566 PRINCIPLES OF GENERAL PHYSIOLOGY 



Willstatter and Stoll (1913, p. 2o) make the following suggestion. Carbon dioxide, 

 attracted to the chlorophyll system by virtue of the magnesium it contains, is reduced by 

 chlorophyll-a (by the agency of light energy), which itself becomes chlorophyll-''. A* \v 

 saw, chlorophyll-/* is an oxidation product of chlorophyll-a, containing one atom more oxygen. 

 Two molecules of chlorophyll-fr might then give off a molecule of oxygen, and become 

 chlorophyll-a again. It is further suggested that this removal of oxygen may be the function 

 of carotin, which thereby becomes xanthophyll. A reducing enzyme finally convert* 

 xanthophyll into carotin again. But this naturally is, at present, purely hypothetical. 



It will be seen how far we are from understanding the process. 



Usher and Priestley (1906) think that hydrogen peroxide is the immediate 

 source of the oxygen given off in photo-assimilation ; this peroxide is decomposed 

 by catalase, present in all green leaves, with evolution of molecular oxygen. It 

 is possible, however, that the hydrogen peroxide detected by them is only the 

 product of the second stage of our hypothetical process, which takes place in rifr 

 and is concerned with the splitting off of formaldehyde from its temporary ass. >ciation 

 with chlorophyll. It may even have nothing to do with the real carbon dioxide 

 assimilation, being possibly concerned with the action of the chlorophyll as an 

 optical sensitiser. It is clear, in any case, that, in the reduction of carbon dioxide, 

 oxygen must be dealt with in some way, and the final net result is that a volume 

 of oxygen equal to that of the carbon dioxide is given off. 



If formaldehyde is taken up in any way by chlorophyll, the latter is shown 

 to be a chemical sensitiser, as well as an optical one. 



Fenton (1914) finds that, under appropriate conditions, formaldehyde and hydrogen peroxide 

 combine to form a compound 2H.CHO.H 2 O.,, which is crystalline and fairly stable at ordinary 

 temperatures. It takes fire if brought into contact with reduced iron or platinum black. It 

 is decomposed by sunlight. 



After all, it seems most likely that the aldehyde which results from the action 

 of light plus oxygen on chlorophyll in vitro may come from actual decomposition 

 of the molecules of chlorophyll, as it does from other organic substances. In such 

 a case it would have no relation to the photo-synthetic process, and be a purely 

 artificial phenomenon. Curtius and Franzen (1912) obtained from leaves afj- 

 hexylene-aldehyde, CH 3 - CH 2 - CH 2 - CH = CHO. The production of such 

 higher aldehydes suggests a possible origin from the phytol of chlorophyll. 



Since chlorophyll is an optical sensitiser and these act by formation of catalysts 

 (Weigert, 1911, pp. 64-70), the possibility must not be disregarded that it may 

 have no other function. If this be so, the formation of formaldehyde from carbon 

 dioxide and water would only be possible in the complex system of the chloroplast, 

 as already suggested above. Although there are certain difficulties in this view, 

 such as the peculiar chemical nature of chlorophyll itself, as an organic magnesium 

 compound, it seems by no means unlikely that it may turn out to be the correct 

 one. If so, the photo-chemical reaction by which carbon dioxide and water are 

 converted into formaldehyde and oxygen, with the taking up of light energy, is 

 effected by other constituents of the chloroplast, perhaps with the aid of iron, as 

 pointed out by Moore (1914), and that the use of the optical sensitiser, chlorophyll, 

 is to enable a sufficient supply of light energy to be available. 



As to the further change of formaldehyde into sugar and starch, this readily 

 takes place under ordinary chemical conditions, as stated above (page 564). At 

 the same time, if the process were as simple as this in the leaf, it would seem that 

 formaldehyde should serve as a means of formation of starch, independent of light. 

 Now, experiments by Miss Baker (1913) show that this is not so, formaldehyde 

 does not serve as carbon food for plants in the dark, although it does so in the 

 light. It is probable, therefore, that light accelerates the polymerisation, so that 

 there is never much free formaldehyde present at one time, thus avoiding the well- 

 known toxic effects of this substance. The energy change is small in the process it' 

 polymerisation of formaldehyde and a catalyst may be produced in the chlorophyll 

 system under the action of light, thus adding another factor to the complex 

 system. 



Timiriazeff (1903, p. 455) has made an interesting calculation, on the basis O f 

 the measurements of Horace Brown and others, to be given presently, of the 

 actual amount of light energy absorbed by chlorophyll. The result is that, if all the 



