THE NUTRITION OF PLANTS 
161 
oaks and their allies, etc. (Figs. 149, 150). 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. 
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. Root-tubercles on 
Vicia Faba.Mter NOLL. 
