18 TEXTBOOK OF PLANT PHYSIOLOGY 



For the successful progress of assimilation, the harmonious and 

 concordant work of both constituents of the green plastids, the 

 stroma and the chlorophyll, is absolutely necessary. This may be 

 seen from the fact that in chlorophyll solutions in different kinds 

 of liquids — alcohol, benzine, and oils — the capacity of decomposing 

 carbon dioxide has not been observed under any circumstances. 

 The colorless plastids (leucoplasts) as well as the yellow and orange 

 chromoplasts likewise do not exhibit the power of assimilation. 



The importance of chlorophyll in the process of carbon assimi- 

 lation will be discussed further on. It may be stated here that it 

 renders the energy of the absorbed light effective in the decomposi- 

 tion of carbon dioxide. In all probability it takes part in this proc- 

 ess also chemically, entering into combination with carbon dioxide 

 and making it accessible to the influence of light. The primary 

 importance of the stroma consists in it being the material basis 

 through which the particles of chlorophyll are distributed and 

 which, in a way not yet understood, makes their functioning pos- 

 sible. It is also the carrier of those ferments which split the first 

 unstable products of the combination of chlorophyll with carbon 

 dioxide, thus promoting their further transformation into carbo- 

 hydrates. Finally, the stroma has still another important func- 

 tion, namely, the manufacture of starch from the sugar originating 

 in the process of assimilation. This function is possessed not only 

 by the green plastids but also by the colorless leucoplasts. The 

 only difference between the two processes is that in the stroma of 

 green plastids the starch is formed from sugar manufactured on 

 the spot, while in the colorless leucoplasts the material which serves 

 for the synthesis of starch is the sugar translocated from the 

 leaves. 



5. Chlorophyll. Its Chemical Composition, Structure, and 

 Properties. — Chlorophyll is one of the most interesting substances. 

 Its presence makes possible the highly important synthesis of an 

 organic compound from the inorganic CO2 and H2O. At the same 

 time, chlorophyll is one of the most complex of organic substances. 

 Its structure and composition have been ascertained only recently, 

 after the efforts of a number of eminent scientists covering a period 

 of over 100 years. The difficulty of studying chlorophyll depends 

 chiefly on its ready denaturation, requiring exclusive treatment 

 with neutral liquids in order to extract it from the plant. Ethyl 

 or methyl alcohol and acetone are the most common reagents used 



