32 PLANT PHYSIOLOGY 



occurs between B and C, in the region where the most light is 

 absorbed by the chlorophyll. 



Engelmann (1882) also devised an ingenious experiment to 

 show the region of greatest carbon dioxide decomposition. A 

 filament of a green alga was placed in a culture of bacteria which 

 are active only in the presence of oxygen and which do not move 

 in its absence. After the bacteria had exhausted the oxygen pres- 

 ent, the filament was placed in a solar spectrum and observed 

 under the microscope. The bacteria were seen to move from the 

 regions between D and F and to collect especially between B 

 and D, with a smaller but noticeable movement towards the blue 

 end in the region between F and G. It is chiefly in these regions 

 then that oxygen is liberated or that carbon dioxide is decom- 

 posed. More recently Dangeard has used a modification of this 

 method with Chlorella and Scenedesmus. Instead of using bacteria 

 as indicators of the oxygen release, he counted the bubbles of 

 oxygen directly as they were given off. These algae respond more 

 quickly than Elodea (Chap. Ill) and there is no inactive tissue 

 to absorb and hold gases (thus resulting in errors) as in Elodea. 

 In all such experiments, however, where a diffraction spectrum 

 is employed, it must not be forgotten that the results obtained 

 in the spectrum are not exactly the same as would occur in inci- 

 dent light, since the intensity per unit area in the different regions 

 is a function of the index of refraction. That is, when different 

 qualities are being compared, one must be sure that the intensities 

 remain the same. 



Timiriazef also determined the relative efficiency of the blue 

 and red ends in decomposing carbon dioxide. In the blue end 

 only 54% as much carbon dioxide is decomposed as in the red, 

 but since the region of absorption in the blue end is nearly four 

 times as broad, the relative efficiency per given area of the blue 

 is only 14%. This is due to the difference in energy absorbed, 

 as shown by the heat effect in the two ends of the spectrum. The 

 blue light of the spectrum is absorbed by chlorophyll but is less 

 efficient because it represents a smaller amount of energy. The 

 relation between the photosynthetic efficiency and the energy 

 absorbed is so close as to warrant the conclusion that the photo- 

 synthetic work accomplished varies directly with the energy absorbed 

 from the light regardless of the wave length. 



In ordinary green plants then the rays between B and C (the 



