520 BIOLOGY OF THE MICRO-OKGANISMS. 



between the yeast and mould fungi. Of mineral sub- 

 stances, potash, phosphoric acid, and calcium are indis- 

 pensable ; the presence of a considerable quantity of 

 acid phosphate of potash (about 20 per cent.) has a 

 markedly favourable influence on growth. 



fJee e o S xy t gen r The mould fuu & and ^ east fun g i bena ve very dif- 

 ferently as regards oxygen. On the whole the presence 

 of free oxygen exerts a very favourable influence on the 

 growth and multiplication of the yeast-cells ; when 

 brought into contact with water containing oxygen or 

 with oxy-hseinoglobin the yeast, as Schiitzenberger has 

 shown, takes up the oxygen very greedily ; and under 

 otherwise similar circumstances the largest quantity of 

 yeast is obtained when a continuous stream of air is 

 passed through the nutrient fluid. Multiplication of 

 the yeast can, however, occur without the access of 

 oxygen, but only when the other nutrient materials are 

 furnished in a suitable form, and when the yeast-cells 

 are able at the same time to cause fermentation. Thus 

 yeast grows actively without the presence of air in a 

 solution of peptone, or in a decoction of yeast containing 

 1 to 10 per cent, of sugar, and *5 per cent, of phosphoric 

 acid; its growth is less energetic when, instead of 

 peptone, meat extract, urea, or ammoniacal salts are 

 mixed with sugar ; and growth does not occur, or 

 only to a very slight extent, when no sugar is present, 

 or when its place is taken by other bodies, such as 

 glycerine or mannite, which do not so readily undergo 

 fermentation. In all cases the multiplication of the 

 yeast in the absence of oxygen goes hand in hand 

 with fermentation of the sugar, and the fermentative 

 activity seems to replace the action of the free oxygen. 

 This possibility of living without oxygen naturally in- 

 fluences also the occurrence and the habitat of the yeast ; 

 thus it can vegetate in the interior of fruits, provided 

 only that these contain fermentescible sugar, and that 

 the other conditions necessary for a development of its 

 fermentative activity are present. 



Influence of With regard to the concentration and reaction of the 

 the concentra- ma t er i a l there are also some differences between mould 



tion or the 

 material. 



