IRRITABILITY : INFLUENCE OF LIGHT 



213 



movements do not invariably have the effect of preventing 

 the organism from moving into the dark. 



{d) Localization of Area of Sensitiveness. — Buder has deter- 

 mined the part of the organism which is sensitive to Hght 

 by the following experiment. A microscope field containing 

 Thiospirillum jenense was completely darkened except for a 

 small rectangular area. By an ingenious arrangement this 

 patch of light could be set in 



any part of the field, and 

 made to move in any part of 

 it. In the illustration two 

 spirilla (A and B) are shown 

 (see Fig. 59). In B the cilium 

 is in front, in A it is behind. 

 In one experiment the lighted 

 patch was made to move to 

 the left at a greater speed 

 than that of the spirilla mov- 

 ing in the same direction, with 

 the result that the line CD 

 overtakes the spirilla from 

 behind. When this line over- 

 takes an organism like A with 



the cilium behind, the response is sooner than when it over- 

 takes one like B with its cilium in front. But in both cases 

 the response, which in this case is a faster movement in the 

 same direction, follows the passing of the line CD over the 

 base of the cilium. Buder thus demonstrated that the base 

 of the cilium is the seat of sensitiveness to changes in light 

 intensity. 



Interpretation of Results. 



The experiments which are illustrated in Figs. 58 and 59 

 show that under normal circumstances shock movements are 

 such as bring the organism back into the light when it enters the 

 dark, or when a dark patch passes over it. The circumstances 

 were exceptional in the experiment illustrated in Fig. 58, and 



Fig. 59. 



