PLANTS IN THEIR RELATION TO HUMAN WELFARE 143 



spore formation; the tiny protoplasmic sphere is called a spore, 



and its dense covering a spore wall. (Fig. 71, D.) In this 



condition bacteria may be blown 



hither and yon as a part of the dust. 



They may be heated even above the 



temperature of boiling water without 



being killed. Whe,n at length they 



settle down on a moist surface that 



will supply them with food, the spores 



burst their thick envelope, assume 



once more their rod-shaped or spiral 



form, and go on feeding, assimilating, 



and reproducing their kind. 



150. Relation of bacteria to soil 

 fertility. Having discussed somewhat 

 the structure and functions of bacteria, 

 we are now to consider the great im- 

 portance of these microscopic organ- 

 isms to human welfare. In the first 

 place, were it not for their never end- 

 ing activity, all life upon the earth 

 would soon cease to exist. Let us see 

 why this is so. When animals or plants 

 die, their bodies fall upon the ground, 

 and were these lifeless masses not taken _ 



FIG. 72. Roots of horse 



care of, the whole surface of the earth bean, showing numerous 

 would long since have been covered root-tubercles. (Stras- 



. , , burger.) 



with a vast number of unbuned or- 

 ganisms. All this dead material, however, as we have seen, 

 is food for the countless bacteria; they cause it to decay, 

 and thus decompose it into simpler chemical compounds 

 that can soak into the earth and then be used in the nu- 



