ADAPTATION OF ORGANISMS 



309 



which these elements must be presented to the organism, in 

 in order to be available for its metabolic processes, are sub- 

 ject to the widest variation. 



We have emphasized and contrasted the nutrition of a 

 typical animal, green plant, and colorless plant, and have 

 seen the reciprocal part which they play in the circulation 

 of the elements in nature, so it is only necessary, with these 

 facts in mind, to cite special cases in order to illustrate the 

 adaptation of special groups of organisms to special condi- 



FIG. 158. Portion of filaments of Begyiotoa alba (a), and two cells of Beggiotoa 

 mirabilis (6) showing enclosed sulfur granules. (From Buchanan.) 



tions of existence. The demands of the so-called Sulfur 

 Bacteria and the Yeasts are in point. 



The Sulfur Bacteria (Beggiotoa) live in water containing 

 sulfuretted hydrogen, from which, by oxidation, they obtain 

 energy and store up within the protoplasm free sulfur in the 

 form of tiny granules. And then by further oxidation they 

 transform the sulfur into sulfuric acid and excrete it. Thus 

 a gas which is poisonous to nearly all organisms is for Beg- 

 giotoa a necessary life condition. (Fig. 158.) 



The Yeasts include a host of microscopic colorless plants 

 which play an important part in the simplification of organic 

 compounds. (Fig. 159.) Being devoid of chlorophyll, 

 Yeasts of course lack photosynthetic powers, though like 

 many other colorless plants they are not dependent upon 



