138 SOILS AND M ANUSES 



bined nitrogen which accumulates in soils. The property 

 of fixing and assimilating the free nitrogen of the air is 

 shared by some of the soil bacteria. 



Moulds. Moulds are usually very abundant in cultivated 

 soils. They are either parasitic or saprophitic in habit, 

 and are often found closely associated with the roots of 

 higher plants. It has been held that this association is 

 of a symbiotic nature, but whether there is any true organic 

 union or not, the effects are highly beneficial. As inde- 

 pendent saprophytes they hasten the decomposition of 

 organic matter probably by the action of enz ernes or 

 soluble ferments and absorb the products. They grow 

 with extraordinary rapidity ; the soft cellular tissues of 

 which they are composed quickly decompose again, and 

 the nitrogen and other constituents become available for 

 the growth of higher plants. It is also possible that by 

 the action of moulds upon organic matter in the soil, the 

 products of decomposition may be, to some extent, 

 rendered directly available to the higher plants, i.e., with- 

 out being first absorbed by the moulds. This is certainly 

 accomplished by bacteria and probably also by yeasts 

 a group of fungi only slightly differentiated from moulds 

 on the one hand, and from bacteria on the other. 



BACTERIA. 



Reproduction. Bacteria are present in the soil in 

 enormous numbers, variously estimated 1 at from half a 

 million to one million per gram. They multiply by simple 

 fission. The process has been observed to take place, under 

 favourable conditions, in about twenty minutes. It has 

 been calculated that sixteen millions might be produced 

 from a single cell in twenty-four hours. They also form 

 spores. These are able to resist extremes of temperature, 



1 In some cases much larger numbers have been observed. 



