510 BIOLOGY OF THE MICRO-ORGANISMS. 



the yeast or the fission fungi. Some mould fungi can 

 grow in extremely dilute nutrient mixtures which only 

 contain traces of the necessary nutrient materials. (This 

 has been especially observed with regard to penicillium.) 

 While, however, in this respect the vital activity of the 

 yeast and fission fungi is almost the same, the mould 

 fungi, on the other hand, have the advantage when we 

 come to deal with a small amount of water and marked 

 concentration of the material. Under such circumstances 

 they are by no means sensitive ; nutrient mixtures from 

 which a great part of the water has been removed by 

 evaporation, or by the addition of salt or sugar, and 

 which have therefore become unsuitable for the nutrition 

 both of the budding and fission fungi, will still be avail- 

 able for the growth of various mould fungi. We have 

 not yet gained exact information regarding the upper 

 and lower limits of the amount of water, and such in- 

 formation is very difficult to obtain, because the quantity 

 varies according to the other constituents of the nutrient 

 medium, and according to the varying species of the 

 mould fungi. In preserving articles of food, we have 

 learnt by experience that salted or smoked meat, 

 for example, which contains 50 per cent, of water, 

 is no longer a suitable soil for fission fungi, but 

 it may, nevertheless, become mouldy; in order to 

 prevent the formation of mould, the amount of water 

 must be reduced to only 10 to 12 per cent. ; if, 

 however, sugar is present at the same time in large 

 quantities, mould formation is prevented, even with 

 as much as 80 per cent, of water. These numbers 

 indicate the lowest limit as regards the amount of 

 water ; the optimum is much higher, perhaps 80 per 

 cent., so far as the dependence of the optimum on the 

 quantity of the other nutrient materials permits us to 

 give definite numbers. All the mould fungi, however, 

 are not in like manner indifferent with regard to great 

 concentration of the material on which they grow. 

 Certain fungi appear to be much more sensitive, espe- 

 cially some which lead a parasitic life, and which only 

 occur in moist seasons and in moist localities. 



