HELIOTROPIC REACTIONS ANIMALS AND PLANTS 231 



the organisms were all scattered equally again in the trough. 

 Fresh organisms were used every day. 



As stated before, one of the two parts of the spectrum was the 

 same in a group of experiments while the other changed in suc- 

 cessive experiments throughout the spectrum. In figure 4 the 

 results of 21 experiments with Euglena are plotted. The part 

 of the spectrum which was stationary was situated at about 

 470 MM which previous experiments had led us to believe was 

 the most efficient wave length. The different degrees of black- 

 ness indicate the denseness of the gathering; the more animals 

 gathered in one spot the darker the oblong representing the ex- 

 periment. We notice that the oblongs at the region 470 mm ai-e 

 with two exceptions much darker than all those at other wave 

 lengths. These results indicate that the greatest efficiency is 

 possessed by the rays between 460 and 490 mm for Euglena viridis. 

 The total of all experiments is represented in diagram II, figure 

 7, where the distribution of this efficiency through the carbon arc 

 spectrum is plotted for this form. The greatest efficiency is in- 

 dicated by the greatest blackness, the greater blackness indicat- 

 ing the denser gathering. 



The results with Chlamydomonas were entirely different. In 

 figure 5, the region between 470 and 480 mm was again constant 

 in each determination, while the region compared with this var- 

 ied in each experiment. It is obvious that in contradistinction 

 to the experiments on Euglena the region between 460 and 480 

 ixjj. was less efficient than the region from 490 to almost 560 ixijl. 



We, therefore, started another series of determinations in 

 which the region about 534 jjlij. was constant (fig. 6). We now 

 found that this region was more efficient than any other region 

 in the visible spectrum. The experiments show that the maxi- 

 mal efficiency lies for Chlamydomonas, approximately in that 

 part of the spectrum which appears brightest to the totally color 

 blind human. 



In the same way long series of experiments were made with 

 the newly hatched larvae of Arenicola, an annelid. The experi- 

 ments in this form suffer from the difficulty that the larvae have 

 a tendency to stick to the glass walls of the trough. We found 



