REACTIONS TO LIGHT 259 



Examples: Shock-movements and orientation in Euglena, 

 Chlamydomonas, Trachelomonas, Chlorogonium, swarm- 

 spores, Volvox and Stentor; orientation in all higher plants, 

 especially those in which the sensory zone is separated from 

 the motory zone, as in the plumules of grasses; and the 

 shock-movements in Edwardsia, Cerianthus, Gonionemus, 

 various moUusks, fly larvae, earthworms, and Amphioxus. 

 Light may have a marked effect on the life processes of all 

 these organisms, but there is no evidence indicating that 

 the slight changes of intensity required to induce reactions 

 affect these processes. The organisms mentioned above 

 are interested, not in the light condition which causes the 

 reaction, but in that which ordinarily follows such a con- 

 dition if the position or the direction of movement is not 

 changed. Euglena for example may respond when the 

 intensity on the colorless anterior end is reduced by a 

 small fraction of a candle-meter. It cannot be of any 

 special importance to the organism to keep this colorless 

 end illuminated, but it is of the greatest importance to 

 keep the green portion back of it illuminated, for light is 

 necessary in the process of photosynthesis. Likewise the 

 slight increase of intensity necessary to cause an earthworm 

 to withdraw into its burrow, or to cause a negative Euglena 

 to give the avoiding reaction, cannot be injurious to either 

 of these organisms, for both thrive in light much stronger 

 than that required to produce these reactions. I have kept 

 earthworms continuously exposed to strong diffuse daylight 

 (150 ca. m.) in excellent condition for weeks, whereas at 

 night a candle-meter of light flashed on them Is often suffi- 

 cient to cause violent contraction. The light condition 

 which causes this response in earthworms is not Injurious, 

 but the Illumination that usually follows if they do not re- 

 spond may be. Then, too, there is another factor involved 

 here. Exposure ordinarily puts the worms at the mercy 

 of the birds which prey upon them. Thus the light may 

 be a sign of an enemy to the worms and in this regard they 

 belong in the following group, for this phase of the response 



