HELIOTROPISM IN EUGLENA 403 



drop preparations were subjected to the light of an incandescent 

 electric light bulb, removed foot by foot, and at each distance the 

 diminution in tropism and shading reaction noted. Tlie tropism 

 and shading reaction were found to weaken and disappear together 

 until at ten feet they were absent in one preparation and could 

 just be made out in the other. Moreover, as the light diminished 

 it could be seen that the individuals which were still heliotropic 

 were the same ones which still gave the shading reaction. Here 

 then we have exactly the kind of a result that Jennings's theory 

 demands. But this does not mean that the shading reaction 

 must always accompany such positive heliotropism. For if plants 

 of this kind be put in the strong arc light for five or ten minutes 

 and then brought back into the weak light, they become positive 

 at once but fail to give the shading reaction for two or three 

 minutes. 



Most of the work on this correlation was done on the negatively 

 heliotropic organisms of Culture A. From the outset they be- 

 haved entirely differently and not at all as required by Jennings's 

 theory. They were not so sensitive to light but would show a 

 feeble negative heliotropism at one or two feet from a glowing 

 carbon filament. To light stronger than this they showed a 

 decided negative heliotropism which became more pronounced as 

 the light became stronger. But in bright diffuse daylight, and 

 even occasionally in direct sunlight no illuminating reaction could 

 be obtained. It was only in the arc light which had been strongly 

 concentrated with a lens that shock-movements were produced 

 by sudden illumination. When first subjected to this strong light 

 all the individuals gave the illuminating reaction with the greatest 

 precision. When, however, a strong light was allowed to act 

 continuously for half an hour or an hour, the illuminating reaction 

 was seen to become weaker and weaker until in some cases it 

 entirely disappeared in nearly all of the individuals. 



We see, then, that the shading reaction may be strongly devel- 

 oped where there is no heliotropism; and also that both positive 

 and negative heliotropism can be regularly obtained without any 

 differential reactions at all. It is obvious that no constant corre- 

 lation exists between the heliotropism and the differential reac- 



