6 Carbon Assimilation. 



Chlorophyll a 



b 



Carotin ... 

 Xanthophyll 



In the chloroplasts these pigments are also mixed with various 

 colourless substances ; fats, waxes and salts of fatty acids. Thus in 

 ail alcohdlic extract of dried leaves chlorophyll is accompanied by 

 about six times its weight of other substances, 



WiUstatter has worked out methods for freeing chlorophyll 

 extracts from these accompanying colourless substances, and also 

 methods for isolating each of the four pigments. In all that follows, 

 when we speak of chlorophyll we refer to the green pigments freed 

 from the yellow ones. 



In thus being able to obtain pure pigments a very great advance 

 is made. All the earlier experiments on the reactions taking place 

 in the green leaf were made with extracts, such as alcoholic 

 extracts of leaves, which contained many substances besides the 

 pigments. 



Yet a further complication becomes obvious from Willstatter's 

 investigations, and this is also a fact which has not yet received its 

 due attention in physiological researches. Moreover it is a fact not 

 only important in this branch of plant physiology but in all cases 

 where plant substances are extracted and purified. This is that 

 by the methods of extraction the state of matter in which the 

 substance generally exists may be altered. For this reason it may 

 become difficult to draw conclusions from the behaviour of the 

 extracted substance as to the function of the substance in its natural 

 condition in the plant. 



From Willstatter's researches it is clear that solvents which 

 dissolve the pure extracted substance do not extract the substance 

 from the dried leaf. For instance, the pure pigment is readily 

 soluble in acetone, ether andJ£enzef. If the dried powder of nettle 

 leaves is placed in pure acetone it can remain there for half an hour 

 without the acetone becoming at all coloured. But if a little water 

 is added the colour immediately becomes intensely green. Neither 

 ether nor benz«l becomes coloured quickly when powdered nettle 

 leaf is added. Yet both are immediately coloured strongly green 

 when a few drops of water are added. This behaviour of chlorophyll 

 suggests that chlorophyll in the leaf is in a different state of matter 

 from extracted chlorophyll. 



The extracted pigment is soluble in petrol-ether as long as it 



