FUNGI 



393 



There are a number of reasons why it does, but the prin- 

 cipal reason is thought to be that such practice keeps the 

 soil from becoming too poor in nitrates. You remember 

 that leguminous crops, like clover, increase the nitrogen 

 supply on account of the work of the bacteria in their 

 tubercles. 



The effects which true fungi produce upon the fertility 

 of soil are not yet well understood, but it is known that 

 their hyphae penetrate everywhere in soil 

 which contains organic matter, and they 

 produce many chemical changes in such 

 soil. Certain soil fungi become closely 

 attached to the roots of other plants, 

 forming a growth which is called myco- 

 rhiza. (The word means root fungus. 

 See Figure 190.) 



This mycorhiza, which is very common 

 upon the roots of forest plants, is believed 

 to be an advantage both to the fungi and 

 to the roots on which they grow. The 

 fungi are believed to derive nourishment 

 from the roots, and the roots are believed 

 to use the filaments of the fungi some- 

 what as they use their own root-hairs. On account of this 

 direct connection with true fungi of the soil, the power of 

 roots to absorb water and solutes is greatly increased. 



Fig. 190. — Mycorhiza. 

 The tip of a beech 

 root covered with a 

 close mass of fungal 

 hyphae. 



F. Lichens. — Probably you have noticed on rocks and 

 trees brownish and greenish patches which are formed by 

 structures having the texture of very thin leather. They 

 stick closely to whatever they grow upon. These structures 

 are lichens. (See Figure 191.) Lichens are the other group 



