Some Photochemical Experiments with Pure Chlorophyll. 343 



intermediate product in this synthesis. Attention has consequently been 

 directed to the detection of formaldehyde either in leaves themselves or in 

 chlorophyll-containing systems outside the plant. 



It is not possible in this paper to deal with all the work that has been 

 published on this question. Below we give in tabular form a summary of 

 these investigations which will serve to make clear the incompleteness of the 

 researches and the lack of concordance in the results obtained. 



In regard to experiments conducted outside the plant an important fact, 

 with which we are now acquainted, was overlooked, namely, that the crude 

 chlorophyll used in such experiments was really a very impure substance. 

 Our knowledge of the chemistry of chlorophyll is now well advanced as a 

 result of Willstatter's researches. We know that the chloroplasts contain 

 two green and two yellow pigments. These four pigments together with a 

 much larger quantity of colourless substances are extracted by the solvents 

 used by the earlier investigators, and are present in the crude chlorophyll 

 obtained by evaporating down such an extract. A good criterion of pure 

 chlorophyll is that it is insoluble in petrol-ether, and even in a moderately 

 impure condition is precipitated by it. It is therefore significant that 

 most investigators, including even the latest workers, Wager (1914) and 

 Ewart (1915), have used chlorophyll soluble in petrol-ether. In view of this 

 it is possible that the various results obtained by those workers may be 

 due to some extent to the impurities present in the crude chlorophyll and 

 not to the chlorophyll itself. 



In order to distinguish in the reactions observed by previous workers 

 between those due to chlorophyll itself and those due to impurities either in 

 the pigment or in the reagents, we have employed pure chlorophyll. We 

 have also borne in mind the possibility of the photochemical production of 

 formaldehyde by the solvents used. 



So long as our knowledge of the chloroplast is as incomplete as it is at 

 present, particularly as regards the distribution and components of the 

 heterogeneous system which it constitutes, it is impossible, from experiments 

 conducted outside the plant, to draw conclusions in regard to photochemical 

 reactions taking place in the leaf. 



Willstatter has shown that the molecule of both " chlorophyll a " and 

 " chlorophyll b " consists of a chromogen complex combined with two ester 

 groups so that the formula of " chlorophyll a " can be written 



(MgN 4 C 32 H3oO ) (COOCH 3 ) (COOC 2 oH 39 ) 



and that of " chlorophyll b " 



(MgN 4 C 32 H 28 2 ) (COOCH3) (COOC 20 H 39 ). 



VOL. LXXXIX. — B. 2 F 



