198 VEGETABLE PHYSIOLOGY 



roots of these plants grow down into soil which is infested 

 with the mycelia of different fungi, with which they become 

 entangled. The hyphas of the fungi continue to grow 

 together with the root, and form an investment 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 cases perforate the external cells of the 

 roots and form a network in the interior. From the out- 

 side of the investing mantle hyphse 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 myco- 

 rliiza ; 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 relation- 

 ship is shown by 

 Monotropa,a, member 

 of the Heath family 

 which possesses no 



FIG. 98. A, EPIPHYTIC MYCORHIZA OF Fagus 

 sylvatica ( x 2) ; B, TIP OF BOOT PABTIALLY chlorophyll. MonO- 



DENUDED OF THE INVESTING MANTLE ( X 30). , 



(After Pfeffer.) tropa possesses a 



rhizome, from which 



rise sub-aerial stems from ten to twenty centimetres high, 

 bearing succulent membranous leaves. From the rhizome 

 are given off crowded masses of roots which are covered 

 with a mycorhizal mycelium, and are embedded in humus. 

 There being no chlorophyll apparatus, Monotropa is de- 

 pendent entirely on the mycorhiza for its nourishment. 



