-membrane until 

 its elasticity equals the pressure, then the wall resists all further attempts at 

 deformation, and as a result the cell becomes more rigid. Increase in rigidity 

 owing to stretching is well exemplified by the behaviour of a thin-walled caout- 

 chouc balloon, which, when blown out, maintains a constant shape but which 

 without such extension of its wall is by no means firm. Osmotic pressure as an 

 agent in the production of rigidity is, however, but little employed in the plant ; 

 it occurs in the lower forms which live in water or in moist air, but in the higher 

 plant only in young parts still capable of growth. Later on, secondary thicken- 

 ing in the cell- wall takes on the duty of maintaining rigidity ; such secondary 

 thickening would be distinctly disadvantageous in vigorously growing organs. 

 Certainly dependence on osmotic pressure for the maintenance of rigidity in such 

 organs has its dangers, for on a warm summer's day they become limp, that 

 is to say their rigidity has been destroyed by excessive evaporation. 



It is of importance for us now to know to what extent the cell-wall may be 

 osmotically stretched. This is determined by the amount of contraction that 

 takes place when turgor is neutralized. Turgor may be arrested by wilting, 

 by killing the cells in hot water, or by plasmolysing them. It will then be seen 

 that all growing ceU-walls are markedly stretched, so much so that, as a rule, a 

 contraction in length from 3 per cent, to 20 per cent, and about 10 per cent, 

 reduction in diameter take place when turgor is abolished (De Vries, 1877 ; 

 ScHWENDENER and Krabbe, 1898). If we stretch the walls of plasmolysed cells 

 by means of a weight until they have attained the same length that they had in the 

 turgid condition we are obviously able to determine the amount of the osmotic 

 pressure in the cells, apart from the values of the osmotic activity of cane sugar, 

 obtained by experimental apparatus, and apart from the use of the plasmolytic 

 method. 



In full-grown cells the extensibility of the cell-wall is so limited that an 

 observable contraction after plasmolysis scarcely exists. Exceptional cases are 

 known, however, for fully grown cells exist which exhibit highly extensible walls. 

 Such cells occur in the leaf articulations, and we shall see later what an important 

 part they play in the movements of many leaves. At present it need only be 

 noted that such cells occur also in the stamens of such plants as the Cynareae. 

 Indeed, these cells are among the most extensible known in the whole vegetable 

 kingdom, for Pfeffer (1892, p. 234) found that when plasmolysed they con- 

 tracted to half their original length ; only in the seeds of Haemanthus perhaps 

 have cells with still more extensible cell-walls been observed (Hildebrand, 1900). 



The determination of osmotic pressure, it may be said here, cannot be 

 carried out offhand by the plasmolytic method in cells which exhibit as great 

 extensibility as those of the Cynareae do. Plasmolysis indicates to us, indeed, 

 the occurrence of osmotic pressure in the cell that has contracted to half the 

 length it was in the turgescent condition, and in which consequently there is 

 relatively twice as much osmotic material as at first. The value for the osmotic 

 pressure so obtained must be in this case estimated at one half of that. It 

 would appear also that in cases where the contractions amount only to 10-20 

 per cent, of the original lengths, corresponding corrections must be made on 

 values determined by plasmolysis, corrections which can only be reached by 

 exact calculation of the decrease in the cell volume in each case. 



From the action of osmotic pressure on the cell- wall just described there 

 arises the possibility of movements of the cell. Looking first of all at a single 

 cell we can see that a simple elongation, that is to say a rectilinear movement, 

 will take place either when osmotic pressure increases, or when the wall becomes 

 more extensible. Similarly, reduction in osmotic pressure, accompanied by con- 

 traction of the cell- wall, will also lead to shortening in a rectihnear direction. 



