INFLUENCE OF LIGHT UPON LIFE-PROCESSES 431 



The validity of the Bunsen-Roscoe law in the heliotropism of organ- 

 isms has been established in a variety of investigations. Thus Blaauw 

 has determined the time required to produce heliotropic curvature in 

 the seedlings of Avena sativa by varying intensities of illumination 

 with the following results : 



Duration of Candle-meters 



Candle-meters. illumination. X seconds. 



26.3 

 20.6 

 21.9 

 18.6 

 19.1 

 16.2 

 17.2 

 18.3 

 19.7 

 22.4 

 23.9 

 21.6 

 24.8 

 27.5 

 24.2 

 21.8 

 16.9 

 18.9 

 18.0 

 24.7 

 20.5 

 22.8 

 19.0 

 19.8 

 16.4 

 26.5 



It will be seen that the product of the intensity into the duration of 

 the illumination approaches the constant value of 20, and indeed, 

 when one considers the very great range of intensities employed and 

 the inherent variability of living material, the degree of constancy 

 observed is really astonishing. A remarkable instance of the applica- 

 bility of this law to the Heliotropism of animals is afforded by the 

 experiments of Loeb and Wasteneys, upon the polyps of Eudendrium. 

 These hydranths are exceedingly variable in their response to light and 

 it was accordingly necessary to make a great number of measurements 

 and treat the results statistically. The polyps were exposed to three 

 different intensities of light, a light of definite strength being stationed 

 at three different distances, namely 25 cm., 37.5 cm. and 50 cm. from 

 the organisms and they were exposed to the light for such periods as 

 to render the product i X t a constant. Under these circumstances 

 it was found that some among any group of polyps underwent helio- 

 tropic curvature, while others did not. The percentage of bent polyps 

 was determined in each case, and if the Bunsen-Roscoe law were valid 

 it is evident that these percentages should be the same for all three 

 intensities of light, i. e., the percentages undergoing bending at the 

 distances 25, 37.5 and 50 cm. from the light should be the same or 

 1 : 1 ; 1, The actual ratios were determined for each of the possible 



