126 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 
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- 
