204 VEGETABLE PHYSIOLOGY 
and Azobacter) which apparently have the power of fixing 
atmospheric nitrogen. 
Many of our forest trees, among which the members of 
the Cupuliferee are conspicuous, exhibit another symbiosis 
which is of the greatest interest and importance. The 
roots of these plants grow down into soil which is infested 
with the mycelia of different fungi, with which they become 
entangled. The hyphe of the fungi continue to grow 
together with the root, 
B A and form an invest- 
ment over it, which 
ig IN some cases met 
with in the form of an 
open network, and in 
others in that of a 
dense feltwork (fig. 
98). The fungi in 
some ‘cases perforate 
the external cells of 
the roots and form 
a network in the in- 
Fig. 98a, Eprenyito Mycoruiza or Fagus terior. From the out- 
sylvatica ( X 2); 3B, Trp OF RooT PARTIALLY side of the investing 
DENUDED OF THE INVESTING MantTLE (x 30). : 
(After Pfoficr.) mantle hyphe grow 
out into the soil in 
a similar way to the root-hairs of ordinary plants. These 
take the place of the root-hairs, which cease to be developed, 
and serve the purposes of the roots as absorbing organs 
for the water and the salts of the soil. The fungus is bene- 
fited by drawing its own nutriment from the cells of the root 
into which it has penetrated. The fungoid web or mantle 
is known as a mycorhiza ; it is present not only on the roots 
of the Cupulifere, but on those of Poplars, and many Heaths 
and Rhododendrons. 
A curious case of this kind of relationship is shown by 
Monotropa, » member of the Heath family which possesses 
no chlorophyll. Monotropa possesses a rhizome, from which 
