PARAMECIUM 



75 



the animals in the drop will sooner or later encounter the acid, 

 and having once entered are unable to escape, just as in the case 

 described above. A group is therefore formed in the acid, illus- 

 trating what is called a positive 

 reaction and in this case positive 

 chemotropism. This experiment 



a 



B 



.Q-.|Q1. 



I 



FIG. 35. Diagrams showing Paramecia 

 collected about a bubble of CO2 in 

 the optimum concentration ; a is a 

 bubble of air, b of CO2. A shows 

 the preparation two minutes after 

 the introduction of the CO2 ; B, two 

 minutes later ; C, eighteen minutes 

 later. (From Jennings.) 



ny cases, as above, the passage 

 weak solution causes no reaction. 



FIG. 34. Path followed by a sin- 

 gle Paramecium in a drop of 

 acid. (From Jennings.) 



may be repeated using a J 

 per cent solution of common 

 salt in which are placed a 

 number of specimens. If a 

 drop of T V per cent solution 

 of the same chemical is now 

 added, the Paramecia will 

 swim into and directly across 

 it, but on reaching the bound- 

 ary between the two solutions 

 on the other side of the drop, 

 the avoiding reaction will be 

 given. Soon the weaker so- 

 lution will contain all of the 

 animals which, having once 

 entered, cannot escape. In 

 from a strong solution to a 

 For certain substances, how- 



