198 PLANT LIFE 
owe their faculty of “ fixing” free nitrogen 
to the energy which the chlorophyll enables 
them to obtain from the sunlight. This is 
the motive power which enables the machinery 
of the green leaf to maintain its output of 
carbohydrate, and it is from this carbohydrate 
that the power or energy is more immediately 
derived which enables the bacillus to perform 
the tremendous operation of forcing free 
nitrogen into combination, and thus to build 
up from the raw materials the stuff from which 
protoplasm itself can be made. Although 
the leguminous root ultimately profits by 
its relations with the bacillus in thus ac- 
quiring a costly food in exchange for a cheap 
one, there is no indication of any degeneration 
of leaf structure on the part of the flowering 
plant. It even becomes almost unthinkable 
that it could occur, inasmuch as the continuous 
supply of carbohydrate from the green parts 
is a prime condition of the nitrogenous 
synthesis. The importance to the organic 
world of these plants which bring nitrogen 
into combination in a form that can be 
utilised by living beings is overwhelming. 
For apart from some means of maintaining 
the supplies of nitrogenous food, life itself 
would ultimately cease to be possible in the 
world. 
There are many other instances of remark- 
able associations of two or more plants, in 
which each is in turn more or less parasitic 
on the other, or, at the least, lives on the 
