4] PHOTOTAXIS AND PHOTOPATHY 199 



by STRASBURGER (78, p. 574) on Ulothrix spores, which are 

 positively phototactic in a weak light. While responding to 

 such a light, they do not, however, turn at once when a light 

 of repelling intensity is thrown upon them. So, also (p. 600), 

 when Hsematococcus is responding to indigo light, the inter- 

 position of red glass does not at once cause it to turn from its 

 path. In all these cases, the immediately preceding condition 

 of light continues to exert an action which modifies the response. 



Closely allied are the results obtained by VERWORN ('89, 

 p. 50) upon the diatom, Navicula brevis, which is attuned to 

 only the faintest light. When, however, a culture had been 

 reared by a window for two weeks, the attunement to light 

 had been so raised that now a slight degree of positive photo- 

 taxis took place in diffuse light. We have in these facts 

 examples of a phenomenon which we have observed in the 

 action of other agents. It is one expression of the acclimatiza- 

 tion of organisms to the peculiar conditions of their environ- 

 ment. We have just seen that every organism has its optimum 

 intensity of light for metabolism and response, and that this 

 optimum is very varied; but, throughout, one law holds. 

 Organisms which are accustomed to live in strong light have 

 a high optimum intensity; and those accustomed to live in 

 a weak light have a low optimum intensity. This relation 

 is, indeed, so close as to raise the suspicion that the normal 

 intensity of the light has determined the optimum. And this 

 suspicion is confirmed by the experimental evidence just cited. 

 Now, since the position of the optimum is usually advantageous, 

 we may conclude that light can so modify protoplasm as to 

 adapt it for the conditions in which it is living. 



We now pass to the consideration of the effect of tempera- 

 ture upon response. This effect was noticed by STRASBURGER 

 ('78, p. 605) in the swarm-spores of Hsematococcus, Ulothrix, 

 etc., which, at a temperature of 1"6 C. to 18 C., gather at the 

 side of the drop next to the window. If, now, they are sub- 

 jected to a temperature of 40 C., the intensity of the light 

 being constant, they migrate to the opposite side. On the 

 other hand, at a temperature of 35, the -f aggregation is more 

 complete than at 16 to 18. Control experiments with emul- 

 sions satisfied STRASBURGER that this change is not due to 



