ASSIMILATION OF CARBON BY PLANTS 181 



The selective absorption of light by the chlorophyll indicates 

 that the assimilation process is not equal in the different parts 

 of the spectrum. It was established long ago that in the 

 green rays, which are feebly absorbed by chlorophyll, assimi- 

 lation is comparatively insignificant. Senebier noted, early in 

 the nineteenth century, that when leaves were placed under 

 double-walled bell jars and the space between the walls filled 

 with colored liquids of different composition, in the yellow-red 

 half of the spectrum, under a soluticyi of potassium bichromate, 

 the plants were assimilating more intensely than in the blue- 

 violet half under a solution of copper sulphate in weak aqueous 

 ammonium hydroxide solution. 



Detailed studies on assimilation in different wave lengths were 

 performed by Timiriazev, who proved that the maximum of 

 assimilation lies in red-orange region (between the Fraunhofer 

 lines B and C), which is most completely absorbed by chlorophyll. 

 The blue-violet rays, though strongly absorbed, have a con- 

 siderably smaller photosynthetic effect. Although violet wave 

 lengths represent large energy quanta, they are not so efficient 

 in the decomposition of carbon dioxide in the green leaf as the 

 smaller quanta of red rays. For the same number of calories, 

 the red wave lengths carry a greater number of quanta than blue 

 rays; these induce the breaking down of a larger number of 

 molecules and show a higher photosynthetic efficiency. 



Photosynthesis is not the only means of creating organic sub- 

 stance. There are plants belonging to the group of bacteria 

 whose source of energy for the sjaithesis of such compounds is 

 not solar light but exothermic processes of oxidation of different 

 substances, e.g., ammonia, hydrogen sulphide, etc., which serve 

 the bacteria as a source of energy. As such may be mentioned 

 the nitrifying bacteria, which oxidize ammonia to nitrites and 

 further to nitrates; sulphur bacteria, which oxidize hydrogen 

 sulphide to sulphuric acid; hydrogen bacteria, which oxidize 

 hydrogen to water; etc. The energy of oxidation is then utilized 

 for the decomposition of carbon cUoxide, for the building of 

 organic substances, and for the formation of compounds of their 

 protoplasm. Their synthetic activity, being based on the 

 transformation of one form of chemical energy into another^ 

 is therefore termed ''chemosynthesis." Of most importance in 

 this group are the nitrifying bacteria, which live in soil and 

 transform the ammonia formed in the process of the decay of 



