TROPIC MOVEMENT INDUCED BY CERTAIN RAYS I95 



Having before us all the operative factors, I now go on 

 to explain the characteristic effects in the different regions 

 of the spectrum. In the infra-red the* energy is at its 

 maximum, but there is no photosynthesis, the energy of 

 the ray being utilised in phototropic stimulation. At B, 

 where there is the maximum absorption, the photosynthetic 

 activity is at its maximum. After this rise of activity 

 there is a continuous fall, which is parallel to the decline in 

 energy of the rays. The slight dark absorption-band in 

 the blue-violet does not produce any increase of photo- 

 synthesis ; how then is the absorbed energy utilised ? In 

 seeking for an answer to this question, I made the 

 following experiments upon the phototropic activity of the 

 different rays. 



Tropic Movement induced by Certain 

 Rays of the Spectrum 



I took a number of long-stalked leaves of Tropaeolum, 



which are very sensitive to the action of light. A row of 



these, mounted in small phials containing water, were 



exposed to the spectrum. In ordinary leaves the lamina 



is so large that it would intercept more than one region 



of the spectrum. This difficulty was overcome by growing 



the Tropaeolum plants in pots, and restricting the supply of 



food-material. The result of this was that the lamina was 



reduced to the size of about i cm. in diameter : so each 



leaf in the row could thus be acted on by monochromatic 



light in the different parts of the spectrum. Exposure of 



the leaves to the spectrum for about an hour brought out 



the differences in the phototropic action of the various rays. 



It was found that there was little or no curvature produced 



in the leaves placed in the red, orange, yellow or green 



light ; there was, however, considerable phototropic bending 



of the leaf under the infra-red rays to the left of A ; the 



phototropic response in the blue-violet between F and G 



was even greater. Records of responses in the infra-red 



