PHYSIOLOGY 



167 



PLASMOLYSIS, or the contraction and separation of the protoplasm from 

 the cell walls, occasioned by the withdrawal of water, the tension of 

 the cell walls is decreased, and the cell becomes 

 flaccid and collapses (Fig. 167), although com- 

 pletely surrounded by an aqueous solution. If 

 placed in pure water, however, the previous 

 turgescence of the cells can be restored, that is, 

 if their protoplasm has not been too strongly 

 affected by the action of the solution. If the 

 protoplasm has been killed in the process, it 

 becomes permeable to water, and it is no 

 longer possible to set up an internal pres- 

 sure. Fresh sections of Beets or Carrots, 

 placed in water, give up none of their sugar 

 or colouring matter ; but after the protoplasm 

 has been killed (by cooking or freezing), the 

 sugar and colouring matter at once escape into 

 the surrounding water, and the sections lose 

 their firmness and rigidity. 



Through a knowledge of the strength of a solution 

 necessary to produce plasmolysis, a means is afforded 

 of measuring the internal pressure within plant cells. 

 For example, if a solution of saltpetre with an osmotic 

 pressure of 5 atmospheres (a 1 per cent solution, 

 according to Pfeffer's investigations, gives rise to a 

 pressure of about 3J atmospheres) is just sufficient to 

 overcome the turgidity of a plant cell (which in the 

 case of stretched elastic cells shows itself by the limit 

 of contraction being, reached), then, conversely, the 

 cell sap exerts upon water an equivalent eudosmotic 

 pressure. The force required to forcibly stretch a 

 flaccid or plasmolysed organ to its original length 

 furnishes also a rough means of estimating the pressur 

 tissues. 



Fig. 167. — Internodal cell of 

 Nitrfla. F, Fresh and 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 

 6.) 



e developed in turgescent 



In the tension produced by turgidity we see how purely physical 

 processes determine the rigidity of plants. These PHYSICAL processes 

 are, however, dependent upon the vital functions of plants, inasmuch 

 as they can only be called into action by living protoplasm. Living 

 plant cells have thus power to regulate the physical effects of osmotic 

 pressure by increasing or diminishing, or even suddenly overcoming 

 their turgidity (c/. Movements of Irritability, p. 269). It will also be 

 apparent, in considering the operation of other physical forces, that 

 the primary and essential result of the vital action is to give rise to 

 the operation of physical processes, to favour, constrain, or vary them 

 in such a way that they become of service to plant life. 



Tension of Tissues, — The rigidity of parenchymatous tissue, 



