THE BEHAVIOR OF BACTERIA 37 



under natural conditions the bacteria rarely if ever have opportunity 

 to react to the separated spectral colors. 



Besides the purple bacteria, a green form, Bacterium chlorinum, is 

 known to assimilate carbon dioxide and to collect in light, in the same 

 manner as do the purple species. 



The precise method by which bacteria react to heat and cold has 

 been little studied. Mast (1903) has shown that Spirilla do not react 

 at all to changes in temperature. If a portion of the preparation con- 

 taining them is heated, they continue to pass into this region just as 

 before, though they may be at once killed by the heat. They may pass 

 also into a cold region, where motion gradually ceases. 



The reaction to the electric current, like that to heat and cold, is in 

 need of a thorough examination. Verworn found that when subjected 

 to a continuous current some bacteria pass to the anode, others to the 

 cathode. 



When placed in a vertical tube, some kinds of bacteria pass upward 

 to the top, in opposition to the force of gravity, while others gather at 

 the lower end (Massart, 1891). The factors on which this reaction to 

 gravity depends, and the precise way in which the reaction takes place, 

 are unknown. 



Bacteria often react to contact with solids by settling down and 

 becoming quiet on the surface of the solid, which is usually some food 

 body. Bacterium termo thus forms dense collections on the surface of 

 such an object as a fly's leg. 



3. GENERAL FEATURES IN THE BEHAVIOR OF BACTERIA 



We find that the chief reactions of bacteria, so far as they have been 

 precisely determined, take place through a single movement, a tem- 

 porary reversal of the direction of swimming. This reaction is so simple 

 as to be comparable to a reflex action as we find it in an isolated muscle. 

 Whether the bacteria collect in a certain region or avoid it depends on 

 what it is that produces this reversal of movement. The reaction is 

 caused as a rule by a change in the environment of the organism. This 

 change is usually brought about by the movement of the bacterium into 

 a region differing from that which it previously occupied, but it may be 

 due to an active alteration of the environment, as when light is suddenly 

 cut off. For the reaction to occur with the result of a general movement 

 of the organisms into a certain region, it is not necessary that different 

 parts of the body should be differently stimulated, as we found to be the 

 case in Amoeba. The only requirement for producing a general move- 

 ment of the organisms in a certain direction is that movement in any other 



