§4] PHOTOTAXIS AND PHOTOPATHY 201 



('93, pp. 94, 96) for information on this subject. Negatively 

 phototactic Polygordius larvee were placed in sea water to 

 which 1% to 1.3% NaCl had been added. They now appeared 

 positively phototactic. Positively phototactic individuals, on 

 the other hand, placed in sea water diluted with 40% to 60% 

 fresh water became negatively phototactic. Similar results 

 were obtained with Copepoda. Thus increased concentration 

 rendered + phototactic (raised light attunement), while dimin- 

 ished concentration rendered — phototactic (lowered light at- 

 tunement). Increased concentration works, therefore, upon 

 Polygdrdius and Copepoda, according to Loeb, like diminished 

 temperature. 



Finally, the chemical condition of the medium has an impor- 

 tant effect on photopathy, as can be judged from certain 

 observations of Engelmann ('82% p. 391). Various chloro- 

 phyllaceous Ciliata, e.g. Stentor viridis and Paramecium bur- 

 saria, are photopathic only when the oxygen supply in the 

 medium is below the normal. In such media they are strongly 

 photophil. This case is clearly not a case of phototaxis, as we 

 have just seen (p. 187). The response is advantageous since it 

 brings these organisms into the sunlight, where chlorophyll can 

 produce oxygen. 



To recapitulate : the sense of response in phototaxis is modi- 

 fied by previous subjection to light, by temperature, and by 

 concentration. These agents modify the attunement of the 

 organism. Any quantitative experiments upon phototaxis 

 must therefore take all of them into account. Certain chloro- 

 phyllaceous organisms exhibit + photopathy, but only in an 

 insufficiently oxygenated medium. 



From the foregoing considerations we conclude that for 

 every phototactic organism there are three ranges of intensity 

 to be distinguished : the positively phototactic range in which 

 the organism moves towards the light ; below this, the indiffer- 

 ent range extending to darkness ; above it, the negatively 

 phototactic range extending up nearly or quite to the point of 

 light-rigor. The limits of these ranges vary with both external 

 and internal conditions. 



h. The Effective Rays. — We have hitherto considered chiefly 

 the action of white light, merely referring casually to the 



