30 
On the Supposed Extracellular Photosynthesis of Carbon Dioxide 
by Chlorophyll. 
By ALFRED J. Ewart, D.Sc., Ph.D., F.LS. 
(Communicated by Professor J. Bretland Farmer, F.R.S. Received October 11,— 
Read December 5, 1907.) 
In two recent papers, Usher and Priestley have brought forward evidence 
to show that chlorophyll is able to assimilate carbon dioxide and produce 
formaldehyde outside the plant, and that hydroxyl is the other product and 
is decomposed into water and free oxygen in the presence of a particular 
ferment. It is, unfortunately, necessary to point out that their work is to 
some extent vitiated by certain oversights and by one serious inaccuracy. 
In their first paper* they confirm Bach’s statement that formaldehyde is 
produced when light acts on a solution of uranium in the presence of carbon 
dioxide and water, although formic acid is much more abundant. In a 
later papert they contradict this result, stating that formic acid alone is 
present and no formaldehyde, thus confirming Euler’s} criticism of Bach’s 
results. 
Usher and Priestley consider that formaldehyde is an intermediate product, 
but bring forward no satisfactory proof. Their most striking experiment on 
extracellular photosynthesis is that made by painting a solution of chloro- 
phyll on gelatine films and exposing them to light in the presence of carbon 
dioxide. The chlorophyll was bleached, which Usher and Priestley consider 
to be due to the formation of hydrogen peroxide, being apparently unaware 
of the fact that chlorophyll is bleached by sunlight in the presence of 
ordinary free oxygen or in air deprived of all carbon dioxide, and that the 
absence of hydrogen peroxide from living cells has been definitely established 
in a number of cases.§ " 
In regard to the apparent detection of formaldehyde in the gelatine films, 
this is not surprising, because they contained in all probability an “ aldehyde ” 
substance before the chlorophyll was painted over them, and further show 
the same amount whether the films are exposed to light in the presence or 
absence of carbon dioxide. All the forms of French leaf gelatine (Coignet’s) 
of cooking (transparent and opaque), Nelson’s, and of impure commercial 
gelatine turn pink in the presence of a decolorised solution of rosaniline, 
* ‘Roy. Soc. Proc.,’ B, vol. 77, 1906, p. 369. 
t+ Loe. cit., B, vol. 78, p. 368. 
t ‘Ber. d. Deutsch. Chem. Ges.,’ 1904, vol. 37, p. 3415. 
§ Cf. Pfeffer’s ‘Physiology,’ Eng. edit., vol. 1, pp. 545—546. 
