28 TEXTBOOK OF PLANT PHYSIOLOGY 



wave length. The faculty of fluorescence points to a considerable 

 photochemical activity of chlorophyll. 



The mechanism by which chlorophyll renders the radiant 

 energy absorbed by it effective in the process of decomposition of 

 carbon dioxide, is not clear to us. As has been said in Art. 4, 

 chlorophyll plays the role of a sensitizer, i.e., it has the faculty of 

 rendering the energy of the absorbed rays effective for the process of 

 CO2 decomposition, which in itself is not subject to the influence 

 of visible light. According to modern photochemistry, radiant 

 energy does not move in a continuous stream but in separate jerks 

 called quanta. Every substance subjected to photochemical 

 decomposition absorbs the radiant energy in a strictly defined 

 quantity — one or more quanta per molecule. The quanta are con- 

 stant magnitudes for each kind of rays, but they vary with the 

 length of the wave; the greater their length, the smaller the quan- 

 tum. Therefore, the number of gram-calories of radiant energy 

 which is absorbed in the photochemical transformation of one 

 gram-molecule of substance is not the same for the different rays of 

 the spectrum : the greater the length of the wave, the smaller the 

 number of gram-calories. For rays with X486 (blue) this num- 

 ber will be 58.470 Cal., for X589 (yellow) 48.240, for X800 (infra red) 

 35.510. The greater magnitude of the quanta in ultra-violet rays 

 is demonstrated by their considerable photochemical effect, for 

 instance, in the decomposition of silver salts in photography, and 

 the relatively weak photochemical action of red and yellow light 

 is due to the small size of the quanta in these rays. 



The process of decomposition of carbon dioxide is, however, 

 not photochemical in itself. By illumination l alone, the break- 

 ing down of the molecules cannot be induced. This is due to the 

 fact that carbon dioxide, being perfectly transparent, absorbs 

 almost no radiant energy, and that for the decomposition of 1 

 gram-molecule of carbon dioxide, according to the formula 

 CO2 — * C + O2, 98 Cal. are required, or almost twice as much 

 as are produced by the blue rays and three times as much as are 

 produced by the red rays. 



It must be said, therefore, that the reduction of carbon dioxide 

 in the chloroplast is a complicated process, having besides the 

 photochemical phase also a purely chemical one. It must be 



1 This is evidently intended to refer to the visible spectrum, since C0 2 

 shows an absorption spectrum in the ultra-violet. — Editors' Note. 



