THE BIOLOGY OF BACTERIA 5 



We shall now take up a number of points in the biology 

 of bacteria which call for early attention, and which are 

 mostly the outcome of comparatively recent work on the 

 subject. 



The Place of Bacteria in Nature. As we have seen, for a 

 considerable period of time after their first detection these 

 unicellular organisms were considered to be members of the 

 animal kingdom. As late as 1838, when Ehrenberg and 

 Dujardin drew up their classification, bacteria were placed 

 among the Infusorians. This was in part due to the powers 

 of motion which these observers detected in bacteria. It 

 is now, of course, recognised that animals have no monopoly 

 of motion. But what, after all, are the differences between 

 animals and vegetables so low down in the scale of life? 

 Chiefly two : there is a difference in life-history (in structure 

 and development), and there is a difference in diet. A plant 

 secures its nourishment from much simpler elements than is 

 the case with animals ; for example, it obtains its carbon 

 from the carbonic acid gas in air and water. This it is 

 able to do, as regards the carbon, by means of the green 

 colouring matter known as chlorophyll, by the aid of which, 

 with sunlight, carbonic acid is decomposed in the chlorophyll 

 corpuscles, the oxygen passing back into the atmosphere, the 

 carbon being stored in the plant in the form of starch or 

 other organic compound. The supply of carbon in the 

 chlorophyll-free plants, among which are the bacteria, is 

 obtained by breaking up different forms of carbohydrates. 

 Besides albumen and peptone, they use sugar and similar 

 carbohydrates and glycerine as a source of carbon. Many 

 of them also have the capacity of using organic matters of 

 complex constitution by converting such into water, carbonic 

 acid gas, and ammonia. Their hydrogen comes from water, 

 their nitrogen from the soil, chiefly in the form of nitrates. 

 From the soil, too, they obtain other necessary salts. Now 

 all these substances are in an elementary condition, and as 



