MYCORRHIZAS 



457 



The complete storj^ of nitrogen in relation to plant life involves a whole 

 series of events, some of which occur in the cells of micro-organisms of the soil 

 and some in the tissues of the higher plants. This series of events is frequently 

 referred to as the "nitrogen cycle." The interrelations of the various processes 

 concerned in the nitrogen cycle are summarized diagrammatically in Fig. lOO. 



[ ANIMAL \ 

 I PROTEINS ) 



Tl_I ^^^— — "^^ 



COj.HjQHjSi 



ch4,e:tc. 



Fig. ioo. The nitrogen cycle. 



Mycorrhizas. — The roots of many species of plants are regularly in- 

 fected with the mycelium of fungi. Such a root, together with its associated 

 fungal hyphae, is called a tnycorrhiza (literally "fungous-root"). In the 

 ectrotropic mycorrhizas the mycelium is chiefly external to the root, invest- 

 ing it with a web-like mantle of hyphae. Some hyphae also penetrate into 

 the root, infecting principally the cortex. In the endotropic mycorrhizas the 

 hyphae are intracellular, being found principally within the cells of the epi- 

 dermis and the cortex. Roots which become infected with mycorrhizal fungi 

 stop elongating and often branch extensively. Mycorrhizas are therefore 

 usually short and stubby as compared with uninfected roots on the same plant. 

 Many authorities believe that mycorrhizas are present on the roots of the 

 majority of vascular species. 



