178 PLANT PHYSIOLOGY 



magnitudes for each kind of ray, but they vary in energy values 

 with the length of the wave; the greater their wave length, the 

 smaller the energy of each quantum. The energy of the quantum 

 of light energy absorbed must be as large as, or larger than neces- 

 sary to decompose the carbon dioxide. For this purpose, only 

 the quanta represented by the ultraviolet are large enough to 

 do this without chlorophyll. But chlorophyll has the ability 

 to absorb 4 quanta at one time, so that the energy of activation 

 of the chlorophyll can be made up of 4 small quanta, which then 

 suffice for the decomposition of the carbon dioxide. Hence, 

 the photocatalyst chlorophyll makes effective the small quanta 

 found in the visible part of the spectrum. For light with a wave 

 length of X 486 (blue), this number will be 58.470 cal.; for X 589 

 (yellow), 48.240; for X 800 (infrared), 35.510 cal. 



That the participation of chlorophyll in photosynthesis is not 

 only of a physical but also of a chemical character is shown by 

 the ease with which it decomposes when exposed to light. Alco- 

 hol and benzine solutions of chlorophyll when brought into 

 contact with air are readily discolored by sunlight and even by 

 diffused light. This may be due to the acceleration of the oxida- 

 tion processes under, the influence of light. Chlorophyll is much 

 more stable in the living plastids of the plant, where photo- 

 synthesis is proceeding under strong illumination. The causes 

 of this stability have not been discovered. Some authors are 

 of the opinion that as the chlorophyll decomposes in the living 

 plastid, it is re-formed. Some suppose that chlorophyll is not 

 in a free state in the plastids but is combined with the protein 

 substances of the stroma, which makes it more stable. Still 

 others assume that chlorophyll is in a colloidal condition and 

 therefore possesses different properties from those of a true 

 solution. In favor of the last two theories is the observation that 

 colloidal solutions of chlorophyll, obtained by highly diluting 

 the alcohol solutions with water or by triturating the leaves with 

 water, are much more stable when exposed to Ught than the usual 

 solutions in alcohol or benzine. 



42. Quantitative Relations between Absorbed and Stored 

 Energy in the Plant. Decomposition of Carbon Dioxide by 

 Energy Obtained from Oxidation. Chemosynthesis. — Of great 

 importance is the question of how much the leaf profits from the 

 radiant energy that it receives, what part of the latter is stored 



