SOIL FUNGI 283 



growth of the plant is influenced injuriously by the infection of the 

 fungus. The fungus decomposes starch readily and its growth is 

 dependent on the presence of the roots of the plant. When the plant is 

 grown in culture, uninfected by the fungus, it will thrive better than 

 when grown under equal conditions in the presence of the fungus. 

 Boulet 182 found endotrophic mycorrhiza on the roots of various fruit 

 trees and suggested that the fungus appears to live as a parasite on the 

 host; this was believed to have generally a beneficial effect on the host, 

 except when the essential organs of the roots are attacked. The possible 

 parasitism of mycorrhiza (to Picea), under certain conditions, has been 

 suggested also by Eexhausen. W. B. McDougal 183 found that the 

 fungi (belonging to the genus Cortinarius) which form ectotrophic 

 mycorrhiza on the roots of Picea rubra are of no benefit to the trees 

 concerned; they form no symbiotic associations with the trees, but are 

 instances of parasitism of fungi on the roots of the trees; no harm, 

 however, may be caused by this parasitism. This is of course merely a 

 theoretical consideration. 



This obligate relation between mycorrhiza and host plant is certain 

 for such plants as orchids and Ericaceae. The mycorrhiza are 

 considered in this case to be not parasitic forms but true mutualistic 

 symbionts. The trees are not injured, whereas the fungi develop 

 better than in pure culture, and are finally digested by the active juices 

 of the plant. Dufrenoy 184 suggested that symbiosis is a form of par- 

 asitism, in which equilibrium exists between the invading power of the 

 fungus and the resisting power of the host; this is profitable to both, so 

 long as it is maintained, but eventually it is a disadvantage or death to 

 either one, if sufficient advantage is gained by the other symbiont. 

 Rayner 185 also considered that an equilibrium becomes established 

 between the attacking mechanism of the fungus and the protective 

 mechanism of the plant. When this equilibrium is broken down, as in 

 the growth of the plant under unfavorable conditions {Calluna vulgaris 



182 Boulet, V. Sur les rnycorrhizes endotrophes de quelques arbres fruitiers. 

 Compt. Rend. Acad. Sci., 150: 1190-1192. 1910. 



183 McDougal, W. B. On the mycorrhizas of forest trees. Amer. J. Bot., 1: 

 51-74. 1914. Mycorrhizas of coniferous trees. Jour. Forestry, 20: 255-260. 

 1922. 



184 Dufrenoy, J. The endotrophic mycorrhiza of Ericaceae. New Phytol., 

 16: 222-228. 1917. 



185 Rayner, M. C. The biology of Calluna vulgaris. II. The calcifuge habit. 

 J. Ecol., 9: 60-74. 1921; The nutrition of mycorrhiza plants: Calluna vul- 

 garis. Brit. Jour. Exp. Biol., 2: 265-292. 1925. 



