336 



PLANT LIFE. 



chiefly to the heaths and orchids. The third form grows 

 upon alders, bayberry, etc. 



460. (c) Root tubercles of Leguminosse. — 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 



*IG. 375- 

 Fig. 374.— Tip of a rootlet of beech {Fagus sylvatica) with fungus mantle, the 

 hyphae acting as absorbing organs in place of root hairs. Magnified too diam. — After 

 Frank. 

 FlG. 375. - Mycorhiza of orchids. A, diagram of a longitudinal section of a root ; /, /, 

 the cells of cortex filled with hyphae of fungus; i-, stele. Magnified about 20 diam. 

 />', a bit of longitudinal section of root of Ffeottia, near the tap. <■, epidermis ; /, a 

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

 hyphae 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 away 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 under is explained by their ability 

 thus to accumulate nitrogen from the air. 



461. 3. Between plants and animals. -Mutualism also 



