400 FRANK W. BANCROFT 



tion had been kept over night in the dark the photochemical sub- 

 stances involved had changed at different rates so that it was now 

 easy to obtain drops in which 99 + per cent were decidedly nega- 

 tive but in which they all reacted to shading and not to illumina- 

 tion. This experiment was repeated many times, and never 

 failed to give the results described. Some times this condition 



CULTURE 



R HEL + + + + + ++ + + + OOP / Ti^, ,-)\ 



D SM. 33S3SSSS6SSSSSS3011 If ILIbM I J 



/\ HEL.OOO O O O O O O fi ipurl 



^■^^S.M, OOOOO OOOOOOOOOOII 1 I I F iLlbni } 



A 



5M. 5 s s s 3 I I r^ (LIGHT) 



r HEL . + + + + + 



LS.M I I I I I 



Fig. 4 Diagram to show the correlation between the sign of the heliotropism 

 and the character of the shock-movements. Distances along the horizontal line 

 indicate distances from the light. On the left, then, we have a weak light and on 

 the right a strong light. Above the line is indicated the sign of the heliotropism 

 {Hel.}. Below the line is indicated the character of the shock-movements (S.M.). 

 S = shock-movements produced by sudden shading, and not by sudden illumi- 

 nation; I = shock-movements produced by sudden illumination, and not by sud- 

 den shading; F = shock-movement mechanism becomes fatigued if light lasts for 

 half an hour or an hour; O = absence of reaction; B = correlation in Culture B; 

 A = correlation in ordinary individuals of Culture A; Lower A = correlation in 

 exceptional samples of Culture A in which the shading reaction appeared; C = 

 correlation in exceptional sample from Culture B. 



would last but a short time at any one light intensity, and then 

 would soon be obscured by some of the individuals becoming neu- 

 tral or positive. But in these cases a stronger light would soon 

 bring it on again. 



When the conditions were right for this experiment the same 

 individual was often seen to swim away from the light, to give a 



