EVOLUTION OF BACTERIA 87 



bacteria, each showing a rod-Hke but cellular form with a 

 deeply staining chromatin or nuclear mass; the arrows point 

 to cells showing these chromatin granules. This organism is 

 chemically more complex in that it can secrete a powerful 

 tryptic-like enzyme which enables it to utilize complex poly- 

 pep tids and proteins (casein). Also it is an obligatory aerobic 

 type, being unable to function in the absence of free oxygen. 



It was only after the chlorophyllic, carbon-storing true 

 plants had evolved that the second great group of parasitic 

 nitrifying bacteria arose to develop the power of capturing and 

 storing the nitrogen of the atmosphere through life association or 

 symbiosis with plants, also of deriving their carbon, not from 

 inorganic compounds, but from the carbohydrates of plants. 

 Such users of atmospheric nitrogen and of plant carbon include 

 three general types: B. radicicola, associated with the root 

 formation of legumes (compare D, Fig. 11), Clostridium (anaer- 

 obic, i. c., independent of free oxygen), and Azotohacter (aerobic, 

 i. e., requiring free oxygen).^ 



It seems that the early course of bacterial evolution was in 

 the line of developing a variety of complex molecules for per- 

 forming a number of metabolic functions, and that the visible 

 cell differentiation came later.- Step by step the chemical 

 evolution and addition of increasingly complex actions, reac- 

 tions, and interactions appear to correspond broadly with the 

 structural evolution of the bacterial organism in its approach 

 to the condition of a typical cell with its cell-wall, protoplasm, 

 and chromatin nucleus. 



To sum up, the existing bacteria exhibit a series of primor- 

 dial physicochemical phases in the capture, storage, and utiHza- 

 tion of energy, and in the development of products useful to 

 themselves and to other organisms and of by-products which 



^Jordan, Edwin 0., 1908, pp. 484-491. -I. J. Kligler. 



