MYCORRHIZA 259 



SO prevent their leaching out when present in excess over the needs 

 of green plants. 



It has been shown that under aerobic conditions in acid soils the 

 decomposition of cellulose is carried out entirely by molds, but in 

 neutral or basic soils both molds and bacteria take part in this 

 decomposition. Only when the soil becomes completely anaerobic 

 do molds play no part. The amount of nitrogen available to the 

 fungi is frequently the limiting factor of cellulose decomposition. 

 .The soil fungi are also very active in ammonification. INIany of 

 the molds will produce ammonia from proteins more rapidly than 

 the most active of the ammonifying bacteria. Thus the molds are 

 among the primary agents concerned in the decomposition of dead 

 plant matter in the soil. On the other hand, Heukelekian and Waks- 

 man have shown that about one third of the not inconsiderable non- 

 nitrogenous material decomposed is built up into mold mycelium. A 

 proportion of the organic matter of the «oil consists of this mycelium 

 living and dead. It may be again rendered available to green plants 

 upon the death and disintegration of the fungi. Thus the molds tend 

 to stabilize the supply of plant food in the soil. Although various 

 claims have been made, there is little to lead one to believe that 

 molds play any part in the oxidation of ammonium salts to nitrates, 

 or to the fixation of atmospheric nitrogen. 



Further information on soil fungi may be obtained from the mono- 

 graph of Nicthhammer " or from the briefer discussions of Waks- 

 man ®^ or Brierley." 



Mycorrhiza. In connection with soil fungi, mention should also 

 be made of a relationship between certain fungi and the roots of 

 higher plants, called the mycorrhizal relationship. It is found that 

 the roots of many plants are invested by a fine network of mycelium 

 (ectotrophic mycorrhiza), which in some cases also penetrates the 

 root tissues or cells in their roots (endotrophic mycorrhiza). Many 

 species of fungi have been shown to be able to cause mycorrhiza in 

 some plant or other. The normal roots become much modified in 

 appearance. The association is obviously of benefit to the plant and 

 hence can be given as an example of symbiosis in the sense that it is 

 an association of two organisms with mutual benefit. 



A recent work by Bjorkman,* unfortunately unavailable to the 

 authors, has appeared which seems to give a new and very logical 

 concept of the function of the mycorrhiza. Through the personal 

 courtesy of Dr. L. G. Romel the outstanding features of this theory 

 have been made available to us. It seems that the fungi enter the 

 plants in the roots but do not develop further unless for some reason 



