SOIL MICRO-ORGANISMS 391 



The cereal, cruciferous, leguminous and solanaceous 

 plants are not associated with mycorhiza. Mycotropic 

 plants are usually those that live in a humus soil 

 filled with the mycelia of fungi. It is thought that 

 the mycorhiza aid the higher plants to obtain nutri- 

 ment that they must strive for in competition with 

 the fungi. 



Mycotropic plants are also able to grow with a very 

 small transpiration of moisture, as is well known to 

 be the case with many conifers; and this restricted 

 transpiration would doubtless result in lack of nutri- 

 ment were it not for the assistance of the mycorhiza. 



261. Plant roots. The roots of plants assist in pro- 

 moting productiveness of the soil both by contributing 

 organic matter and by leaving, upon their decay, 

 openings which render the soil more permeable to 

 water and which also facilitate drainage and aeration. 

 The dense mass of rootlets, with their minute hairs 

 that are left in the soil after every harvest, furnish 

 a well-distributed supply of organic manure, which is 

 not confined to the furrow slice, as is artificially incor- 

 porated manure. The drainage and aeration of the 

 lower soil, due to the openings left by the decomposed 

 roots, are of the greatest importance in heavy soil, 

 and the beneficial effects of clover and other deep- 

 rooted plants are due in no small measure to this 

 function. 



II. MICRO-ORGANISMS OF THE SOIL 



Of the micro-organisms commonly existing in 

 soils, the great majority belong to plant rather than 



