Febeuaey 2&, 1915] 



SCIENCE 



329 



question of the relative efficiency of the differ- 

 ent parts of the spectrum for both forms with 

 sufficient accuracy. We selected, therefore, a 

 different method which allowed us to compare 

 the relative efficiency of two narrow parts of 

 the spectrum. A carbon are spectrum, 23 cm. 

 wide, was thrown on a black screen SS (see 



M 



Ml 



Pig. 1. 



Fig. 1) with two slits a and h in the two differ- 

 ent parts of the spectrum which were to be 

 compared in regard to their heliotropic effi- 

 ciency. The two beams of light passing 

 through the slits are reflected by the two 

 mirrors M and M^ into the square glass trough 

 in such a way as to strike the same region g 

 of the back waU. of the trough. The glass 

 trough is surrounded by black paper except at 

 B and iJ„ where the two beams of light enter 

 from the mirrors. Before the experiment be- 

 gins, all the organisms are collected in the 

 region g hj a. special arrangement which need 

 not be described here. As soon as the spectrum 

 is turned on, these organisms are simultane- 

 ously exposed to two different beams of light 

 which come from the two mirrors M and M^. 

 When one type of light, e. g., that from M, 

 is much more efficient than the other coming 

 from Jfj, practically all the organisms are 

 oriented by the light from M and move toward 



this mirror, collecting in the region B. When 

 the relative efficiency of the two types of light 

 is almost equal the organisms move in almost 

 equal numbers to B and B^. By using as a 

 standard of comparison the same region of 

 the spectrum and successively altering the posi- 

 tion of the other slit in the spectrum we were 

 able to ascertain with accuracy the relative 

 efficiency of the different parts of the spectrum 

 for the two forms of organisms. When the two 

 parts of the spectrum which are to be compared 

 are very close to each other it is necessary to 

 deflect the beams with the aid of deflecting 

 prisms, before they reach the two mirrors. It 

 turned out in these experiments that for 

 Euglena the region of maximum efficiency was 

 in the blue between A ^462 and A = 492;u,/t; 

 while for Chlamydomonas it was in the green 

 or greenish-yellow between A ^529 and 

 A = 539/A/i. In other words, Euglena behaved 

 like the seedlings of oats and like Eudendrium, 

 both of which had their maximum of efficiency 

 in the blue (in the carbon are spectrum) ; 

 while Chlamydomonas behaved like Daphnia. 

 We may remark incidentally that earlier ex- 

 periments by Loeb and Maxwell* on Chlamy- 

 domonas had led these authors to the same 

 conclusion. 



From the viewpoint of Hess, which seems 

 to have met the approval of several German 

 physiologists, we should be forced to conclude 

 that the unicellular green organism, Chlamy- 

 domonas, has sensations of brightness, that 

 it is totally color-blind and that it is not helio- 

 tropic; while the unicellular green organism, 

 Euglena, has no sensations of brightness, is 

 not color-blind and is heliotropic. The con- 

 fusion created by this mode of reasoning is 

 increased if we consider that Chlamydomonas 

 is usually claimed by the botanist and Euglena 

 by the zoologist. 



We are inclined to put a different interpre- 

 tation upon our observations, namely, that 

 heliotropic reactions may be determined by 

 two different types of photosensitive substances 

 or by the same type of photosensitive substance 

 in two modifications. One of these types of 

 substances or modifications has its maximum 



i Univ. Calif. Puil, Physiology, III., p. 195, 

 1910. 



