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. Engimann found that Bacterium termo gathers 
near the margin of the cover-glass where there is more 
oxygen than near the center. If green alge 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 
