430 BIOLOGY AND HUMAN LIFE 



cannot be measured in money. We may consider the good or 

 the harm that the different kinds of organisms^ do to us, even if 

 we cannot measure it, just as we accept the oxygen of the air, 

 water, and sunshine without any business transactions ; these 

 are without price but indispensable. 



317. Schizomycetes. The fact that bacteria cause the decay 

 or decomposition of organic matter can be used directly in the 

 preparation of sponges for commerce. The sponges are allowed 

 to lie in tanks of water until the dead cells are completely de- 

 stroyed by bacteria, leaving only the familiar horny skeletons. 

 These are washed clean and dried for market. A similar proc- 

 ess, employing different species of bacteria, brings about the 

 retting (really, "rotting") of the soft portions of flax and hemp 

 stalks, and so allows the bast fibers to become separated. 



The action of bacteria is used in making vinegar out of cider, 

 wine, or other liquids containing alcohol. The bacterial fer- 

 ments cause a partial oxidation of the alcohol into the acid of 

 vinegar. In making sauerkraut and other kinds of pickles, as 

 well as in the curing of silage, bacterial fermentation is used. 

 In the work of the dairy, from the souring of milk to the curing 

 of cheese, bacteria are used at several points. Cheeses of the 

 Cheddar type and various cream cheeses, as well as butter, de- 

 pend for their flavor upon the particular species of bacteria 

 present during the souring. 



It is very likely that bacteria play a large part in the curing of tobacco 

 and in the making of hay, although the problems connected with these 

 processes have not been thoroughly worked out as yet. In the prepara- 

 tion of indigo dye from the extracts of certain plants of the bean family 

 it is likely that an oxidizing ferment from certain bacteria performs an 

 essential part of the work. In the preparation of hides for tanning, cer- 

 tain of the changes are brought about by bacterial fermentation. 



^For the general plan of classification see Chapter VII. Many plants and 

 animals are related to human welfare by the mere fact that they fit into the 

 general life cycle whereby the material of the earth, water, and air is con- 

 stantly circulating from one kind of protoplasm to another. Here the relation- 

 ship is not always specific; that is, one kind of plant or animal could serv'e 

 as well as several others (see Fig. 176). 



