SYMBIOSIS. 



2S5 



to the heaths and orchids. The third form grows upon 

 alders, bayberry, etc. 



365. {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. 214). The 

 presence of these bacteria, in a 

 way yet unexplained, certainly en- 

 ables the plant to use free nitrogen 

 from the atmosphere, while other 

 plants are required to obtain it 

 from the soil in combination with 

 other things. 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. 



366. 3. Between plants and 

 animals. — Mutualism also occurs 

 between plants and animals. 

 Various species of plants attach 

 themselves to animals by which 

 they are carried about. The plant 

 is thus aided in obtaining the ma- 

 terials for food, and not infrequently the plant conceals the 

 animal from another which seeks it as prey. In this way 

 certain crabs are hidden by algae attached to them. 



Fig, 



— Mycorhiza of orchids. 

 A bit of longitudinal section of 

 root of Neottia, near the tip. e, 

 epidermis ; fi. a series of cortical 

 cells filled with fungus. Into the 

 cell a (nearer the tip of root) the 

 hyph« are just entering; in the 

 cells abovCj /, recently entered, 

 they have only formed a small 

 knot about the nucleus. Magni- 

 fied about 200 diam. — After 

 Frank. 



B. Helotism. 



367. I. Fungi and algae. — Helotism exists between fungi 

 and algae, constituting the bodies known as lichens, in which 



