170 



PLANT PHYSIOLOGY 



than a. The transformation of one into the other, however, 

 may be accompHshed neither by oxidation nor by reduction, the 

 difference between them thus proving to be more complex than 

 simple oxidation or reduction. They differ somewhat also in 

 their color, chlorophyll a being of a bluish-green, and chlorophyll h 

 of a yellowish-green tinge. The amount of chlorophyll a is 

 usually greater than that of chlorophyll h. In general in higher 

 plants, for every 3 molecules of chlorophyll a there is present 

 1 molecule of chlorophyll h. This ratio varies somewhat in 

 different plants. The total amount of chlorophyll in a plant is 

 not great, constituting, on an average, only about 1 per cent of the 

 dry weight. To secure such quantities of chlorophyll as are 

 required for exact chemical investigations, great quantities of 

 plant material have to be used. 



Chlorophyll may be considered a complex ester of a dicar- 

 boxylic acid, chlorophyllin, in one carboxyl of which hydrogen 

 is replaced by the methyl radical, in the other, by the radical 

 of an unsaturated monatomic alcohol with a rather long carbon 

 chain, called ''phytol," and of the formula C20H39OH. The more 

 detailed formula of chlorophyll a may be represented as follows : 



C32H2oON4Mg< 



COOCH. 

 COO-C.oH 



20-L-L39 



Phytol extracted in a pure form is a colorless hquid. 



The structure of the central nucleus of chlorophyll is of great 

 interest. Omitting details that would lead into physiological 

 chemistry 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- 



— Cv 



— C' 

 — C- 



c 



\ 



N 

 >N Mg- 



-C 



I 



c- 



-c- 



■•N< 



c 



^c— 



^c— 



-c— 



