.iSSIMILATION OF CARBON BY PLANTS 169 



decompose the first unstable products of the combination of 

 chlorophyll with carbon dioxide, thereby promoting their further 

 transformation into carbohydrates. The stroma has still another 

 important function: it contains enzymes for the synthesis of 

 starch from the sugar that originates in the process of pKbto- 

 sjnithesis. This function is possessed not only by the green 

 but also by the colorless plastids, e.g., the starch-forming leuco- 

 plasts in the cells of potato tubers and other underground storage 

 organs. The only difference between the processes of starch syn- 

 thesis in chloroplasts and leucoplasts is that in the stroma of green 

 plastids the starch is formed from sugar found in them, while in 

 the colorless leucoplasts the material that serves for the synthesis 

 of starch is the sugar translocated from the leaves. The process 

 of starch formation in both plastids must be regarded as second- 

 ary, not connected directly with photosynthesis. 



Chlorophyll is one of the most interesting of substances, for 

 its presence makes possible the highly important synthesis of an 

 organic compound from the inorganic CO2 and H2O. It is the 

 only pigment capable of doing this in higher plants. At the 

 same time, chlorophyll is one of the most complex of organic 

 substances. Its structure and composition have been ascer- 

 tained only recently after the efforts of a number of eminent 

 scientists, covering a period of over 100 years. Among these, 

 the classical works of Willstatter (1910 to 1915) and his coworkers 

 are outstanding. The difficulty of studying chlorophyll depends 

 chiefly on its quick decomposition, requiring special treatment 

 with neutral solvents to extract it unchanged from the plant. 

 Ethyl and methyl alcohol and acetone are the most common 

 reagents used for extraction, after which the chlorophyll is trans- 

 ferred to benzine or petroleum ether to be purified. 



According to the investigations of Willstatter, the purest 

 chlorophyll, freed from the admixture of yellow pigments, is 

 composed of two very similar substances, called ''chlorophyll 

 a" and "chlorophyll 6." The chemical composition of chloro- 

 phyll may be expressed by the following formulas : 



Chlorophyll a C55H7205N4Mg 



Chlorophyll h C55H7o06N4Mg 



Evidently the difference between the two kinds of chlorophylls 

 is that 6 has 2 atoms of hydrogen less and 1 atom of oxygen more 



