22 THE TROPISMS 



does not as in Paramcecium occur independently of orienta- 

 tion, but the orientation is only indirectly produced instead 

 of being the result of appropriate direct reflexes. 



The movements of organisms toward or away from certain 

 stimulations, even where they are quite directly caused, 

 may be brought about in a variety of ways. They are by 

 no means always the result of a forced orientation. Never- 

 theless such movements are commonly described as tropisms. 

 Whether or not the tropic response is effected in one or the 

 other of the ways we have described it may be just as much 

 the result of reflex action and just as little the outcome of 

 intelligence and volition. 



CHEMOTAXIS 



Movements toward or away from chemicals are naturally 

 widespread. Many kinds of bacteria are markedly che- 

 motactic. Englemann found that Bacterium termo gathers 

 near the margin of the cover-glass where there is more 

 oxygen than near the center. If green algae are present the 

 bacteria congregate around them as long as they are exposed 

 to sunlight and are therefore giving off oxygen. If the slide 

 is kept in the dark so that no more oxygen is produced and 

 the oxygen present becomes evenly diffused the bacteria 

 become uniformly scattered over the field. Englemann 

 in an ingenious experiment also showed that if a spectrum 

 were thrown on a long thread of the alga Cladophora the 

 bacteria would congregate most abundantly in the red end 

 of the spectrum where most oxygen is given off. The ex- 

 periment affords a delicate test of the amount of oxygen 

 evolved under the influence of the various kinds of light. 



Bacterium termo and other species were found by Pfeffer 

 to be attracted to various salts if they were employed in 

 weak solution, as well as a variety of other substances of the 



