REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 141 



We can thus distinguish an optimum intensity of light, in which 

 Euglena tends to remain. Movement toward either a greater or a less 

 intensity of light causes the avoiding reaction, with its trial of different 

 positions and directions of movements, 

 till a position or direction is found 

 which leads toward the optimum, or 

 retains the optimum intensity undi- 

 minished. Or, in other words, after 

 Euglena receives an amount of light 

 which we might call "enough," it 

 avoids more light, and also less light. 



That degree of light in which it tends 



Fig. 96. — Diagram to illustrate the 

 results of Famintzin's experiment. The 

 light comes from the direction indicated 

 by the arrows, while the opposite side of 

 the vessel is shaded, as indicated by the 

 dots. The Euglenae gather in the inter- 

 mediate region, across the middle. 



to remain seems to be about the 

 amount which is most favorable to its 

 life activities. Euglena requires light 

 for assimilating carbon dioxide by the 

 aid of its chlorophyll, just as do higher plants. If confined to dark- 

 ness, it soon ceases activity, contracts into a sphere, and becomes en- 

 cysted. On the other hand, direct sunlight is very injurious to it ; if 

 long continued it causes the organism to fall to the bottom and die. 

 Euglena avoids both the higher and the lower intensities that are 

 injurious to it. 



C. Negative and Positive Reactions compared 



Thus in both negative organisms (Stentor) and positive organ- 

 isms (Euglena), the determining cause of the reaction is a change in 

 the intensity of light, and the reaction takes place by the usual method of 

 the performance of varied movements, subjecting the animal succes- 

 sively to different conditions. When the sensitive anterior end is sub- 

 jected alternately to light and shade, the organism "tries" other direc- 

 tions of movement till it finds one where such changes are not pro- 

 duced. In Stentor it is an increase in light that causes this reaction ; 

 in Euglena is it usually a decrease that causes the reaction, though 

 when the light is very strong an increase may have the same effect. 



D. Reactions to Light in Other Lnjusoria 



The reactions of other infusoria to light are similar in character, 

 so far as known, to those of Stentor and Euglena. In only a few other 

 cases have details of the avoiding reaction been worked out as thoroughly 

 as for the two species mentioned. But all that we know of the reac- 

 tions of infusoria to light is consistent with the method of reaction known 



