2OO Elementary Biology. 



differently from the cell-wall, and hinders many substances 

 from passing inwards which the cell- wall would allow to 

 pass through. 



The chemical nature of the cell-contents influences to 

 a very great extent the nature and rapidity of the endosmotic 

 flow. It is believed that the organic acids and their salts, 

 always present in living cells, exert an attractive influence 

 on the water, although probably other substances as well 

 have a like, though not so great an, affinity for water. After 

 exosmosis of the water of the cell-sap, the protoplasm 

 contracts, leaving a space between it and the cell-wall. 

 This phenomenon is known as plasmolysis, and takes place 

 when the cells are treated with a solution of a substance 

 having a strong affinity for water. 



We have now to examine the laws governing the absorp- 

 tion of gases by plants, 



From the composition of the atmosphere given at p. 41 

 it will be seen that it is a mixture of several gases, the 

 principal of which, from a biological point of view, are 

 oxygen and carbonic acid. It must be remembered that 

 these gases are in no way chemically united, but are simply 

 mixed completely and equally, in obedience to the law of 

 diffusion, viz. that gases mix equally and in all proportions. 

 The method by which these gases are absorbed by the plant 

 cell is the same as that by which solids are absorbed. No 

 gas can be assimilated unless it be first of all in solution in 

 water or in cell-sap. The solubility of gases in water is, 

 therefore, a subject of extreme importance in a discussion 

 of the nutrition not only of aquatic, but also of terrestrial 

 plants (p. 42). 



(a,) Absorption of water and dissolved salts. As 

 already seen (p. 196), the soil consists of a mixture of a 

 great many chemical compounds obtained from the dis- 

 integration of rock masses, and also from the ultimate 

 decay and disintegration of animal and vegetal matter. 

 The particles of which the soil is made up are surrounded 



