ASSIMILATION OF CARBON BY PLANTS 171 



The atoms of nitrogen in their turn constitute a part of the 

 heterocyclic structure of the pyrrole group, which is characterized 

 by a five-menibered ring of atoms: 



HC— CH\ 



II >NH 



HC— CH/ 



This estabUshes a certain chemical relationship between 

 chlorophyll and the blood pigment hemoglobin in whose structure 

 the pyrrole groups also occur. 



The structure of chlorophyll has been established through a 

 detailed study of the successive disruption of its molecules under 

 the influence of alkaUes, acids, etc. The action of alkalies induces 

 in the first place the saponification of the ester groups, i.e., the 

 splitting off of phytol and methyl alcohol. 



The products of reaction are the alkah salts of chlorophyllinic 

 acids, retaining the undisturbed structure of the central nucleus 

 as well as the color and fluorescence of chlorophyll. The action 

 of acids results in the splitting off of magnesium from the chloro- 

 phyll molecule and in the formation of a brown product without 

 any fluorescence. This brown color is frequently observed when 

 leaves containing a considerable amount of acid in their cell sap 

 are killed, e.g., leaves of sorrel or Begonia, from which it is ahnost 

 impossible to obtain unmodified chlorophyll. Willstatter suc- 

 ceeded in showing that, when carefully treated with acid, the 

 molecule of chlorophyll remains almost unchanged, only hydrogen 

 replacing magnesium. This substituted chlorophyll he called 

 ''phaeophytin." The green color may be restored if the 

 hydrogen is replaced by magnesium, zinc, copper, or iron. When 

 acted upon by stronger acids, the hydrolysis of the other groups 

 and the further breaking down of the complex molecule of 

 chlorophyll take place. 



It is interesting and important to note that having investigated 

 over 200 species of plants, WiUstatter has found in aU of them 

 the same chlorophylls. Hence, this most important substance 

 seems to be uniformly the same in all plants. From this, the 

 cdnclusion may be drawn that the fundamental chemistry of the 

 process of carbon assimilation is also similar in all chlorophyll- 

 bearing organisms. 



Besides chlorophylls, the green plastids of the leaf contain two 

 other classes of pigments, carotenes and xanthophylls. These 



