210 LIGHT AND PROTOPLASM [Cn. VII 



straight line, since the muscles of the two sides now act simi- 

 larly. Thus orientation of the organism is effected. The same 

 explanation, which is modified from one of LOEB ('93, p. 86), 

 will account, mutatis mutandis, for positive phototaxis. 



Such an explanation can serve only for elongated organisms. 

 The case of the amoeba is quite different. Here we must think 

 of the protoplasm as being modified by a light ray so as to flow 

 centrifugally especially in that ray, perhaps through peculiar 

 molecular disturbance wrought by the ray. 



As for photopathic response, that is probably to be accounted 

 for on the ground upon which ENGELMANN has explained the 

 arrangement of Bacterium photometricum in the microspec- 

 trum ; on the ground, namely, that increased brightness causes 

 a movement forwards, that a diminution in brightness causes a 

 movement backwards, or vice versa, thus resulting in the accu- 

 mulation of the organisms in the darker or lighter parts of the 

 field. 



To summarize, then, light acts directly either through differ- 

 ence in intensity on the two sides of the organism, or by the 

 course the rays take through the organism. Difference in 

 intensity of light may also determine the position of organisms 

 with reference to light by virtue of the irritation produced by 

 rapid change of intensity. 



SUMMARY OF THE CHAPTER 



The study of the effect of light on protoplasm must be made 

 quantitative as well as qualitative, and demands the use of appa- 

 ratus for determining the quality and intensity of the light 

 employed. The reactions produced by light upon protoplasm 

 are undoubtedly of a chemical character, and, indeed, experi- 

 ments with non-living organic compounds show that it has an 

 important effect in synthesis, in analysis, in substitution, in the 

 production of isomeric or potymeric conditions, and in fermen- 

 tation. Since protoplasm consists of a large number of kinds 

 of organic substances, we should expect light to produce far- 

 reaching results, the more so as it can penetrate deep into the 

 tissues of the organism. 



The effect of light upon the general functions of organisms is 



