SECT, ir 



PHYSIOLOGY 



183 



a stretching of the cell-Avall. Naturally this condition cannot be 

 maintained ; in the end the same result must be reached as in the 

 former case, viz. a uniform distribution of the water and salts through 

 the whole space. When this is attained the increase in volume of the 

 cell is again lost. A third case is essentially distinct : when the 

 membrane consists of a substance which is permeable to water but 

 impermeable to salts. If such a semi-permeable substance is employed, 

 a condition of tension must again result, but in this case it is 

 permanent, since diffusion of the salts outwards can never take place. 



In the vegetable cell itself the cell-wall is completely permeable. 

 The layer of protoplasm applied to it, on the other hand, is more or 

 less semi -permeable, at least so long as it is 

 living. As a result of this there is a one-sided 

 passage of water into the vacuole without any 

 corresponding passage outwards of salts. A 

 further result is the pressure of the cell-contents 

 on the protoplasmic sac and through it on the 

 cell -wall. The protoplasm becomes stretched 

 under this pressure (turgescence, osmotic pres- 

 sure) without much resistance, but the cell-wall, 

 by virtue of its elasticity, exerts a considerable 

 counter-pressure. In many cases the cell-wall 

 is stretched by the pressure some 10 per cent 

 to 20 per cent, in extreme cases even 50 per 

 cent, and it contracts when the pressure ceases. 

 By the distension the cell- wall becomes more rigid, 

 just as a thin india-rubber balloon when air is 

 forced into it resists changes of shape. The 

 increase of rigidity of the plant, by reason of 

 the tui'gor-pressure or turgescence, is very im- 

 portant ; it is the simplest, and in many cases 

 the only waj^, in which the cell becomes rigid. 

 This is dependent naturally upon the presence 

 of a sufficient supply of water ; if a distended 

 cell is taken from the water and allowed to 

 give up water in the air, the stretching of 

 the wall disappears, and with this the rigidity. 

 The cell will wither. With a fresh supply 

 of water the turgescent condition can be 

 restored. 



The internal pressure of the cell can be 

 diminished in other ways than by the evaporation of the water ; for 

 instance, when the protoplasm is killed and its semi-permeable nature 

 destroyed, or when a prick is made in the cell. In both these cases 

 the shortening of the cell referred to above would take place (Fig. 176). 

 The same result can be obtained without destroying the internal 



Fif,. 176. — Internodal cell of 

 NitcUa. F, Fresli an J tur- 

 gescent ; p, with turgor 

 reduced, flaccid, shorter 

 and smaller, the proto- 

 plasm separated from the 

 cell - walls in folds ; ss, 

 lateral segments. ( x circa 



