296 THE PROTOZOA 



Light as well as heat rays frequently have a similar directive 

 effect upon Protozoa, a phenomenon called phototaxis by Strasburger, 

 which is explained as follows by Verworn : " A ray of light extends 

 through space from a source of light, in a straight direction, and 

 diminishes in intensity with the distance, hence any two points in 

 the line of the ray possess different intensities ; the point that is 

 nearer the source has the greater, that which is farther away has the 

 less, intensity. A ray of light, therefore, fulfils very completely the 

 conditions that are necessary to the appearance of unilateral stimula- 

 tion ; in fact, it is extremely difficult to establish conditions under 

 which an organism is stimulated by light uniformly on all sides. As 

 a result of this, stimulation by light calls out very pronounced direc- 

 tive effects." l A difference of opinion exists, however, as to the 

 general effect of light upon organisms ; Klebs ('85), on the one hand, 

 asserted that all protoplasm is sensitive to light, while Verworn, after 

 trying in vain to get reactions on the part of certain organisms, con- 

 cluded that such is not the case. It appears from numerous investi- 

 gations that the most phototactic forms are the flagellated cells, 

 Strasburger ('78) observing that swarm-spores of different plants are 

 attracted toward a light of a certain intensity, and move away from 

 a light of greater intensity, while he and Cohn ('64) determined the 

 fact that rays having a short wave-length, especially the blue and the 

 violet, are more effective than those of the longer wave-length, such 

 as the red. Engelmann ('82) saw Euglena gather in heaps along the 

 region of the line F of a microspectrum, and he also established 

 the fact that colorless forms, such as Chilomonas paramcecium, or the 

 colorless swarm-spores of. the Chytridiae, are positively phototactic in 

 weak, and negatively phototactic in strong, light. 



Among the Infusoria, light reactions are much less marked than 

 among the Mastigophora, and light seems to be quite unnecessary for 

 their existence. 2 Nevertheless, in one form, at least, positive reactions 

 have been demonstrated by Verworn ('89), who showed that Pleu- 

 ronema chrysalis reacts vigorously to light stimuli through cobalt 

 glass, while light through red glass gives no effects, an experiment 

 which shows that it is not the longer but the shorter light vibrations 

 that are effective. 



The results with Rhizopoda have been somewhat more definite 



1 Lee, 1900, p. 447. 



2 The slight effect of light upon Ciliata is shown by the following table from Maupas 

 ('88). The organisms were kept for one month in the light and one month in darkness, 

 and during that time they increased by division as follows : 



Colpidiuvn colpoda in darkness 48 times ; in light 46 



Glaucoma scintillans " " 98 " " " 99 



Paramcecium bursaria " " 9 " " " 9 



Stylonychia pustulata " " 48 " " " 50 



