THE NUTRITION OF PLANTS 



161 



can absorb 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 manufac- 

 ture. The delicate branching fila- 

 ments (hyphae) of the fungus wrap 

 the rootlets with a mesh of hyphaa 

 and penetrate into the cells, and it 

 is evident that the fungus obtains 

 food from the rootlet as a parasite. 



(3) Boot-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. 

 151). 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. 151. Eoot-tubercle9 on 

 Vitia Faba.— After Noll. 



