130 THE COLOURLESS PLANT CELL. THE YEAST PLANT 



of juice appearing on the surface of a fruit with ruptured 

 skin, and the like. But the yeasts, like the wheat plant 

 or the apple tree, are best known in the " domesticated " 

 or cultivated condition. There are two main uses to 

 which yeast is put by man the fermentation of sugary 

 plant juices to make alcoholic drinks, such as beer, 

 wine, or cider, and the " raising " of bread by means 

 of the carbon dioxide given off by active yeast. 



" Brewers' yeast " is a frothy, viscous, cream- 

 coloured liquid with an " alcoholic " smell. " Bakers' 

 yeast " is a similarly coloured putty-like substance. 

 A little of either placed in a drop of water under the 

 microscope is seen to consist of myriads of yeast cells, 

 but while in bakers' yeast these are mostly single, in 

 brewers' yeast they are united in chains which are often 

 branched, the result of rapid reproduction by budding 

 (Fig. 14, A and B). 



Nutrition. Yeast can live in a culture solution of 

 salts containing the essential elements of the food of 

 protoplasm, of which the most complex is ammonium 

 tartrate (NH 4 ) 2 C 4 H 4 O 6 , an organic salt containing 

 carbon, hydrogen and oxygen as well as nitrogen. The 

 other elements can be obtained from simple inorganic 

 salts such as potassium phosphate, K 3 P0 4 , calcium 

 phosphate, Ca 3 (PO 4 ) 2 , and magnesium sulphate, MgSO 4 . 

 With these alone, therefore, it can construct proteins. 

 But it grows very slowly in such a medium, for the 

 amount of energy it can obtain from ammonium 

 tartrate is small. If sugar be added to the solution 

 it grows and multiplies much faster, for from the sugar 

 it can obtain a large amount of energy. 



Reproduction. Yeast reproduces itself primarily by 

 budding (Fig. 14, A and B) . A tiny area of the cell wall, 

 usually at one end of the oval cell, is thrust out by the 



