204 



VEGETABLE PHYSIOLOGY 



and Azobader) which apparently have the power of fixing 

 atmospheric nitrogen. 



Many of our forest trees, among which the members of 

 the Cupuliferce 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 hyphae of the fungi continue to grow 



together with the root, 



S A and form an invest- 



ment over it, which 

 is 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 peases perforate 

 the external cells of 

 the roots and form 

 a network in the in- 

 terior. From the out- 

 side of the investing 

 mantle hyphae 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 Cupuliferae, but on those of Poplars, and many Heaths 

 and Ehododendrons. 



A curious case of this kind of relationship is shown by 

 Monotropa, a member of the Heath family which possesses 

 no chlorophyll. Monotropa possesses a rhizome, from which 



Fro. 98. A, EPIPHYTIC MYCORHIZA OF Facjus 

 sylvatica ( x 2) ; B, TIP OF HOOT PARTIALLY 



DENUDED OF THE INVESTING MANTLE ( X 30). 



(After Pfeffer.) 



