46 



FOUNDATIONS OF BIOLOGY 



or flagella, as in the case of the common Spirillum of decay- 

 ing vegetable infusions. (Fig. 14.) 



Reproduction is by a process of cell division which, under 

 favorable conditions, may occur as often as every half hour. 

 The vast multitude of cells thus produced before long exhaust 

 the food supply and contaminate with excretion products 

 the medium in which they are living, so that further growth is 

 inhibited. Under these circumstances the protoplasm within 

 the cell wall ordinarily assumes a spherical form and secretes 



FIG. 14. Types of flagellation in Bacteria. 1, without flagella (atrichous forma) ; 

 2, 8, 4i 5, with flagella (trichous forms). (From Buchanan.) 



a protecting coat about itself, and thus enters upon a resting 

 state. In this spore form the Bacteria can withstand drying 

 and variations in temperature to which in the active state 

 they would readily succumb, and thereby the organisms tide 

 over periods of unfavorable conditions and are ready to start 

 active life again when the opportunity presents itself. (Fig. 

 160.) 



B. CYCLE OF THE ELEMENTS IN NATURE 



We have seen that carbon dioxide is the source from which 

 green plants derive the carbon which they synthesize into 

 carbohydrates, fats, and proteins. Animals directly or in- 



