182 LIGHT AND PROTOPLASM [Ch. VH 



may occur when there are in the vessel chlorophyllaceous 

 organisms producing oxygen in the sunlight. The oxygen, 

 more abundantly formed on the sunny side of the vessel, becomes, 

 then, a means of attraction to other (chemotactic) organisms, 

 whose position seems thus to be determined directly by relative 

 brightness. 



A good example of this kind of pseudophototaxis is described by Engel- 

 MANN ('81'). He found that the Schizomycetes in a certain drop of water, 

 partially illuminated, were aggregated toward the illuminated side. Exami- 

 nation revealed the presence of a chlorophyllaceous schizomycete — Bacte- 

 rium chlorinum — in the drop, and the apparent phototactic appearances were 

 easily accounted for as follows : Under the influence of light the Bacterium 

 chlorinum secreted oxygen, and this acted chemotactically to attract the 

 bacteria, which thus moved, at the same time, towards the illuminated 

 area. That it was the oxygen produced in the sunlight rather than the 

 light itself which attracted, was evinced by the fact that, when the supply 

 of oxygen is abundant in all parts of the drop, or if the Bacterium chlorinum 

 is removed, no aggregation takes place at the bright point. 



2. Distribution of Phototaxis and Photopathy. — a. Protista. 

 — We now come to the consideration of the cases of true pho- 

 totaxis and photopathy, and shall first discuss the distribution 

 of the phenomenon in the different groups of Protista. Of the 

 Protista we may take up first the chlorophyllaceous forms. 



Flagellata and Swarm-Spores. — In no other group does 

 phototaxis show itself more clearly than in this. The earliest 

 studies were made here, but despite the ease of gaining results 

 they were mostly fragmentary and uncritical. A simple ex- 

 periment of Nageli ('60) had, indeed, showed conclusively 

 that swarm-spores are responsive to light. A glass tube three 

 feet long and held vertically was filled with alga- water. When 

 the upper end of the tube was enveloped by black paper the 

 organisms moved to the lower end, and conversely. A diffi- 

 culty was encountered, however, in the fact that when zoospores 

 were placed in a plate by a window, the organisms gathered 

 at the edge next the window, which, since the edge of the plate 

 threw a shadow there, was the darkest part of the surface. In 

 consequence some authors had concluded that swarm-spores 

 shun the light ; whereas Cohn asserted, all too briefly, that 

 they move in the direction of the rays and toward the source 

 oftlight. Finally, Famintzin ('67) had discovered that swarm- 



