I40 ROOTS AND UNDERGROUND STEMS 



Plants also obtain their supply of the various mineral 

 salts needed by them from solutions in the soil water which 

 they absorb through their roots. Different species, and 

 even different varieties of the same species, absorb these 

 substances in very different proportions, and upon this 

 fact, much more than upon the form of roots (Sec. 173), 

 depends the principle of the rotation of crops in farming. 



199. Plants can not choose their Food. — Substances are . 

 often found in plants which appear to be useless, and some, 

 as zinc and lead, which are positively harmful. This shows 

 that the roots are not able to choose their own nourishment, 

 but absorb whatever is present in the soil in soluble form 

 and can penetrate their cell walls. It is not safe, there- 

 fore, to conclude merely because a substance is found in a 

 plant that it should constitute a part of its food. Neither 

 can we always be sure that because a plant will grow in 

 a certain soil this is the best soil for it, since its presence 

 there may be due merely to adaptation or toleration, and 

 it might do better if given a chance somewhere else. All 

 these circumstances present matter for careful discrimina- 

 tion by the farmer. 



200. Food Manufacture. — The proportion of ash found 

 in green plants increases from the roots upwards to the 

 leaves, thus showing that the latter are the organs in 

 which the manufacturing or building-up process takes 

 place, and its products are most abundant there. The 

 first article of food to be recognized is starch, but others 

 also occur and are distributed to the parts where they are 

 needed. Of course solid substances like starch and the 

 various ashes that we find in the structure of plants can 

 not pass through the walls of the cells unchanged, but 

 must be reduced to the form of a solution. In the case of 

 substances that are insoluble they must first be transformed 

 to soluble ones and then reformed into their original con- 

 stitution, so we see that the nutrition of plants is a very 

 complicated process, involving repeated chemical changes 

 and redistributions of material. 



