THE FOOD OF PLANTS 



89 



oaks and their allies, etc. (Figs. 76, 77). The delicate 

 branching filaments (hyphae) of the fungus spread through 

 the soil, wrap the rootlets with a mesh of hyphae, and pene- 

 trate into the cells. It seems 'clear that the fungus obtains 

 food from the rootlet as a parasite ; but it is also thought 

 that the hyphal threads, spreading widely through the soil, 

 are of great service to the host plant 

 in aiding the rootlets in absorbing. 

 If this be true, there is mutual ad- 

 vantage in the association, for the 

 small amount of nourishment taken 

 by the fungus is more than compen- 

 sated by its assistance in absorption. 



(3) Root-tubercles. On the roots 

 of many legume plants, as clovers, 

 peas, beans, etc., little wart -like 

 outgrowths are frequently found, 

 known as " root-tubercles " (Fig. 

 78). It is found that these tuber- 

 cles are caused by certain Bacteria, 

 which penetrate the roots and in- 

 duce these excrescent growths. The 

 tubercles are found to swarm with 

 Bacteria, which are doubtless ob- 

 taining food from the roots of the 

 host. At the same time, these Bac- 

 teria have the peculiar power of 

 laying hold of the free nitrogen of 

 the air circulating in the soil, and 

 of supplying it to the host plant 

 in some usable form. Ordinarily 

 plants can not use free nitrogen, 



although it occurs in the air in such abundance, and this 

 power of these soil Bacteria is peculiarly interesting. 



This habit of clover and its allies explains why they are 

 useful in what is called "restoring the soil." After ordi- 



Fio. 78. Root-tubercles on 

 Vicia Faba.-A.fter NOLL. 



