METABOLISM OF ORGANISMS 



39 



cytoplasm. A cell wall chemically similar to protein is usually 

 present. Some forms show active movements by means of thread- 

 like prolongations of the cytoplasm, or flagella, as in the case 

 of the common Spirillum of decaying vegetable infusions. 



Reproduction is by a process of cell division which, under very 

 favorable conditions, may occur as often as every fifteen minutes. 



B / I C 



Fig. 14. -- Chief types of Bacteria. A, cocci; B, bacilli; C, spirilla. 



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. In 

 many species, under these circumstances the protoplasm within 

 the cell wall assumes a spherical form and secretes a protecting 

 coat about itself, and thus enters upon a resting state. In this 

 spore form the Racteria can withstand drying, variations in tem- 

 perature, and other conditions — in certain cases even strong 

 carbolic acid — to which in the active state they would readily 

 succumb, and thereby the organisms tide over periods of unfavor- 

 able conditions and are ready to start active life again when the 

 opportunity occurs. It is certainly fortunate for Man that the 

 great majority of disease-producing Racteria are unable to form 

 spores. (Fig. 200.) 



2. 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 carbo- 

 hydrates, fats, and proteins. Animals directly or indirectly feed on 

 plants, so that the ultimate source of the carbon of animals is 

 likewise the carbon dioxide of the atmosphere. Although both 

 plants and animals by their respiratory process are continually 



