PLANTS AND ANIMALS 67 
gases are respectively taken in and given out in such proportions 
that the amount of carbon dioxide in the air does not rapidly grow 
larger, and the amount of free oxygen rapidly get smaller, as 
would undoubtedly be the case if the results of breathing were not 
compensated. 
The action and reaction between organisms and the air also 
involve chemical processes which have to do with nitrogen, and in 
which a leading part is played by various bacteria which live in 
the soil, Green plants get the nitrogen which they require for 
feeding purposes in the form of dissolved nitrates, which are 
derived from more than one source. It is a familiar fact that 
ordinary earth or soil, such as is to be found in a garden, is more 
or less dark in colour, largely as the result of the presence of 
organic matter. This partly consists of the remains of organisms 
which have died and decayed, and partly of substances derived 
from the nitrogen-containing excreta of animals. The rotting, 
decomposition, or breaking down of such materials is the result 
of chemical changes brought about by certain bacteria in the 
presence of oxygen, with production of ammonia compounds. 
Another set of bacteria convert these compounds into salts known 
as nitrites, from which nitrates are then produced by the action of 
still another group of bacteria. The nitrates serve as food to 
green plants, which in their turn are devoured by animals. We 
thus see that by the death and decay of organisms material is 
produced which helps to build up the bodies of new generations. 
This, however, is not the only source of nitrates in the soil, for 
what are known as xztrzfying bacteria are there present, which 
possess the remarkable power of abstracting free nitrogen from 
the air, and causing it to enter into combination. There is 
another arrangement by which, in leguminous and a few other 
plants, the same end is attained. If, say, a pea- or bean-plant is 
dug up, and the earth washed away from its roots, these will be 
found to bear a number of rounded thickenings. Within each 
such “root-tubercle” live a number of microscopic fungi (possibly 
bacteria) that appropriate the free nitrogen of the air which circu- 
lates in the soil, employing it to build up nitrates. We have here 
a striking example of Mutualism (symbiosis), ze. the intimate 
association of two organisms for their common benefit. The 
leguminous plant has a supply of nitrates ready to hand, while 
the tubercle-fungus is sheltered, and no doubt nourished. 
