24 THE TROPISMS 



band or ring whose diameter varies with the size and strength 

 of the drop. As the solution diffuses and becomes diluted 

 at the center, the ring widens and extends toward the 

 center and after a time closes in to form a solid group. If 

 two species of protozoa having a different attunement to 

 carbon dioxide are used they may form two rings, one out- 

 side of the other. 



The tendency of Paramcecium to form groups may be 

 explained as due, in part at least, to then* positive chemo- 

 taxis to carbon dioxide. If water containing Paramoecia 

 is allowed to stand undisturbed for some time most of the 



individuals will be found in 

 groups or clusters as if they 

 were drawn together by their 

 social proclivities or by some 

 object of common interest. But 



FIG. 2. Showing the positive the cause of the association is 



chemotaxis of Paramos cium to 



a drop of a weak solution of car- much simpler. As Paramcicea 



give off carbon dioxide there 



is a greater quantity of it in a region where several 

 Paramoecia happen to be associated. Positive chemotaxis 

 to this substance tends to keep the cluster together as well 

 as to retain other individuals w r hich happen to come into 

 the region, until finally most of the Paramoecia in the vicinity 

 are assembled in the group. Other infusorians, such as 

 Oxytricha and Loxocephalus, form associations which are 

 not due to carbon dioxide or to any other acids since they 

 show no positive chemotaxis to these substances. There 

 is probably some substance which produces the grouping, 

 although it has not yet been identified. 



The chemotaxis of the lower organisms is to a certain 

 extent related to the welfare of the individuals, although in 

 only a general sort of way. The negative reaction to strong 



