REVERSION IN ORIENTATION TO LIGHT 



369 



and the lamps it was possible to obtain quickly a wide range in 

 illuminations. The beam of light, before it entered the dark- 

 room, passed through a heat-screen consisting of a Pfeifer warm- 

 ing-stage containing distilled water. This screen was so ar- 

 ranged that a stream of water continuously flowed through it so 

 as to prevent excessive heating. The observations were prac- 

 tically all made in six rectangular glass aquaria which were des- 

 ignated observation aquaria. These aquaria were 2.7 cm. wide, 

 2.7 cm. long and about 1 cm. deep. They were constructed from 

 pieces of the best-quality microscope slides and Khotinsky 

 sealing-wax. 



Fig. 1 Diagram representing arrangement of apparatus, a, rectangular ob- 

 servation aquaria; I, gas-filled steriopticon lamp; b, beam of light; w, wall be- 

 tween dark-room and room containing lamp; h, he'at-screen containing running 

 distilled water or saturated solution of chlorophyl in 96 per cent alcohol; t, dead 

 black table, 7 m. long; s, light-screens. 



RELATION BETWEEN ILLUMINATION AND REVERSION IN 

 ORIENTATION 



Both Volvox and Pandorina are very sensitive to light and they 

 orient fairly precisely. Like a considerable number of other 

 similar organisms, they have ordinarily been found to be posi- 

 tive in weak and negative in strong light. This has often been 

 observed in previous work (Mast, '07, '11, '18) and it was re- 

 peatedly observed in the experiments performed in connection 

 with this work. The following detailed description of a typical 

 series of observations clearly illustrates the relation between this 

 change in the sense of orientation and the intensity of illumination. 



On August 8, 4.40 p.m., colonies of Pandorina which had been 

 in darkness for nearly four days were exposed in strong direct 

 sunlight. At first they were very inactive and there was no indi- 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 27, NO. 3 



