CH. l] NUTRITION. 5 



amount of yeast does not simply depend on the sugar, 

 but also on the presence of certain other substances 

 which must be supplied to the plant in solution. 

 The reason is obvious : the cells contain nitrogen, sulphur, 

 phosphorus, potassium, lime and magnesium, and these 

 must be supplied in the culture-fluid. The solution used 

 for the growth of yeast is known as Pasteur's solution 

 and has the following constitution 1 : 



Potassium phosphate 20 parts 



Calcium phosphate 2 



Magnesium sulphate 2 



Ammonium tartrate 100 



Cane sugar 1500 



Water 8376 



10000 parts. 



The two chief points to notice are the conditions in 

 which carbon and nitrogen are supplied to the plant. Car- 

 bon is supplied as sugar and the yeast-cell cannot assimilate 

 carbon unless it is presented to it in an organic compound. 

 Yeast therefore resembles animals in regard to its carbon 

 supply, since like an animal it depends on a substance 

 (such as sugar) which has been manufactured in the leaves 

 of another plant, the sugar-cane. 



But in regard to nitrogen the yeast differs from 

 animals : no animal could live if its only nitrogenous food 

 were an ammonia compound, whereas the yeast is able to 

 make use of the ammonium-tartrate. 



1 Practical Biology (Huxley and Martin), 1888. 



