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 nitrifying 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), i.e. 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. 



