776 . BIOLOGICAL EFFECTS OF RADIATION 



on stomatal opening. Darwin's experiments (8) also brought out the 

 marked influence of the red end, but he could obtain no secondary maxi- 

 mum in the other end of the spectrum. Lloyd (23) used, instead of a 

 spectroscope, as Kohl and Darwin had done, two color filters, to obtain 

 different spectral regions. The first, a bichromate solution, transmitted 

 the region between 5400 A and 7000 A; the second, a copper ammonium 



~ o 



sulfate solution, transmitted the region between 4800 and 4200 A. 

 Stomata opened behind both filters, but the influence of the red part of 

 the spectrum was stronger than that of the blue. More recently Sayre 

 (35) has carried out a more extensive investigation with more accurately 

 recorded light conditions. He used, with a sunlight source, eleven Com- 

 ing glass filters transmitting various regions of the visible, ultra-violet, 

 and infra-red. He found that stomata did not open in wave-lengths 

 longer than 6900 A but could not determine the exact limit of effectiveness 

 in the blue-violet end of the spectrum, as light intensity became the 

 limiting factor. Other regions of the visible transmitted by his filters 

 seemed equally effective. He used no filters, however, which transmitted 

 only the green and yellow regions. In no one of the investigations 

 mentioned above was the intensity of radiation falling on the stomata 

 controlled or measured. Hence, while we may assume from these 

 investigations that stomata will open in either the blue or the red end of 

 the spectrum, they do not give sufficient data to enable us to determine 

 the relative effectiveness of the two regions. 



Information on this subject has been supplied by the recent work 

 of Sierp (45) who has carried out a very careful investigation on the 

 opening of stomata in different regions of the visible spectrum. Great 

 precautions were taken not only with the photometric methods but also 

 with the choice and treatment of experimental material. The light 

 source was a "Kinobox" lamp, 500 watt, 110 volt. Radiation from it 

 passed through a 1-cm. layer of 6 per cent CuSOi and then through one 

 of five filters (Schott und Gen.), before entering a small circular opening 

 in the experimental chamber. The CUSO4 layer absorbed most of the 

 infra-red, and the filters each transmitted a different region of the 

 visible spectrum. The maximum transmission of energy through the blue 

 filter occurred at wave-length 4360 A. For the green fijter the maximum 

 transmission point was 5090 A; for the yellow, 5460 A; for the orange- 

 yellow, 5780 A ; and for the red, 6440 A. The total energy actually falling 

 on the stomata under observation was measured by means of a Moll 

 thermopile. With the blue filter, which was the least transparent 

 one, in one series of experiments, this total energy was found to be 

 0.095 cal./cm.Vmin. Hence the total energy transmitted by the other 

 more transparent filters was cut down, by the insertion of photographic 

 plates in the path of the light, to that of the blue filter. In another series, 

 the total energy in all cases was equalized to 0.056 cal./cm.Vmin., which 



