3l8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



Claviceps microcephala turn toward the source of light 

 nearly as rapidly in the red as in the blue, and Brefeld 

 obtained similar results for Pilobolus microsporus and 

 Pilobolus crystallinus. 



Some of the contradictory results mentioned above are 

 evidently due to the fact that the authors did not take into 

 account the effect of the time of exposure and the inten- 

 sity of the light. If the blue, for instance, is more active 

 than the red, the maximum curvature will take place in 

 the blue in weak illumination, while a minimum curvature 

 will take place in this region in very strong illumination; 

 for plants either become negative in high light intensity or 

 fail to respond. Recently Blaauw (1909) made a thorough 

 investigation of the reactions of plants in different regions 

 of the spectrum with these facts in mind. He calculated 

 the relative efficiency of different rays in terms of energy 

 contents and time of exposure of the reacting organ, and 

 found for oats seedlings, Avena sativa, that in medium light 

 intensity and time of exposure there is a slight reaction from 

 the red end of the spectrum to the green, 500^^, then a 

 rapid increase to a maximum in the indigo 465^'', and a 

 decrease to zero well in the ultra-violet. For equal energy 

 and time of exposure the reaction is 2600 times greater in 

 the region of maximum efficiency than in the red, yellow 

 and green, twice as great as between the violet and the 

 ultra-violet, 390^^, and only four times as great as in the 

 ultra-violet at 365''^. 



In case of the fungus Phycomyces, Blaauw found the 

 effect of red and yellow relatively much greater than in 

 Avena. In other respects the distribution of efficiency in 

 the spectrum was found to be similar in the two forms. 



In spectra of very high intensity produced by means of 

 a grating it was found that the minimum rate of curvature 

 for Phycomyces is in the indigo, and that there are two 

 regions of maximum rate of curvature, one in the red, the 

 other in the violet. This, the author maintains, is due to 

 the fact that Phycomyces is positive in weak light (100- 



