ALTERATIONS WHICH THE NUTRIMENT UNDERGOES. 553 



the proteid substances, and this is, in fact, the only 

 mode of origin when the fungi are fed, for example, 

 only on peptone or albumen. 



We must imagine, then, that the further fate of the 

 non-nitrogenous substances is that they are in part 

 employed for building up portions of the organisms 

 (cellulose, fat) ; in part they combine with the nitro- 

 genous molecules, and thus furnish the proteid-like 

 materials ; in part, lastly, they are split up in the 

 protoplasm, chiefly in the form of carbo-hydrates, and 

 further destroyed by the oxygen, thus furnishing true 

 excretory products. Some of them can, like the nitro- Non-nitrogeu- 

 genous materials, act on the one hand as plastic material, products, 

 and on the other hand as excreta ; for example, the 

 organic acids (which are scarcely utilised when better 

 carbonaceous nutrient materials are present at the same 

 time) are made use of and given off if the nutrient 

 material be poor in carbon, but otherwise suitable. 

 Among the non-nitrogenous constituents of the fungi 

 there are, however, some which must be looked on as 

 true excreta, and which always act as such. Thus, 

 oxalic acid, formic acid, &c., cannot again play a part 

 as nutritive compounds ; and carbonic acid more espe- 

 cially cannot be utilised by the fungi, and hence must 

 in all cases be looked on as excrementitious matter. 

 As, however, carbonic acid is always produced by all 

 the fungi, we have in its separation the chief reason 

 for the gradual impoverishment of a nutrient mixture. 

 Some aromatic products appear also to be formed 

 to a slight extent in the tissue change of the fungi ; 

 for example, phenol, skatol, indol, &c. As a rule, 

 these bodies are bad nutrient materials, and indeed, 

 at a certain degree of concentration, can act as 

 poisons, and hinder the development of the fungi, 

 even when other nutrient compounds are present ; thus 

 they for the most part behave as true excretory 

 products. 



Oxygen is the element which, in addition to the Tissue change- 

 materials mentioned, takes the most active part in the 

 changes which occur in the bodies of the fungi. As 



