312 PRACTICAL COURSE IN BOTANY 



Among others, the germs of pneumonia, diphtheria, and 

 tuberculosis are often found in the mouth, nose, and sputum 

 of perfectly healthy persons. There are also a number 

 of bacteria that are regular inhabitants of the mouth, some 

 of which are the cause of decayed teeth and foul breath. 

 One form of bacterium, concerned in the production of in- 

 flammation and abscesses (Staphylococcus) is so constantly 

 present on the human epidermis that one authority has 

 declared it impossible to sterilize the skin so thoroughly 

 as to free it entirely of this microbe. It is ordinarily not 

 harmful unless it comes in contact with open wounds and 

 abrasions. 



353. The economic importance of bacteria. — It is hard 

 to say whether these organisms concern us most on account 

 of the damages attributable to them on the one hand, or 

 the benefits we owe them on the other. If they were all 

 as harmful as the pathogenic kinds, life would hardly be 

 possible on the globe, while without their presence life 

 as we know it would have ceased to be possible long ago. 

 They are nature's great army of scavengers, the sole agents 

 of decomposition, without which dead organic matter would 

 be subject only to the slow changes by which the rocks 

 and mineral matter of the earth's crust are disintegrated, 

 and the undecomposed bodies of the vast procession of 

 plants and animals that have existed since life first began 

 on our globe would long ago have cumbered its surface to such 

 an extent as to render impossible the continued develop- 

 ment of life such as we know. 



354. Sterilization is the process of ridding a substance 

 of living microorganisms. To do this effectively, the pro- 

 cess must be repeated several times at intervals, so as 

 to give any spores that may have survived previous applica- 

 tions time to pass into the vegetative state, when their 

 power of resistance is diminished and they are more easily 

 destroyed. The incubation period, as the time required 

 for the germination of the spores is called, is different for 



