REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 129 



The blue Stentor tends to gather in shaded regions, and when sub- 

 jected to Ught coming from one side it moves away from the source of 

 light. Thus, if a glass vessel containing Stentors is placed near a win- 

 dow, the animals swim away from the source of light, and are soon found 

 to be collected on the side opposite the window. 



How is this result brought about? Just what is the cause of the 

 reaction to Ught, and what is the behavior of the Stentors in reaching 

 the shaded regions ? 



In arranging experiments which shall answer these questions, let us 

 first try the effects of sudden strong changes in the intensity of the light 

 affecting the animals. This may be done 

 by placing a fiat-bottomed glass vessel con- 

 taining many Stentors in a shallow layer of 

 water on the stage of the microscope in a 

 dark room. From beneath, strong hght is 

 sent directly upward through the opening 

 of the diaphragm by means of the substage 

 mirror, while all other hght is completely 

 excluded. In this way a circular area in 

 the middle of the field is strongly illumi- 

 nated, while the remainder of the vessel 

 containing the Stentors is in darkness.^ 



The Stentors in the darkness swim 

 about in all directions, but as soon as one 

 comes to the lighted area it at once re- 

 sponds by the avoiding reaction — it swims backward and turns toward 

 the right aboral side (Fig. 86, 1-4). Thus its course is changed and 

 it does not enter the hghted area. Since every Stentor reacts in this 

 way, the hghted area^ remains empty. Usually the avoiding reaction 

 occurs as soon as the anterior end of the Stentor has reached the 

 hghted region. In other cases the entire Stentor passes completely into 

 the hghted area, then reacts in the usual manner, thus passing back 

 into the dark. 



Fig. 86. — Reaction of Stentor 

 at passing from a darli to a light 

 region (1-4). 



' By using a projection lantern as the source of light the field of the microscope is 

 projected on the ceiling, or, by the use of a mirror to reflect the light at right angles, on 

 the ordinary projection screen. When thus projected, the behavior of the Stentors is 

 observable with the greatest ease. 



^ The light is passed first through a thick layer of ice water, in order to remove the 

 heat as far as possible. The fact that the reactions are not due to heat is shown in the 

 following manner. Specimens of Paramecium, an organism which is more sensitive to 

 heat than Stentor, but is not sensitive to light, are mingled with the Stentors. The Para- 

 mecia pass into the lighted region without hesitation, showing that this region is not 

 heated sufficiently to affect them ; the heat then cannot affect the Stentors. 



