DIV. II 



PHYSIOLOGY 



225 



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 pressure) without much resistance, but 

 the cell wall, by virtue of its elasticity, exerts a considerable counter- 

 pressure. This puts a limit on the absorption 

 of water by the cell. It ceases when the 

 amounts of water entering and forced through 

 the distended membrane in a unit of time are 

 equal. 



It is not necessary to go further into the 

 question of the water-content of protoplasm. 

 It is also necessarily limited, since the proto- 

 plasm is under pressure on the one side from 

 the cell sap, and on the other from the cell 

 wall. 



The distension of the cell wall is often con- 

 siderable and depends on the amount of the 

 internal pressure and the elastic properties of 

 the cell wall. 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. 

 When the cell is pricked or the protoplasm 

 killed, the pressure is removed and the wall con- 

 tracts (Fig. 237). 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 turgor pressure or 

 turgescence, is very important ; it is the simplest, 

 and in many cases the only way, 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 wilts. With a fresh supply of 

 water the turgescent condition can be restored. So long as a cell does 

 not possess its maximum water-content it acts as a suction-pump, the 

 degree of suction depending on the deficiency in water. Under such 

 circumstances it will be evident that cells with highly-concentrated cell 

 sap will develop the greatest power of suction. 



Many chemists regard every molecular watery solution as having 

 a definite osmotic pressure, whether this is actually effective towards 



Q 



FIG. -J37. Internodal cell of 

 Nitetta. F, Fresh and tur- 

 gescent ; p, with turgor 

 reduced, flaccid, shorter 

 and narrower, the proto- 

 plasm separated from the 

 cell walls in folds ; ss, 

 lateral segments. ( x circa 

 6. After NOLL.) 



