128 



PLANT LIFE. 



into food for the roots. (7) Bacteria which, as will be 

 seen in a subsequent chapter, form in themselves several 

 distinct groups. 



At every cultivation period all these seven popula- 

 tions are affected, and their relative arrangement is 

 disturbed. Birds, such as the partridge, starling, peewit. 



Fig. 13. — Nitko(;p:n Circui,ation. The free (n) nitrogen of air becomes 

 nitrates through bacteria (B5) of soil or (r6) those of leguminous root tubercles, 

 and as such enters the plant. The nitrogenous matter of an animal's body or its 

 excretions will become free nitrogen by the bacteria (B4) of denitrification in the 

 absence of oxygen, or in presence of oxygen by (B3) other bacteria will become 

 ammonium compounds. The la'ter become first nitrites by work of (R2) bacteria 

 and then nitrates by work of (ui) bacteria, and as such enter the plant. (See also 

 p. 202.) 



crow, and seagull destroy millions of insects in every 

 season ; and man and the larger animals also interfere 

 in their distribution. Under the best natural con- 

 ditions, there is a distinct tendency for the soil to 

 improve with every season; almost the entire organic 

 matter formed by the year's working remains in the 

 soil for the benefit of future generations ; for example 

 the nitrogen from a dead bird may traverse suc- 

 cessively the worm, two or three kinds of soil 

 bacteria, and the roots and leaves of plants : it may 



