THE FOOD OF PLANTS 



89 



enough for their needs from the soil without this assistance, 

 and, if so, the fungus becomes of vital importance in the 

 nutrition of such plants. In the case of some of these 

 plants it seems that the soil fungus is not merely passing 

 into their bodies the soil water with its dissolved salts, but 

 is contributing to them organized food, thus diminishing 

 the amount of necessary food manu- 

 facture. The delicate branching fila- 

 ments (hyphse) of the fungus wrap 

 the rootlets with a mesh of hyphae 

 and penetrate into the cells, and it 

 is evident that the fungus obtains 

 food from the rootlet as a parasite. 



(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- 



FIG. 78. Root-tubercles on 

 Vicia Faba.Mier NOI.L. 



