HELIOTROPIC REACTIONS — ANIMALS AND PLANTS 229 



elusion that the bhie is more effective than the green, yellow, 

 red, indigo, and violet; but they do not permit a more definite 

 statement. 



Engelmann states that he found a strong gathering of Euglena 

 in the blue between 470 and 490 mm in a spectrum.'-^ 



When we made similar experiments with Chlamydomonas pisi- 

 formis, which is also a chlorophyll-bearing unicellular organism 

 like Euglena, we noticed that the gathering went much farther 

 towards the yellow ending at about X = 5G0 or 570 mm- The 

 region of maximal gathering seemed to be at about X = 520 mm- 

 A similar result had previously been obtained with Chlamydo- 

 monas by Loeb and Maxwell.^'' 



The method is still less definite with larger and rapidly moving 

 animals, and j^et it is mainly by such experiments that Hess 

 tried to prove that the most efficient part of the spectrum for 

 heliotropic animals is identical with that which appears brightest 

 to the totally color blind. 



IV. EXPERIMENTS WITH THE TWO-BEAMS METHOD 



With the aid of two slits (fig. 1) two narrow strips of the spec- 

 trum were cut out. Their width was such that in the green part 

 of the spectrum the difference in the wave length of the extreme 

 rays that passed through was about 10 mm- With the aid of 

 prisms and mirrors these two parts of the spectrum were made to 

 converge to one spot in the trough where the organisms had 

 previously been collected. It was ascertained which of the two 

 beams of light was more powerful. 



In detail the experiments were as follows. The trough was 

 surrounded with black cardboard in which there was one open- 

 ing at that spot where it was intended the animals should col- 

 lect. Then an incandescent lamp was turned on in front of this 

 opening which caused all the organisms to collect at that spot 

 of the trough. When this happened, the spectrum was turned 

 on and the incandescent lamp turned off and the animals were 

 exposed to the two beams of light a and h selected for com- 



18 Engelmann. Arch. f. d. ges. Physiol., 1882, 29, 387. 



i« Loeb and Maxwell. Univ. Cal. Pub., 1910, Physiol., 3, 195. 



