NUTRITION OF FUNGI. 173 



importance as a foodstuff for fungi, and is assimilated by 

 them in very varied forms. Some fungi are capable of ab- 

 sorbing and fixing nitrogen in its free state ; others obtain 

 their nitrogen in the form of inorganic compounds (ammonium 

 salts, nitrites, nitrates), including species which can only 

 absorb nitrogen from inorganic substances. The most im- 

 portant group of fungi, however, are dependent on organic 

 compounds containing nitrogen, such as amides, peptones, 

 albumoses, etc. The first two groups must obviously have 

 access to a supply of carbon, and this is often the case with 

 the third group, for in many cases the nitrogen bound up in 

 the organic substance can only attain its full value as a food 

 in conjunction with other sources of carbon. Both elements, 

 however, usually occur together in the same chemical sub- 

 stance, and it is moreover impossible to draw a sharp dis- 

 tinction between any of the above-mentioned groups. In 

 regard to the relative food value of the substances themselves, 

 it may be noted that, on the whole, the ammonium salts 

 constitute a better source of nitrogen than the nitrates. The 

 amides also play an important part in the nutrition of many 

 fungi. 



The greatest diversity exists as to the sources from which 

 the fungi may obtain their carbon. Sugar, tartaric, acetic, 

 oxalic, and carbonic acids are amongst them. In these, as in 

 other cases, the value of the nutritive matter varies according 

 to the other conditions of nourishment. The amount of 

 aeration is an important consideration, since oxygen facili- 

 tates the absorption of certain substances, and is stimulating 

 to some species, but acts restrictively on others. Temperature, 

 like aeration, may react in either direction. The nature of 

 the nitrogenous compound, too, has a definite bearing on the 

 availability of the source of carbon. In the case of yeasts, 

 acetic acid forms an excellent carbon food for the Myco- 

 dermse. Citric and tartaric acids form specially good food 

 for certain saccharomycetes, and, again, malic acid can be 

 assimilated in considerable quantity by certain species. Many 

 bacteria also take up organic acids in the presence of nutritive 

 salts. Glycerine and mannite are good sources of carbon for 

 the moulds. It is, however, the carbohydrates which, as is 



