THE METABOLISM OF THE SULPHUR BACTERL4 41 



hindered rather than aided growtli. Bavendamm obtained the 

 same negative results in his experiments with pure cultures, 

 and considered that the sulphur bacteria derived their carbon 

 from the carbonic acid of the atmosphere. He observed that 

 the addition of carbon compounds made no difference to the 

 growth of Chroniatiiim Wanningii and of Lamprocystis. The 

 question cannot, however, be held to be completely settled by 

 the negative results of these two investigators. They have 

 conclusively proved that the CO2 of the atmosphere com- 

 pletely suffices for the carbon requirements of the sulphur 

 bacteria, but it has still to be proved that under other circum- 

 stances these organisms are not able to make use of carbon 

 in any other combination. 



4. Oxygen. — The influence of oxygen on the sulphur 

 bacteria may be considered under three heads : — 



(1) The directive influence of oxygen as a chemiotactic 



agency in effecting the attraction or repulsion of 

 bacteria. 



(2) The possibility of this gas being given off by the purple 



sulphur bacteria in the same manner as green plants. 



(3) The possible necessity of oxygen to the bacteria during 



the process of respiration. 



The first and second of these questions are treated in 

 Chap. XI. The third may now be considered. 



Respiration is a function which is common to all organisms. 

 The majority of organisms require oxygen to effect the de- 

 composition of highly complex substances in order that the 

 plant may benefit by the energy which is thereby liberated. 

 Such plants constitute the majority of the members of the 

 vegetable world, and there are only a few that respire in a 

 different way. The two classes of plants are known respec- 

 tively as aerobes and anaerobes. The anaerobes are confined to 

 a few organisms among the bacteria and some closely related 

 organisms. Winogradsky was the first to call attention to 

 the reaction of the sulphur bacteria to oxygen. He had noted 

 that in his mixed cultures the bacteria invariably retreated 

 from the surface of growth if oxygen were present in the same 



