Mast, Light Reactions in Lower Organisms. 165 



to the reactions of swarm-spores. It is difficult to criticise these 

 experiments, since the light intensity and time of exposure are 

 not definitely stated. However, it seems utterly impossible that 

 the effect upon the optimum in colonies exposed for so short a 

 time could, as Oltmanns states, be observed after three days. 

 For purely a priori reasons we should, nevertheless, expect expo- 

 sure to light to cause the optimum intensity to be higher, provided 

 it is exposed to light in which acclimatization takes place. It may 

 be, then, that the reason why the exposure to light in my experi- 

 ments caused a decrease in the intensity of the optimum, is because 

 the organisms were exposed to very intense light for but a compara- 

 tively short time, in other words, because they did not become 

 acclimatized. If our explanation of the cause of reversal in the 

 sense of reaction is correct, we should expect exposure to intense 

 light for a short time, to lower the optimum. This is expressed 

 in Fig. 14, path B, n n', which indicates that the colony was nega- 

 tive to a much lower light intensity immediately after it had been 

 exposed to light of high intensity than later. In accordance with 

 our assumption, in attempting to explain the reaction represented 

 by this figure it would mean an accumulation of the hypothetical 

 substances {X -) and (^+) during the time of exposure to a supra- 

 optimum intensity. 



There are some indications that when Volvox is negative to 

 light of low intensity, it becomes positive when exposed to a much 

 higher intensity. This is shown by the following observations: 



August 23, 1904, was a bright clear day. At 4 p. m. specimens 

 were collected in a place wiiich had been well exposed to the sun 

 much of the afternoon. Soon after reaching the laboratory, these 

 specimens were found to be positive in light intensities varying 

 from 230 to 1400 candle meters. The colonies not used in these 

 tests were put into a liter jar and placed in strong diffuse sunlight 

 in a west window. Here many of the colonies soon aggregated 

 on the side of the jar farthest from the source of light. At 5.45 

 p. m., after having been in the window about an hour, they were 

 found to be negative to an intensity of 230 candle meters and at 

 6.45 p. m. to an intensity as low as 3 candle meters. They seemed 

 to becoir.e more strongly negative the longer they were. left in the 

 window, although the light from 6.30 p. m. on was quite dim. At 

 the close of the experiment, 7 p. m., certain colonies which had 

 been strongly negative to an intensity of 230 candle meters were 



