336 



PLANT LIFE. 



chiefly to the heaths and orchids. The third form grows 

 upon alders, bayberry, etc. 



460. (c) Root tubercles of Leguminosae. A peculiar case 

 of mutualism appears in the bean family between the roots 

 and bacteria. The latter produce upon 

 the roots small swellings from the size 

 of a grain of wheat to that of a hazelnut 

 (fig. 376). The presence of these 



FIG. 374. 



FIG. 375. 



FIG. 374. Tip of a rootlet of beech (Fagus sylvatica) with fungus mantle, the loose 

 hyphz acting as absorbing organs in place of root hairs. Magnified 100 diam. After 

 Frank. 



FIG. 375. Mycorhiza of orchids. A, diagram of a longitudinal section of a root ; p, p, 

 the cells of cortex filled with hyphs of fungus ; j, stele. Magnified about 20 diam. 

 B, a bit of longitudinal section of root of Neottia, near the tip. e, epidermis ; /, a 

 series of cortical cells filled with fungus. Into the cell a (nearer the tip of root) the 

 hyphs are just entering ; in the cells above /, recently entered, they have only formed 

 a small knot about the nucleus. Magnified about 200 diam. After Frank. 



bacteria, in a way yet unexplained, certainly enables the plant 

 to use free nitrogen from the atmosphere, while other plants 

 are required to obtain it in combination from the soil. The 

 enrichment of the soil by growing clover and similar crops 

 upon it and plowing them tinder is explained by their ability 

 thus to accumulate nitrogen from the air. 



461. 3. Between plants and animals. Mutualism also 



