‘Supposed Extracellular Photosynthesis of Carbon Diowide, etc. 31 
with or without gentle warming, and it is on this reaction that Usher and 
Priestley mainly rely. 
The best mode of obtaining the reaction is to soak the gelatine in water, 
melt, cool, and then pour on a little of the decolorised rosaniline, when a 
pink colour appears near the surface of the gelatine, even if an excess of 
sulphur dioxide is present. It must, however, be remembered that the same 
reaction is given by acetaldehyde, and possibly by other aldehydes also. 
Hence, possibly, may arise the fact observed by Usher and Priestley that the 
methyleneaniline obtained from the distillate from the gelatine had a slightly 
different melting point to that of the pure substance.* Even prolonged 
soaking and washing in water does not remove from the gelatine its power 
of assuming a pink or violet-pink colour with decolorised rosaniline, and it 
is quite evident that this oversight renders Usher and Priestley’s method 
valueless. Repeating their experiments, however, there appeared to be a 
slight but distinct increase in the amount of “aldehyde” after the chloro- 
phyll on the film had been bleached by exposure to sunlight, but quantitative 
estimation was not possible, and the same increase appeared to be shown by 
films exposed to light in the absence of carbon dioxide. 
Usher and Priestley state that chlorophyll films on water also produce 
formaldehyde when exposed to light in the presence of carbon dioxide. I 
find this method extremely difficult to apply, and have not been able to obtain 
more than a suggestion of the possibility of the presence of formaldehyde in 
the subnatant water by the magenta test, and the presence or absence of 
carbon dioxide appeared to be immaterial. Hence a trace of formaldehyde 
might have been present in the original chlorophyll, the green colour of 
which masked the test when first applied. 
Conclusive results were, however, obtained by using the paler bases of 
Vallisneria leaves. These showed no trace of formaldehyde when freshly 
examined, whether killed by heat, chloroform, or ether, but developed a 
strong pink coloration when placed in decolorised magenta after eight hours’ 
exposure to sunlight. All green parts gave similar results, although 
wherever a tissue is dark green it is difficult to be sure of the absence of a 
trace of formaldehyde in the original material. Owing to the poisonous 
character of this substance, however, the amount originally present could not 
be large. After exposure to light, leaves of Vallisnerta give the reaction 
rather better than leaves of Hlodea and shoots of Chara. In the case of grass 
and other leaves, they must either be crushed or chopped after bleaching in 
sunlight, or the epidermis removed in order to allow the reagent to penetrate, 
otherwise the pink colour is only produced at the cut edges of the leaf. 
* Usher and Priestley, ¢bzd., p. 320. 
