272 PRINCIPLES OF SOIL MICROBIOLOGY 



it is now known also for many other plants. Root hairs were found 

 to be frequently absent in ectotrophic mycorrhiza and are replaced by 

 hyphae of fungi. Frank formulated the theory that these absorb the 

 mineral salts and organic nitrogen compounds for the plant, whereas the 

 latter supplies the fungus with synthesized carbohydrates. In the case 

 of the endotrophic mycorrhiza the plant obtains the nitrogen from the 

 fungus in the process of digestion of the mycelium. 



Frank's theories were not generally accepted, especially when endo- 

 trophic mycorrhiza were also found on forest trees and ectotrophic 

 mycorrhiza in soils poor in organic matter. Various other theories 

 have been put forth concerning the nature of mycorrhiza formations 

 and their role in plant nutrition. Some of them were directly con- 

 flicting, largely because of the fact that the causative organisms could 

 not be isolated and cultivated in pure culture and the mycorrhiza 

 produced artificially. It is necessary, however, to differentiate between 

 theories of nutrition based on observation only, and those deduced from 

 pure culture experiments, as those of Bernard and Burgeff on orchids, 

 Melin on Conifers and other trees, Rayner on the Ericaceae and 

 PeyronePs recent work on endotrophic mycorrhiza. 141 



The penetration of the hyphae of the fungus into the root, which 

 results in the formation of an endotrophic mycorrhiza can take place 

 (as described for Lolium temulentum 1 * 2 ) in two different ways: 1. 

 If the hypha reaches the root hair, it may penetrate through its wall and 

 grow through the cavity until it reaches the corresponding epidermal 

 cell; sometimes the hypha may twist round the hair in one or two close 

 loops before penetrating. 2. The filament may enter the root by 

 piercing the epidermis directly. Having reached the interior of the 

 root, the filament twists in a loose spiral-like fashion in the lumen 

 of the epidermal cell. Growth takes place at right angles to the root 

 surface and the next layer of root cells is infected before there is a con- 

 siderable horizontal extension of the mycelium in the root. The 



141 A detailed review of the earlier literature on mycorrhiza is given by Ber- 

 nard, Gallaud, Janse and Rayner. Bernard, N. L'evolution dans la symbiose 

 des orchidees et leurs champignons commensaux. Ann. Sci. Nat. Bot. (9), 9: 

 1-186. 1909; Gallaud, I. Etudes sur les mycorhizes endotrophes. Rev. gen. 

 Bot., 17. 1905; Janse, J. M. Les endophytes radicaux de quelques plantes 

 javanaises. Ann. Jard. Bot. Buitenzorg., 14. 1897. Rayner, M. C. Mycorrhiza. 

 New Phytolog., 25: 1-50, 65-108, 177-190. 1926. 



142 McLennan, E. I. The endophytic fungus of Lolium. II. The mycor- 

 rhiza on the roots of Lolium temulentum L., with a discussion of the physiolog- 

 ical relationships of the organism concerned. Ann. Bot., 40: 43-68. 1926. 



