The Reversal of Tropisms 97 



with the rays where they escape exposure to the 

 stronger stimulus. 



A very striking exhibition of similar behavior 

 is shown by the chloroplasts of the filamentous alga 

 Mesocarpus. According to Strasburger, "the chloro- 

 plasts in the form of a single plate suspended in 

 each cell turn upon their longitudinal axes accord- 

 ing to the direction and intensity of the light. In 

 light of moderate intensity, according to Stahl's ob- 

 servations, they place themselves transversely to the 

 source of light, so that they are fully illuminated 

 (transverse position) ; when, on the other hand, they 

 are exposed to direct sunlight, the chloroplast plates 

 are so turned that their edges are directed toward 

 the source of light (profile position)." Analogous 

 protection against too strong light occurs in some 

 diatoms in which the chloroplasts when intensely 

 stimulated become aggregated in dense clusters. 



Most of the free swimming spores of algae and 

 a great many of the chlorophyll bearing flagellates 

 keep in situations of optimal light intensity owing 

 to the fact that they are positively phototactic in 

 weak light and negatively so in strong light. The 

 particular intensity of light in which any form tends 

 to remain depends not only upon the species, but 

 also upon various conditions affecting the individual 

 organism. That the organism tends to remain in 

 a region of the most advantageous degree of illumi- 

 nation cannot of course be asserted, especially in view 

 of the many unadaptive features shown by tropic 



