132 VEGETABLE PHYSIOLOGY 



CHAPTER IX 



THE ABSORPTION OF FOOD MATERIALS BY A GREEN PLANT 



We have seen that the materials which protoplasm is 

 eventually able to assimilate or incorporate into its own 

 substance, and which, therefore, constitute its food, are of 

 a similar nature to those deposited in seeds and other store- 

 houses of nutriment. We know further that these are not 

 the materials which an ordinary green plant absorbs from 

 the environment in which it lives. We know also that its 

 structure prevents its taking in anything in a solid form, 

 and that, therefore, everything entering it must either be in 

 solution in the water which it is almost constantly absorb- 

 ing through its roots, or must become dissolved in the 

 liquid which permeates the walls of the cells which line the 

 intercellular passages. The only substances that can be 

 taken up under these conditions are certain gaseous con- 

 stituents of the air, and various inorganic salts which are 

 present in the soil. Between such raw materials and the 

 complex products which are needful for the nutrition of its 

 substance there is a great difference, and the manufacture 

 of the latter from the crude materials absorbed constitutes 

 a very important part of the metabolic processes. 



There are several ways in which we may proceed to 

 discover what a green plant absorbs from the soil, two of 

 which especially have been made use of by various observers. 

 The first is known as the method of water-culture. It 

 consists in cultivating plants with their roots inserted in 

 water containing various salts in solution, and observing 



