194 CHAP. XXII. EFFICIENCY OF THE SPECTRAL RAYS 



that photosynthesis depends only on the energy or intensity 

 of radiation. But the curves of energy of radiation and 

 of photosynthesis do not run a parallel course ; for the 

 intensity of radiation is at its maximum in the infra-red 

 to the left of A, where photosynthesis is practically zero. 



The less refrangible visible rays of greater wave-length 

 have been held by others to be the most effective. But the 

 least refrangible visible ray A induces hardly any photo- 

 synthetic action. Finally, it has been thought that it is the 

 amount of energy absorbed, as indicated by the chlorophyll- 

 spectrum, which is the chief factor in photosynthetic action. 

 This view finds partial support in the occurrence of 

 maximum activity at B, which is also the region of greatest 

 absorption in the spectrum : but it fails to explain the 

 considerable amount of photosynthetic activity in the 

 orange and in the yellow, in which there is no great absorp- 

 tion. There is, again, very little activity corresponding to 

 the absorption-band in the blue-violet. 



These anomalies disappear if we take full account of all 

 the factors and their combined effects. 



1. Other things being the same, photosynthesis depends 

 on the energy of radiation. 



2. Photosynthetic activity is greatest in the region of 

 maximum absorption in the spectrum. 



3. When photosynthesis and absorption do not coincide, 

 it may be inferred that the absorbed rays are utilised for 

 some other work. 



Complementary A and D effects. — The third factor in 

 the list given above requires further amplification. It is 

 obvious that the incident radiation can be utilised (1) for 

 increasing the potential energy, or (2) dynamically in 

 movement : the former takes place characteristically in 

 photosynthesis ; the latter in phototropic stimulation. The 

 rays at the opposite ends of the spectrum — the infra-red 

 and the highly refrangible blue-violet — being, as will be 

 presently shown, the most effective in phototropism, are 

 the least effective in photosynthesis. 



