512 S. O. MAST 



between 524 and 574 ^u^t, the results being too indefinite to locate 

 it more precisely. In three of these series the colonies were 

 negative and in two of these the region of maximum effect was 

 near 554 z^^, while in the remaining one it was near 544 mx. 

 These results support the contention that the relation between 

 stimulating efficiency and wave-length is the same for negative 

 as it is for positive colonies. 



In three of the series of tests the spectrometer slits were re- 

 duced to 0.1 mm., thus greatly increasing the purity of the spec- 

 trum and greatly reducing the illumination. In all of these 

 series the effective region in the spectrum was greatly shortened, 

 this region extending only approximately from 510 to 570 n/j,. 

 In two of these series the region of maximum effect was near 544 

 /XM, but in the other one it was nearer 554 /jl/j.. There is conse- 

 quently no evidence indicating that increase in the purity of the 

 spectrum causes any appreciable alteration in the location of this 

 region or that it is shifted toward the violet in low intensity as 

 it is in the case of human beings. That is, there is no evidence 

 indicating that there is a Purkinje effect in Pandorina. 



The distribution of maximum efficiency was calculated only for 

 the results obtained in the three series of tests given in the table, 

 but there is no reason for assuming that such a calculation for 

 the remaining series of tests would yield results materially 

 different. 



Eudorina elegans 



Eudorina and Pandorina were usually found in the same pools, 

 but both were never observed to be very abundant at the same 

 time. Pandorina ordinarily predominated when the pools con- 

 tained much decaying organic material, and Eudorina when they 

 contained little. Thus in fresh pools rich in such material the 

 former generally appeared first, developed rapidly until a cer- 

 tain stage in the disintegration of the organic substance was 

 reached and then they gradually decreased, while the latter ap- 

 peared and began to increase, becoming abundant after the 

 former had practically disappeared. 



Eudorina orients very precisely in light, but like Pandorina it 

 deflects to the left and the degree of deflection varies with the con- 



