SOIL FUNGI 281 



from the soil. 172 The symbiotic saprophytism was believed to be a 

 result of the supplemental capacities of two organisms brought into 

 nutritive contact chemotropically. 173 The cells of the higher plants 

 harboring the fungus generally show a decrease in carbohydrate and an 

 increase in protein content. According to Bernard and Burgeff, seed 

 infection of the embryo by the appropriate fungus is essential for the 

 germination of orchid seeds. Knudson 174 suggested, however, that the 

 role of the fungus may consist merely in supplying organic matter, 

 since the germination of the orchid seeds depends on an available supply 

 of organic matter. When this is added in the form of certain sugars or 

 plant extracts, germination is made possible without the aid of any 

 fungus. The action of the fungus is ascribed to its production of 

 enzymes which liberate products utilized by the embryo. 



2. The fixation of atmospheric nitrogen by the fungi bringing about 

 the formation of mycorrhiza. This subject has received considerable 

 attention 175 because of the important practical application of the phe- 

 nomenon. Much of the earlier work, however, is largely speculative 

 and is of little practical value. According to Rayner, 176 nitrogen 

 fixation by the endophyte of Ericaceae is demonstrated by the fact 

 that these plants grow on poor soils deficient in available nitrogen and 

 that pure culture seedlings of Calluna grow with marked vigor on 

 media free from combined nitrogen. This is especially interesting, since 

 the endophyte belongs to the genus Phoma (P. radicis), which is known 

 to be capable of fixing atmospheric nitrogen. 177 Moller 178 found, how- 

 ever, that ectotrophic mycorrhiza are mostly unable to fix any atmos- 

 pheric nitrogen. The inability to fix nitrogen was also definitely 

 established for forest trees by Melin. 179 



172 See also Shibata, K. Cytologische Studien liber die endotrophen Mykor- 

 rhizen. Jahrb. wiss. Bot., 37: 643-684. 1902; Rexhausen, L. Die Bedeutung 

 der exotrophen Mykorrhiza fur die hoheren Pflanzen. Beitr. Biol. Pflanz., 14: 

 18-59. 1920; Magrou, J. La symbiose chez les plantes. Bull. Inst. Past., 20: 

 169-183, 217-231. 1922. 



173 MacDougal, D. T., 1899 (p. 275). 



174 Knudson, L. Non-symbiotic germination of orchid seeds. Bot. Gaz., 

 73: 1-25. 1922; 77: 212-219. 1924; 79: 345-379. 1925. 



176 Ternetz, C. Assimilation des atmospherischen Stickstoffs durch einen 

 torfbeiwohnenden Pilz. Ber. deut. bot. Gesell., 22: 267. 1904; 1907 (p. 270). 



176 Rayner, M. C. Nitrogen-fixation in Ericaceae. Bot. Gaz., 73: 226-235. 

 1922. 



177 Duggar and Davis, 1916 (p. 270). 



178 Moller, A. Mykorrhizen und Stickstoffernahrung. Ber. deut. bot. Gesell., 

 24: 230-234. 1906. 



179 Melin, 1925 (p. 276). 



