PLAXTS WSJCH CAUSE DECAY 391 



In the mauufacture of beer, etc., the sugar is ob- 

 tained by allowing the grain to germinate until a large 

 part of its starch has become sugar (see page 169), 

 then killing it by heat and extracting the sugar by 

 means of water; the Yeast is then added. In addition 



-\i. Bluck Mould of ^read (Rhizopns), showing myceliimi. 



to the useful Yeasts, there are others which impart a 

 disagreeable taste or odor to the product; and these 

 must be carefully excluded. The use of pure cultures 

 of Y'easts is now becoming general: they are obtained 

 by the same methods as pure cultures of bacteria. 



Another class of plants which resemble the bacteria 

 in causing decay are the Moulds. A very common one 

 which occurs everywhere on decaying fruit, vegetables, 

 bread, etc., is the Black Mould of bread (known as 

 Mucor stolonifer, or RMzopus nigricans). 



