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 im 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 
