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CHAPTER IX 



THE VALIDITY OF THE BUNSEN-ROSCOE LAW 



FOR THE IIELIOTROPIC REACTIONS OF 



ANIMALS AND PLANTS 



We have thus far said little about the identity of the 

 heliotropism of plants and animals. Yet the two phe- 

 nomena are essentially alike. When we keep positively 

 heliotropic sessile plants and sessile animals near a win- 

 dow, both will bend toward the source of light, though 

 the mechanism of bending may not be the same in all 

 details, the bending being produced in the case of the plant 

 (and possibly in certain animals like Eudendrium) by 

 unequal growth in length of the plant on the illuminated 

 and shaded sides; while in the case of higher animals, 

 e.g., Spirographis, it is produced by differences in the 

 tension of the muscles on the illuminated and shaded 

 sides of the animal. Motile plant organisms like Volvox, 

 are driven to the source of light, owing to differences in 

 the tension of the contractile organs on the shaded and 

 illuminated side, and the same is true for animals like 

 insects. 



A further point of coincidence lies in the validity of 

 the photochemical law of Bunsen and Roscoe for the 

 heliotropism of animals and plants. 



The law of Bunsen and Roscoe says that within certain 

 limits the chemical effect produced by light increases in 

 proportion with the product of intensity into the duration 

 of illumination, e.g., Effect = Kit, where i is intensity, t 

 duration of illumination, and K a constant. This is true 



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