THE MATERIAL OF PLANTS 17 



leaves, which cut like knives when drawn care- 

 lessly through the hand. This device may well be 

 a protection against animals, for they will not only 

 not eat such plants, but avoid walking among them. 



The plant may pick up almost any salt in the soil 

 penetrated by its roots, of even such poisonous sub- 

 stances as zinc, antimony, arsenic, or copper. The 

 wood of large numbers of trees growing in the 

 regions which haive copper-bearing rock in the soil 

 may contain as much as one per cent of their dry 

 weight in copper, although this metal is of no use to 

 the plant and is slightly poisonous to it. The same 

 is true in regard to zinc and arsenic. In general, it 

 may be said that no substance is taken up in great 

 proportion unless used as food. 



Animals, especially man, take in substances in 

 food which do not actually enter into the tissues of 

 the body, but promote the digestion and use of the 

 necessary elements. The same will apply in some 

 degree to plants. It is necessary for the proper 

 absorption and use of the indispensable food sub- 

 stances that others should be present with them 

 which are not used. Thus the plant does not use 

 sodium in constructing protoplasm, yet it should 

 have this substance in its food solutions. 



