lOO LIGHT AND THE BEHAVIOR OF ORGANISMS 



This is of course due to the successive illuminating and shad- 

 ing of different parts of the organism owing to its move- 

 ments. There is no evidence indicating that light, acting 

 constantly as a directive stimulus similar to the action of 

 the electric current, has any influence on orientation of 

 Euglena in accordance with the idea of Loeb supported by 

 Torrey. This however does not mean that light does not 

 act constantly on the organism, for it is probable that It 

 does, much in the manner of temperature. The evidence 

 bearing on the point in question is however not conclusive. 

 Euglena does become more active when the Intensity is 

 increased, but it Is Impossible to say whether this increase 

 in activity Is due to absolute intensity or to the change 

 of intensity on certain structures caused by the rotation 

 of the organism. The fact that the movement of the 

 organism does cause changes of Intensity on different 

 structures in It makes the problem as to the effect of con- 

 stant light intensity an exceedingly difficult one to reach 

 experimentally. 



Nageli (i860, p. 102) concludes that In swarm spores the 

 rate of movement Is Independent of the light intensity, and 

 Strasburger (1878, p. 624) comes to the same conclusion. 

 " Die Schnelligkeit der Bewegung wird durch das Licht 

 nicht beeinflusst, doch bewegen sich die Schwarmer je 

 grosser die LIchtintensItat ist, in um so geraderen Bahnen." 

 Pfeffer (1884, p. 375) also is of the opinion that chemical 

 stimulation of fern spermatozolds causes no acceleration of 

 movement. Holmes (1903, p. 323) however says: " It was 

 found that, as the Volvox travelled towards the light, their 

 movement was at first slow, their orientation not precise, 

 and their course crooked. Gradually their path became 

 straighter, the orientation to the light rays more exact and 

 their speed more rapid. After travelling over a few spaces 

 (centimeters), however, their speed became remarkably 

 uniform until the end of the trough was reached." I came 

 to the same conclusion in my study of Volvox (1907, p. 150), 

 but was of the opinion that the Increase In rate of movement 



