Roots. 39 



26. Free Nitrogen made Available. Although about 

 seventy-nine parts in one hundred of the atmosphere con- 

 sist of free nitrogen, plants, with the exception of certain 

 microscopic forms, are not able to use it for food until it 

 has been combined with other elements to form some solu- 

 ble compound, such as nitrates and ammonia. These are 

 obtained largely from the decomposing remains of plants 

 and animals. It has long been known that clover, when 

 plowed under, leaves the soil much richer in nitrogen, 

 and the reason for this is now well understood. There 

 are bacteria residing in the tubercles of the clover roots 

 (see Fig. 13), and they in some way combine the free 

 nitrogen of the atmosphere with the other necessary food 

 constituents provided by the clover plant, and use. the sub- 

 stances thus formed as food. After a time the bacteria 

 become disintegrated, and are apparently absorbed by the 

 clover," which, in this roundabout way, obtains the nitrogen 

 after it has Been combined with other substances to form 

 proteids. Finally, the clover decays and yields its com- 

 bined nitrogen to the soil. By this remarkable cooperation 

 of two widely different kinds of plant life the free nitrogen 

 of the atmosphere is made available to all kinds of plants. 



27. Extent of Roots. The roots with their rootlets and 

 root hairs form a dense plexus threading the soil in all direc- 

 tions. Being buried in the soil, their great extent is not 

 easily apprehended. It has been estimated that if all the 

 roots and rootlets of a single corn plant grown under good 

 field conditions were placed end to end they would cover* a 

 linear mile. The roots of some plants extend to great 

 depths, and these plants are thus able to obtain water and 

 continue fresh and green after the surface soil has become 

 dry and plants with shallow roots have withered away. 

 The roots of alfalfa, for instance, sometimes penetrate the 



