10 PLANT LIFE ON THE FARM. 



portions in the plant, while the quantity in any given 

 sample of the soil from which it must be derived is some- 

 times so small as to elude detection. The plant in this 

 case, or some part of it, is so greedy, if we may so say, for 

 this particular substance, that it absorbs all within its 

 reach, and stores it up in its tissues or uses it in some way, 

 the demand ensuring supply. On the other hand, the soil 

 may contain a large quantity of some particular ingredient 

 which is incapable of being absorbed, or which the plant 

 does not or cannot make use of, and, in consequence, none 

 is found within the plant. The supply is present, but there 

 is no demand. 



The diiferent physical requirements of the plant supply 

 also the explanation of the fact that diiferent plants, grown 

 in the same soil, supplied with the same food, yet vary so 

 greatly in chemical composition. Thus, when wheat and 

 clover are grown together, and afterwards analysed, it is 

 found that while lime is abundant in the clover, it is 

 relatively in small quantity in the wheat ; and silica, 

 which is abundant in the wheat, is absent from the clover. 

 Poisonous substances even may be absorbed, if they are of 

 such a nature as to be capable of absorption ; and so the 

 plant may be killed by its own action by suicide, as it 

 were. 



The entrance of water into the plant and the entrance of 

 those soluble materials which a plant derives from the soil 

 are therefore illustrations of the process of osmosis, and are 

 subjected to all the conditions under which osmosis becomes 

 possible, or under which it ceases to act. The study of these 

 conditions is a question for the physicist, and the full explana- 

 tion of them must be sought in works relating to physics. 

 So the investigation of the substances which are absorbed 



