20 TEXTBOOK OF PLANT PHYSIOLOGY 



interest. Omitting details that would lead us into biochemistry 

 instead of physiology, it is necessary to mention that the atom of 

 magnesium occupies in it a central position and is connected with 

 the atoms of nitrogen according to the formula: 



/C— 



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

 heterocyclic structure of the pyrrol group, which establishes a 

 certain chemical relationship between chlorophyll and the blood 

 pigment, haemoglobin, in whose structure the pyrrol groups also 

 are embodied. Willstatter attributes much importance to the 

 presence of magnesium in chlorophyll, assuming that it is joined 

 with a molecule of carbon dioxide in the first synthetic phase of 

 the assimilation process (see Art. 9). 



The structure of chlorophyll has been established through a 

 detailed study of the successive disruption of its molecules under 

 the influence of alkalies and acids. 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, as well as the rupture of 

 the lactam ring. The green color first becomes brown, but after- 

 wards the ring once more closes and the green color is restored. 

 The products of reaction are the alkali salts of chlorophyllinic 

 acids, preserving 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, resulting not in a fluorescent, but a brown 

 product, which for a long time has been called chlorophyllan. 

 This brown color is frequently observed when leaves containing 

 considerable amount of acid in their cell sap are killed, as for 

 instance, those of sorrel or Begonia, from which it is almost 

 impossible to obtain unmodified chlorophyll. Willstatter suc- 

 ceeded in showing that when carefully treated with acid, the 

 molecule of chlorophyll remains almost unmodified, hydrogen 

 only replacing magnesium. This substituted chlorophyll he 

 called phaeophytin. The green color may be restored if the 



