340 



PLANTS WITHOUT CHLOROPHYLL 



Dead organic matter 



impossible for life to exist on 

 the earth, for green plants would 

 be unable to get the raw food 

 materials in forms that they 

 could use in making food and 

 living matter. In this respect 

 bacteria are of the greatest 

 service to mankind. 



Relation of Bacteria to Fer- 

 mentation. — Bacteria contin- 

 ues the process of fermentation 

 begun by the yeasts. In mak- 

 ing vinegar the yeasts first make 

 alcohol (see page 339), which 

 the bacteria change to acetic 

 acid. The lactic acid bacteria, 

 which sour milk by changing 

 the milk sugar to an acid, grow 

 rapidly only in a warm temper- 

 ature ; hence milk which is 

 cooled immediately and kept 

 cool or which is pasteurized and 

 kept in a cool place will not 

 sour readily. Why? These 

 same lactic acid bacteria are 



useful when they sour the milk for the cheese maker. 



Other Useful Bacteria. — Certain bacteria give flavor to cheese 



and butter, others give flavor to sauerkraut, while still other 



bacteria aid in the '^ curing " 



of tobacco, in the preparation 



of the dye indigo, in the '^ ret- 

 ting" or fermentation of certain 



fibers of plants for the market, 



as hemp, flax, and ramie, in the 



rotting of animal matter from 



the skeletons of sponges, and in 



the process of tanning hides to Microscopic appearance of ordinary 



^ ° milk, showing fat globules and bacteria 



make leather. which cause the souring of milk. 



Soluble Nitrstes 



Dead organic matter is broken down 

 by bacteria so that it may be used again 

 by green plants. 



