324 LIGHT AND THE BEHAVIOR OF ORGANISMS 



Dunkel ihnen unangenehm ware." The process of col- 

 lecting in regions of given light conditions in these forms 

 is therefore precisely the same as that which Jennings 

 found some fifteen years later with reference to the col- 

 lection of Paramecium caudatum and aurelia in regions 

 containing carbon dioxide. 



It is evident that in Paramecium bursaria a reduction of 

 light intensity or a change from regions illuminated by 

 the longer waves to one illuminated by the shorter causes 

 a definite response. Whether or not this response would 

 under certain conditions result in orientation in these 

 organisms as it does in Euglena was unfortunately not 

 ascertained. Nor was it ascertained whether or not they 

 ever become negative, i.e., respond to an increase of inten- 

 sity or to a change from the shorter waves to the longer. 



Engelmann thinks that the reactions of these organisms 

 to light is regulated by the oxygen liberated, that it is a 

 change in the oxygen pressure that produces a stimula- 

 tion. The response w^hich results in aggregation in these 

 organisms is however, undoubtedly, at least indirectly 

 dependent upon the time rate of change of light intensity 

 rather than upon the absolute intensity, and their reaction 

 system and method of collection in a given region are 

 evidently quite different from those in diatoms. 



c. Flagellates with Euglena viridis as a type (1882, 

 P- 395)-^ — The light reactions of the organisms in this 

 class are not primarily dependent upon . oxygen pressure. 

 They respond to light in the same way whether the oxygen 

 pressure is normal or above or below normal, unless it is 

 carried to such extremes that all activity ceases. 



These organisms were found to form dense aggregations 

 in the more highly illuminated regions, just as Paramecium 

 bursaria does when the oxygen pressure is below normal, 

 but in the microspectrum they collect in the blue near 

 the Fraunhofer line F, 470-490^^, not in the red where 



^ Bacterium photometricum (1883, pp. 95-124), a form on which Engel- 

 mann worked later, might also be included here. 



