212 VEGETABLE PHYSIOLOGY 



upon the roots of the leguminous plant. These tubercles 

 are swellings of the cortex of the root, the cells of which 

 are inhabited by a particular fungus, which breaks up in 

 their interior into curious bacterioid bodies. The exact 

 nature of the fungus has not been accurately determined. 

 The soil contains many of these bacterium-like bodies, 

 which make their way into the interior of the leguminous 

 plants by penetrating their root-hairs, and growing down 

 them into the cortex of the root. In the cells of the latter 

 the penetrating filaments bud off the bacterioid bodies in 

 great numbers. The stimulus resulting from the invasion 

 causes a considerable hypertrophy of the roots at the points 

 attacked, and -the tubercles are the result. The fungus 

 appears to have the power of fixing atmospheric nitrogen, 

 bringing it into some combination, the exact nature of 

 which is unknown, but which serves as the starting point 

 of proteid synthesis, either by the green plant or by the 

 intruder. The relationship is clearly of great advantage 

 to both organisms, the fungus obtaining its carbohydrate 

 supplies from the green plant, much as is the case in the 

 lichens already described. 



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 hyphge 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. 102). 

 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 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 



