24 



PHYSIOLOGY OF NUTRITION 



Engelmann was the originator of the bacterial method for the study of 

 photosynthesis. It is well known that many bacteria are active only in the 

 presence of oxygen, and that their movement ceases as soon as there is no 

 oxygen present. If a filament of a green alga is placed in a culture of such 

 bacteria, upon a slide, and if the preparation is protected by a cover glass and 

 darkened, the movement of the bacteria eventually ceases because of lack of 

 oxygen. If a solar spectrum is now projected upon the alga filament, under the 

 microscope, it is seen that the movement of the bacteria is renewed in the neigh- 

 borhood of both of the main chlorophyll absorption bands (Fig. 14), being espe- 

 cially pronounced in the red and appreciably weaker in the blue. It is only in 

 the spectral regions thus undicated, therefore, that an evolution of oxygen 

 occurs, to which the bacteria respond. 



The degree of difference between the efficiences of the blue and red spectral 

 regions was established by Timiriazev. 1 For this purpose he divided the 



aB C D 



Eb F 



I_ 



Fig. 14. — Bacterial movement in the regions 

 of the absorption bands of chlorophyll. (After 

 Englemann.) The dots indicate moving bacteria 

 and the letters denote the Fraunhofer lines. 



700 



600 



500 



WO 



Fig. 15. — AB, distribution of heat 

 energy in the solar spectrum. (After 

 Langley.) 100—14, relative rates of car- 

 bon-dioxide decomposition by leaves in 

 red and in blue light. 



spectrum into two equal parts by means of a cylindrical lens and a prism with 

 a very small angle of refraction. Flat-sided glass tubes containing pieces of 

 leaves of equal area were placed in the bright bands of blue and yellow light 

 thus obtained, and a gas analysis of the tube contents was made after three- 

 quarters of an hour or an hour. If the intensity of carbon dioxide decomposi- 

 tion in the less refrangible (red-yellow) light be taken as 100, then the corre- 

 sponding intensity in the more refrangible (blue) light is 54. Thus the light 

 absorbed by the leaves in the blue half of the spectrum is only about half as 

 effective as that absorbed in the other half. The absorption spectrum of the 

 leaves used in Timiriazev's experiment is presented in Fig. 15. It must be 

 noted, however, that the two absorption bands are not of equal width, the one 

 in the blue-violet region of the normal spectrum being more than three times 

 as wide as the band between B and C. If each of the ratios mentioned above is 



1 Timiriazev, C, Photochemische Wirkung der am Rande des sichtbaren Spektrums liegenden Strahlen. 

 1893. (Russian.)* 



