64 



ELEMENTARY STUDIES IN BOTANY 



case, the loss of nitrogen is stopped and nitrates are allowed 

 to accumulate ; in the latter case, fresh drafts on the air, 

 the ultimate source of nitrogen, are made. There are other 

 reasons why a soil improves by " resting " or by a clover 

 crop, but these will be referred to when the crop plants are 

 studied. In this connection we are merely concerned with 

 the work of Fungi in the soil. 



The true Fungi of the soil, with their network of filaments 

 extending everywhere through the soil, have proved to be 



of much greater importance than 

 was once supposed. These subter- 

 ranean mycelia become connected 

 with the roots of many plants (Fig. 

 50), from which they obtain their 

 food-supply, and in so doing put at 

 the service of the host plant an ex- 

 tensive water-receiving and salt- 

 receiving system. Such plants as 

 orchids and oaks have long been 

 notorious for using subterranean 

 mycelia in this way, but it is found 

 now that this arrangement is of 

 great advantage to any plant. 

 Especially does this habit prevail 

 in forest soil, which is " a living mass of innumerable 

 filamentous Fungi." In effect, this connection established 

 between an oak, for example, and the subterranean mycelia, 

 resembles the establishment of a connection between the pipe 

 systems of a house and those of a city. With such a con- 

 nection, soil water and soil salts far beyond the reach of the 

 root system of a plant become available. Such Fungi are 

 called mycorhiza, a name meaning " root-fungi." 



41. Summary. The Fungi are Thallophytes that can- 

 not manufacture food, and therefore are dependent plants. 

 The dependent habit compels them to obtain food as 



FIG. 50. Mycorhiza : the tip 

 of a beech rootlet enmeshed by 

 a soil fungus. After FRANK. 



