Life of Plants and Animals 191 



the active principle alcohol. The bubbles rising 

 from fermenting substances are due to this gas CO 2 . 

 In the case of bread-making the gas is imprisoned in 

 the sticky dough and consequently causes the spongy 

 consistency of wheat bread. 



Effects somewhat similar to fermentation are doubt- 

 less present in the jar containing the hay or the green 

 algi. The bacteria attack the organic substances, 

 causing their disorganization resulting in the emission 

 of gas which, discolorizing the water, escapes as foul 

 odor. This decay or putrefaction is, therefore, 

 due to bacteria. When such disorganization is pro- 

 duced in living things, disease and death result; and 

 the bacteria which attack living things in this way, caus- 

 ing disease and death, are called pathogenic bacteria. 

 Some minute rod-like forms, called bacilli, are the 

 cause of that dreaded disease consumption. The 

 germs of this disease are present in the air; and, 

 when taken into the body, they develop in those organs 

 that are especially weak and consequently unable to 

 resist their action. Almost any organ may thus, 

 when weakened, become the seat of this deadly dis- 

 ease. But the lungs are most frequently affected, in 

 which case pulmonary tuberculosis results. 



Another rod-like form, the bacillus of typhoid 

 fever, is found in wells and drinking water that has 

 been contaminated with organic filth. The disease 

 attacks the alimentary canal and is often fatal, giving 

 rise to severe and prolonged fevers, doubtless due to 

 toxic or poisonous substances secreted by the germs. 



Other forms of bacteria (nonpathogenic) are useful 

 in promoting digestion of food in the alimentary 

 canal, in giving flavors to articles of diet as butter 

 and cheese, etc. Many forms are associated with 

 the roots of higher plants in the soil, as in red clover, 

 giving to the plant the disintegrated substances, 

 among them nitrogen, resulting from its activity. 



