SPECTRAL COLOR AND STIMULATION 505 



The results of two of the most extensive and accurate of- these 

 series are given in table 8. In the second series given in this 

 table (B) the readings are much smaller than in the first (A). 

 This is due to a great reduction in the luminous intensity of the 

 spectrum caused by bending in the filaments of the lamp, which 

 was not discovered until after the series of readings was com- 

 pleted. Both series indicate clearly that for the species tested 

 the region of maximum stimulating effect and also efficiency is 

 in the green between 500 and 515 mm, that is, considerably nearer 

 the red end of the spectrum than it is for Euglena and Trachelo- 

 monas, but that the rate of decrease in stimulating efficiency in 

 either direction from the maximum is about the same in all of 

 these forms (table 15). This contention is supported by the 

 results of all of the tests made on Chlamydomonas except those 

 which were too indefinite to be of value. 



The conclusions regarding the location of the region of maxi- 

 mum stimulating effect do not agree with that reached by 

 Loeb and Wasteneys ('16). These authors maintain that this 

 region is considerably nearer the red end of the spectrum, i.e., 

 in the green near the yellow at about 535 mm- 



The discrepancy between the results obtained by Loeb and 

 Wasteneys and those obtained by the writer can not be ac- 

 counted for on the basis of the fact that the former used a car- 

 bon arc as a source of light while the latter used a gas-filled tung- 

 sten lamp, for the difference in the distribution of energy in the 

 two prismatic spectra is wrong in direction to explain this dis- 

 agreement. The discrepancy mentioned must therefore b due 

 either to the fact that the two sets of observations were not 

 made on the same species, pisiformis being used by Loeb and 

 Wasteneys and fluvialis (?) and globulosa by myself, or to 

 inefficiency in methods and observations. 



Gonium pectorale 



Specimens of Gonium were frequently found in the clay pools 

 at various times during the summer, but never in abundance 

 and they were always mixed with other forms, so that the obser- 

 vations were continuously hampered by insufficient material, and 

 moreover, orientation was never veiy precise. The results of 



