CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 695 



conditions, showed that the former contained 27 per cent, of silica, and the 

 latter scarcely 7 per cent. This is due to the fact that the Grasses deposit 

 considerable quantities of silica in their cell-walls, whereas Peas are unable to 

 do this to anything like the same extent. 



It not infrequently happens that chemical elements are found to 

 be present in the ash, which are known to be present in the 

 soil in the form of compounds which are insoluble in water. 

 These compounds are brought into solution by various means. 

 For instance, the soil usually contains carbon dioxide, which is 

 evolved from the decomposing animal or vegetable matter which 

 is commonly present, and from the absorbent organs themselves ; 

 and it is Avell known that various substances, such as calcium car- 

 bonate and certain silicates, which are insoluble in pure water, 

 are soluble in water charged with carbon dioxide. Again, the sap 

 which fills the vacuoles and saturates the walls of root-hairs, has 

 an acid reaction, due to the presence of organic acid, and this acid 

 sap will dissolve many substances which are insoluble in pure 

 water. The solvent effect of this acid sap is well demonstrated by 

 means of the familiar experiment with a slab of marble. If a 

 highly-polished slab of marble be fixed in the bottom of a flower- 

 pot, and a plant be grown in the soil above it, the roots will come 

 into contact with the slab and will apply themselves to its surface. 

 On subsequently examining the slab of marble, it will be found to 

 have become corroded where the roots had been in contact with 

 it. The corrosion is due to the solution, by the acid sap of the 

 roots, of particles of the marble. 



The activity of the absorption of water is dependent upon the 

 temperature. For example, the absorption of water by roots be- 

 gins at a minimum temperature ; it increases in activity as the 

 temperature rises to optimum ; any further rise of temperature is 

 attended by a diminished activity of absorption. 



Absorption of Gases. The absorption of gases depends, like the 

 absorption of water and substances in solution, upon diffusion. 

 Supposing an absorbent cell, the sap of which holds, to begin with, 

 no gases in solution, to be brought into relation with a mixture of 

 gases, these gases will be dissolved at the surface in proportion to 

 their solubility and to the amount of each gas present in the mix- 

 ture, that is, the amount absorbed of each gas depends, in the first 

 instance, upon its solubility and its partial pressure. Tlie relative 

 amount of each gas absorbed over a pei'iod of time will further 

 depend upon the extent to whicli it undergoes chemical alteration 



