3i6 



PIGMENTS 



surface of leaves had no deterrent effect on assimilation, but a 

 slight pressure applied to the leaves brought assimilation to a 

 complete standstill. 



THE CONSTITUTION OF CHLOROPHYLL. 



As already stated, chlorophyll was first isolated from its 

 accompanying yellow pigments, the carotinoids, by Willstatter 

 and Hug in 191 1, and in the following year it was shown by 

 Willstatter and Isler that the chlorophyll so obtained was not 

 a single substance, but a mixture of two distinct substances, 

 chlorophyll a and chlorophyll h, in the proportion roughly of 

 three molecules of the former to one of the latter. 



The separation of these two constituents was effected by 

 repeatedly shaking a petrol ether solution of crude chlorophyll 

 with 90 per cent aqueous methyl alcohol, whereby the chloro- 

 phyll h is washed out of the petrol together with a considerable 

 quantity of chlorophyll a* 



The formulae assigned to these two substances are as 

 follows : — 



/COOCH3 



Cs^HsoON.Mg/ 



\COOC20H39 

 Chlorophyll a 



/COOCH3 



\COOC20H3, 

 Chlorophyll h 



from which it may be seen that chlorophyll a contains two 

 atoms of hydrogen more, but one atom of oxygen less than 

 chlorophyll h, and, accordingly, chlorophyll h would appear 

 to represent a more oxidized form of chlorophyll a ; attempts 

 to convert chlorophyll a into chlorophyll h have, however, not 

 been successful. The chief difference between the two modi- 

 fications are given in the following table : — 



Chlorophyll [a and b). 



Analysis agrees with for- 

 mula CssH^PeN.Mg. 



Bluish-black glistening 

 powder, with metallic 

 lustre. 



Appears cry stalline u nder 

 the microscope. 



Chlorophyll a. 

 C^sH^PsN.Mg. 



Bluish-black powder. 



Bluish-black powder. 



Chlorophyll b. 

 CssH.oOeN.Mg. 



Dark green or greenish- 

 black glistening pow- 

 der. 



Dark green or greenish- 

 black glistening pow- 

 der. 



* For further details of the separation see Willstatter and Hug : 

 " Annalen," 191 1, 380, 177. 



