CONDITIONS OF LIFE OF THE MOULD FUNGI. 511 



The reaction of the nutrient mixture has a most im- Influence of 

 portant influence on the growth of the mould fungi, of the nutrient 

 They seem to be most sensitive to an excess of alkali, material, 

 although some forms occur on substrata which have a 

 marked alkaline reaction ; an excess of acid is not nearly 

 so hurtful. Free tartaric acid may be present in the 

 nutrient mixture up to 5 per cent., and free phosphoric 

 acid up to 1 per cent., or more, without preventing the 

 development of mould fungi. This point is of great 

 importance, because in this respect, also, there is a 

 difference between the mould fungi and the majority of 

 the fission fungi, which are very sensitive to excess of 

 acid, and because by the reaction of the nutrient 

 material alone the kinds of fungi which obtain the 

 mastery when growing together can be influenced. But 

 on this point also we can give no definite figures which 

 take into account the differences in the nutrient solution, 

 and the specific properties of the various forms of fungi. 



As has been mentioned, there must be a most favour- 

 able proportion for all the other nutrient materials, and 

 increase or diminution of the quantity will exercise an 

 unfavourable action. Further, any admixture of foreign 

 materials which are not nutritious must render the 

 nutrient mixture to a certain extent unsuitable, even 

 when such materials are not poisonous for the fungi, and 

 do not hinder their growth even when present in great 

 concentration. Finally, poisonous materials which have 

 a specific noxious action on the growth of the mould 

 fungi may be present in the nutrient substratum or in 

 the surrounding air. The action of these substances 

 will be discussed in the chapter on "Disinfection." 



At times the mould fungi can convert insoluble mate- Fermentative 

 rials into soluble, substances which can be absorbed, 

 and this is done by the formation of suitable ferments. 

 For instance, in the case of penicillium and aspergillus 

 niger, the production of a ferment which inverts cane-sugar 

 and maltose* has been demonstrated, and Van Tieghem 

 showed that penicillium and aspergillus niger were 

 able to split up tannin into gallic acid and glucose; the 



* Bourquelot, Compt. rend., vol. 97, p. 1322. 



