CHLOROPHYLL AND ENERGY 171 



is excited. As it has the lowest level of excitation, the other 

 pigments transmit to it the energy absorbed. A single exception 

 occurs with chlorophyll-^ of red algae, which does not 

 transmit the energy received although it is incapable of using 

 it for photosynthesis. 



Chlorophyll-a, with which we are exclusively concerned, 

 plays the role of a photo-catalyst; it is capable of taking 

 hydrogen from a donor at low potential, such as water, and 

 fixing it on a substance at high potential, such as carbon 

 dioxide or an organic acid. It is reversibly oxidized and 

 reduced. 



Emerson's experunents on photosynthesis in intermittent 

 Hght are described in Chapter VIII of Part I {Light and 

 Vegetation). 



As we have said, one of the physical properties of chloro- 

 phyll is fluorescence. The molecule absorbs light of short 

 wave-length and great energy, is excited by it and rids itself 

 of the energy by re-emitting it, but in the form of a photon 

 which is less charged with energy — ultra-violet, or blue, 

 incident light is transformed into red Hght. This reddish 

 glimmer can easily be seen by looking sideways at a solution 

 of illuminated chlorophyll. If a substance capable of receiving 

 the energy absorbed by the chlorophyll is placed in the 

 solution, the fluorescence disappears. 



Kautsky observed this fluorescence in the course of photo- 

 synthesis. At first it is weak because at the beginning the 

 greater part of the energy absorbed by the chloroplasts is 

 used for photosynthesis. As the illumination continues, the 

 fluorescence rapidly increases, for the chlorophyll needs a 

 certain time to use this energy and energy arriving during 

 that time is unused and dispersed in fluorescence. But a 

 regime of continuous utilization is soon established, when this 

 maximum fluorescence decreases and a level similar to the 

 first is stabilized. 



