270 PRINCIPLES OF SOIL MICROBIOLOGY 



Nitrogen-fixation . Various claims have been put forth, at different 

 times, that fungi are able to assimilate atmospheric nitrogen. In most 

 cases the quantities fixed were very small, amounting to a few milli- 

 grams, so that doubt might arise whether this was not due merely to ex- 

 perimental errors. In some cases the mere fact that fungi grew on agar 

 free from nitrogen compounds was taken as an index of positive nitrogen- 

 fixation, the fact being overlooked thereby that some of these organisms 

 can readily assimilate traces of ammonia present in the atmosphere and 

 that various chemicals may contain, as impurities, small amounts of 

 nitrogen. The more careful studies of recent investigators 130-132 have 

 definitely established the fact that common soil fungi are unable to fix 

 atmospheric nitrogen. The only possible exceptions to this rule may 

 be in the case of certain mycorrhiza fungi, 133 especially organisms belong- 



130 Goddard, 1913 (p. 259). 



131 Chambers, C. O. The fixation of free nitrogen by certain fungi. Plant 

 World, 19: 175-194. 1916. 



132 Duggar, B. M., and Davis, A. R. Studies in the physiology of the 

 fungi. I. Nitrogen fixation. Ann. Mo. Bot. Gard., 3: 413-437. 1916. 



133 Peklo, J. Neue Beitrage zur Losung des Mykorrhizaproblems. Ztschr. 

 Garungsphysiol., 2: 275-289. 1913. 



PLATE XV 



Mycorrhiza Fungi 



111. Apparatus for rooting cuttings under controlled conditions: w, cotton,' 

 c, cutting; s, sand; r, glass rod; h, rain water; p, potash tube (from Rayner). 



112. Vessel for study of mycorrhiza formation in pure culture (from Melin). 



113. Hyphae of Tricholoma flavobrunnea, grown in pure culture in symbiosis 

 with birch tree, X 50 (from Melin). 



114. Oblong section of hyphae radiating from mycorrhiza-root, X 500 (from 

 Melin). 



115. Beginning of infection of epidermis of young wheat root by phycomycoid 

 endophyte; p, points of entrance of mycelium into the root; attention is called 

 to the growth of the mycelium between the cells, X 130 (from Peyronel). 



116. Stages of evolution, showing the process of tuber formation as a result of 

 symbiosis: A, Solanum tuberosum; B, Orobus tuberosus; C, Ficaria ranunculoides ; 



D, plantlet of Bletitta hyacinthina inoculated with attenuated Rhizoctonia repens; 



E, plantlet of Bl. hyacinthina inoculated with an active Rh. repens; F, embryo 

 tuber of Cattleya; t = tubers (after N. Bernard and Magrou). 



117. Longitudinal section of a potato root, showing an early stage of fungus 

 infection; to, coiled mycelium; n, cellular nuclei; n', fungus nuclei (from Magrou). 



118. Two infected cells of a potato root, the lower cell showing large bodies 

 resulting from disintegration by phagocytosis and the upper cell showing non- 

 disintegrated mycelium which attacks the cell (from Magrou). 



119. Mycorrhiza cells from young root of seedling of Calluna vulgaris showing, 

 at right, "clumping" at early stage of digestion and, at left, digestion process 

 (from Rayner). 



