58 PRESIDEXTIAL ADDRESS — SECTION C. 



The two green pigments, chloroiihyll a and b, are fundament- 

 ally' similar in tlieir chemical constitution. Willstatter and Stoll 

 found no change in their relative proportion, even during inten- 

 sified assimilation under their experimental conditions, nor in 

 their total amount. They infer that both function similarly 

 in photosynthesis. The empirical formulae only differ in 

 the replacement, of two atoms of hydrogen in a by an atom of 

 oxygen in h. Along with this difference goes a slight difference 

 of colour and absorption spectrum; but we can refer to both 

 together as chlorophyll in treating of the construction of their 

 molecules as elucidated by Willstatter. 



Chlorophyll is a complex organic compound containing in the 

 molecule one atom of magnesium, but no iron, notwithstanding 

 the fact that in the absence of iron plants fail to form a normal 

 amount of chlorophyll. The magnesium is readily removed by 

 acids, even by weak organic acids. As the characteristic colour 

 of the chlorophyll disappears at the same time it is clear that its 

 behaviour towards light, which is fundamental to its role of 

 absorbent of radiant energy in photosynthesis, must depend on 

 this magnesium atom and its mode of union with the rest of 

 the molecule. 



The colourless phacopJnjtin wdiich is obtained from chloro- 

 phyll by removal of the magnesium does not othei'wise differ in 

 constitution from the chlorophyll in any fundamental respect. 

 On carefully graded treatment with alkalis each gives a series. 

 of less complex products, one series containing magnesium and 

 green in colour, the other free from magnesium and not green. 

 Each of the foi-mer when treated with dilute acid gives the 

 corresponding member of the magnesium-free series. 



Although the magnesium is not removed by treatment with 

 alkali and the products obtained are green, the first stage of the 

 treatment is -marked by an evanescent change of colour from 

 green to brown. Willstatter and Stoll attribute this to a change 

 m the magnesium complex whereby a condition of greater- 

 stability is reached : the brown phase represents the intermediate 

 imlinking, probably of a closed ring which is reformed in a 

 different \Aay with reappearance of the chlorophyll green. 



In the final product of saponification by alkali, namely, 

 aetiophyllin, the magnesium is still present. Willstatter and' 

 Stoll represent it as united to the nitrogen atoms of two salt- 

 forming pyrrol rings and probably also more loosely to the nitro- 

 gen of two other pyrrol-hke complexes, which constitute with the 

 other two pyrrol rings the central nucleus of the molecule. It is 

 an extraordinary coincidence that the nucleus of the haemoglobin 

 of blood is very similar in construction, but contains iron m place 

 of magnesiimi. 



Around this nucleus are attaclied various side chains of 

 carbon and hydrogen. In chloropyhll itself there are also two 

 carboxyl groups, in one of which the hydrogen is replaced by a 

 methyl group, in the other by the radicle of a complex alcohol" 



