REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 145 



If the experiments were made with swarm spores that were nega- 

 tive to the intensity of light used, they gathered of course in the shadow 

 instead of in the light. If a board was placed across the middle of the 

 vessel from right to left, such swarm spores formed a collection in the 

 partly shaded region at the edge of the board (as in Fig. 96), where they 

 found the optimum degree of illumination. They were repelled both 

 by the strong light and by the deep shadow. 



Thus it is clear that in the swarm spores, as in Euglena and Stentor, 

 a change in the intensity of illumination produces reaction. But a 

 certain amount of change is required before any effect is produced. 

 If the intensity of illumination changes only very gradually from one 



Fig. 98. — Diagrams to illustrate the results of some of Strasburger's experiments with 

 positive swarm spores (original). A, the margins of the drop are shaded (as indicated by the 

 dots); the organisms gather in the lighted centre. B, a broad band of shadow lies trans- 

 versely across the drop; the organisms swim toward the light, but are stopped by the shadow. 

 Thus two groups are formed, one at the side of the drop next the light, the other in a corre- 

 sponding position at the edge of the shadow. 



region to another, the difference in intensity between succeeding points 

 is insufficient to cause reaction. Hence under these circumstances the 

 organisms remain scattered and move about without reaction. Stras- 

 burger showed this in the following way. He used a hollow wedge- 

 shaped prism, 20 cm. long, tilled with a partly opaque solution of humic 

 acid in ammonia. Through this the light was passed. At the thin 

 end of the wedge nearly all the light was transmitted ; at the thick end 

 little or none, and there was a gradual transition from light to dark 

 between the two ends. This prism was placed over the drop contain- 

 ing the swarm spores, and the light was allowed to fall directly from 

 above (Fig. 99, X). The drop being very small in comparison to the 

 length of the wedge-shaped prism, there was of course but little differ- 

 ence in the illumination of its two sides, and the transition from one 

 to the other was very gradual. Under these conditions the swarm 

 spores remained scattered throughout the drop. The change in pass- 



