PLANT NUTKITIOST. 15 



relative proportions in the plant, while the quantity in 

 any given sample of the soil from which it must be de- 

 rived, is sometimes 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 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 different physical requirements of the plant sup- 

 ply also the explanation of the fact that different 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 

 analyzed, 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 ab- 

 sorbed, 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 os- 

 mosis becomes possible, or under which it ceases to act. 

 The study of these conditions is a question for the physi- 

 cist, and the full explanation of them must be sought in 

 works relating to physics. So the investigation of the 

 substances which are absorbed with the water, of the 

 food materials, and their transformations within the 

 plant, is the work of the chemist, and their history must 

 be sought in chemical books. 



