CHAP. II.] CHEMISTRY, ETC., OF PLANT LIFE. 59 



soili that rarely contains more than three per cent, of 

 substances in solution, consequently the latter passes 

 into the substance of the plant by endosmose. 



It is a well-known fact that the ash of different kinds 

 of plants growing close together in the same soil or water, 

 may and does differ considerably in composition ; this 

 at first sight suggests a certain selective power possessed 

 by plants, absorbing certain substances and rejecting 

 others; when all the facts are analyzed, however, there 

 is no evidence of any selection due to the presence or 

 influence of the life of the organism, and the entire 

 selective process can be fully explained by the laws of 

 diffusion as previously explained. A given substance 

 held in solution by the water in contact with the root of 

 the plant will continue to diffuse into the plant utitil 

 equilibrium is restored ; if the substance is not consumed 

 and chemically changed by the plant, the equilibrium 

 once established remains permanent, and no more of the 

 substance enters the plant, whereas if the substance is 

 at once chemically changed on entering the plant, as in 

 the case of carbonic dioxide already explained, a constant 

 inflow is maintained in the endeavour to restore equi- 

 librium. Since the chemical work done by different 

 plants is varied in its nature, the difference in composi- 

 tion of the ash of different plants can be readily under- 

 stood. 



The above explanation will illustrate the object of the 

 rotation of crops as practised by the farmer. The various 

 food substances derived from the soil by plants are as a 

 rule only soluble in very small proportions in rain water, 

 the eflBcacy of which to promote this condition depends 

 in many instances on the presence of small proportions 



