ABSORPTION-SPECTRUM OF CHLOROPHYLL 191 



verge of response so that it is possible to detect and measure 

 the effect of incident radiation of even feeble intensity. 



Before describing the effects of different rays of the 

 spectrum on photosynthesis I must refer to two important 

 factors which affect photosynthesis, namely, the absorption 

 of particular rays by the chlorophyll and the utilisation of 

 the energy of certain of these rays in phototropic stimulation. 



The Absorption-Spectrum of Chlorophyll 



Spectroscopic observations were made to ascertain what 

 absorption-bands are produced by the interposition of a 

 single leaf of Hydrilla in the path of light. The spectrum 

 showed (1) a feeble general absorption and numerous faint 

 dark lines throughout ; (2) an intense dark band between 

 the red and the orange on both sides of B (680 /a/a), the 

 darkest region being slightly to the left of B ; and (3) a less 

 dark absorption-band in the blue-violet region between F 

 and G. This second band was inconspicuous when the 

 spectrum was bright with strong light ; but with feeble light 

 it was more noticeable. 



Typical Experiments on the Photosynthetic 

 Efficiency of the Different Spectral Rays 



The following are the results of an experiment upon 

 the relative effectiveness of light of different wave-lengths, 

 made on the heterostatic method. For facility of com- 

 parison the maximum activity, occurring at B, is taken 

 as 100. Photosynthesis was found to be initiated even 

 in the invisible infra-red region : the activity was, how- 

 ever, feeble, being 7-6 at about 770 /a/a. At A (760 /a/a) 

 the activity increased to 12 ; at a (720 /a/a) it rose to 35. 

 The maximum activity of ioo was attained at B (680 /a/a). 

 After this, photosynthetic action underwent a decline to 70 

 at C (656 fifi), to 39-6 at D (590 /a/a), to 15 at E (527 /a/a), to 

 6-8 at F (486/4//,), and to 2-0 at G (430 /a/a). 



I give in the following table results obtained with three 



