126 VEGETABLE PHYSIOLOGY 



CHAPTEE 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 

 storehouses of nutriment. We know further that these 

 are not the materials which an ordinary green plant takes 

 into itself from the environment in which it lives. We 

 know also that its structure prevents its taking in any- 

 thing in a solid form, and that everything entering it must 

 either be in solution in the water which it is almost 

 constantly absorbing 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 constituents 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 

 what effect upon their growth and development is pro- 



