564 VITAL ACTIONS OF THE LOWER FUNGI. 



by the fact that the occurrence of definite products of 

 fermentation is absolutely dependent on the presence of 

 definite fermentescible substances, without which, how- 

 ever, the life of the bacteria in question can go on very 

 well. We have also to bear in mind the marked influence 

 exerted by the presence or absence of oxygen on the 

 nature of the materials formed ; further, that the com- 

 position of the nutrient medium and the excess of nitro- 

 genous or non-nitrogenous substances determine the 

 relative quantity in which the various products usually 

 appear. Finally, in some cases abnormal alterations in or 

 mere accidental admixtures with the nutrient medium 

 lead to the transitory appearance of unusual products. 

 In the same way we can observe in the higher plants 

 the formation of large quantities of amides when there 

 is no assimilation of carbon ; further, the formation of 

 benzoic acid when hippuric acid is given to the plants 

 as the nitrogenous nutrient material. In like manner, 

 the mould fungi, for example, are able to form gallic 

 acid and glucose from tannic acid, and it is probable 

 that the above-mentioned (p. 528) splitting up of the 

 nitrates occurs in the same way. More elaborate 

 researches will, without doubt, lead to the discovery of 

 many of these more or less accidental products of tissue 

 change occasioned only by deviations in the nutrient 

 medium, and -disappearing again with alteration in the 

 nature of the soil. 

 Constancy of These differences in the excreta do not in the least 



the products . . . . 



of the tissue prevent us from utilising certain characteristic products 

 theSrient 6 ^ tissue change as a means of recognising the different 



conditions species of bacteria. For so long as the external con- 

 remain the * '. . . T 



.same. ditions remain the same, the same products of tissue 



change constantly accompany the specific form. We 

 do not find that other fungi suddenly acquire the power 

 of yielding products which are characteristic of a certain 

 species of bacteria ; and where normal conditions are to 

 some extent maintained, it happens just as seldom that 

 this species of bacteria loses its characteristic properties, 

 and furnishes other products of tissue change instead. 

 Thus we find that the production of colouring matter, 



