564 PRINCIPLES OF GENERAL PHYSIOLOGY 



The fact that light energy is stored up shows at once that we have to deal with 

 a process that is not a catalytic one. The reduction of carbon dioxide at ordinary 

 temperatures is effected against chemical forces. As Weigert points out (1911, 

 p. 99), this indicates that chlorophyll itself takes part in the reaction and that 

 the considerable increase in potential which occurs is due to the interaction with 

 other parts of the chloroplast, as indicated above. This raising of potential is a 

 common phenomenon in physiological processes (see the paper by Weigert, 1908, 

 p. 464). 



In living organisms, as we know, the process is a reversible one, since carbo- 

 hydrate is oxidised with production of carbon dioxide and water, but it is not 

 necessary that the same intermediate stages should be passed through ; in fact it 

 does not seem probable that they are. If, however, we take the simplest form of 

 the equation given above, making x=\, formaldehyde is one of the products on 

 the right-hand side, and this is oxidised by oxygen, at all events by " active " 

 oxygen, giving out light as a phenomenon of chemi-luminescence (see page 

 557 above). According to Trautz (1905, p. 101), this light is of a reddish colour, 

 in fact, of the same wave length as the position of the main chlorophyll absorption 

 band. Thus, the equation might be written, with the inclusion of light as a 

 part of the reversible system : 



CO 2 + H 2 O + light energy of definite wave length ~ > HCHO + O 2 . 



It was suggested by von Baeyer (1870) that formaldehyde is the first product 

 of photo-synthesis, and Usher and Priestley (1906) found that an aldehyde is to 

 be detected as a product of the action of light on films of chlorophyll in the 

 presence of moist carbon dioxide. There is also reason to expect formaldehyde 

 to be produced, since Bach (1893) obtained formic acid by the action of light on 

 carbon dioxide in presence of solutions of uranium salts, and Moore and Webster 

 (1913) have obtained formaldehyde by the action of ultra-violet light upon 

 colloidal solutions of uranium hydroxide or ferric hydroxide. Moreover, formalde- 

 hyde is readily polymerised to higher carbohydrates. Loew (1889) obtained, by 

 the action of magnesium oxide and lead on formaldehyde at 60, a sugar which 

 he called formose this was afterwards shown by Emil Fischer (1890) to be inactive 

 fructose. Berthelot and Gaudechon (1910) obtained formaldehyde by exposing a 

 mixture of carbon dioxide and hydrogen, or water and carbon monoxide, to 

 ultra-violet light, and Walther Loeb (1905) by exposing moist carbon dioxide 

 to the silent electric discharge. In both cases, a series of intermediate reactions 

 took place, and the conditions are, perhaps, rather far from those of the green leaf, 

 although, as we have seen, the photo-chemical process, in Luther's view, is 

 fundamentally an electric one. The alternating electric field of the silent discharge 

 is not very far removed from that of light, but, of course, the frequency of the 

 alternations is very much less. 



Now, if the aldehyde in Usher and Priestley's experiments were actually 

 derived from the carbon dioxide present, a great step would have been taken, but 

 we have already seen reason to doubt whether such a reaction is possible by the 

 aid of chlorophyll alone. Schryver (1910) showed that an aldehyde is only to 

 be obtained from chlorophyll after it has been exposed to light, but that the 

 production does not depend on the presence of carbon dioxide. Recent work 

 by Wager (1914) and by Warner (1914) confirm this result as to the production 

 of some aldehyde from chlorophyll by light in the absence of carbon dioxide ; 

 they find also that oxygen is necessary, and that it is used up. Wager is doubtful 

 whether the aldehyde formed is formaldehyde, since the colour given with 

 Schryver's reagent is different from that given by pure formaldehyde. The 

 aldehyde produced under these conditions is a result of the photo-chemical 

 oxidation of chorophyll itself, which becomes bleached. Wager shows that the 

 amount of aldehyde produced is proportional to the amount of absorption in 

 the different regions of the spectrum. He could not detect any disappearance 

 of carbon dioxide when chlorophyll films were exposed to light in its presence. 

 but admits the possibility that his method might not have been delicate enough 

 to detect a minimal disappearance. An important fact shown is that, when 



