682 REPORT—1899, 
result in a complete filtering-out and utilisation of the whole of the rays of the 
right period for producing decomposition of carbon dioxide. 
Fig, 3, 
Ee ate 
“3 
SUS 
i e 
LES See 
ow os 
O—-tore 
This maximum efficiency expressed in calories per square metre per hour is 
66,300, corresponding to the heat of formation of about 16°5 grams of carbo- 
hydrate. Under the most favourable conditions we have employed up to the present 
we have not obtained a larger production than about 3-0 grams of carbohydrate per 
square metre per hour, or about 18 per cent. of the theoretical maximum ; but this 
was in air containing only 16°4 parts of carbon dioxide per 10,000, which must be 
very far below the true optimum amount. 
The brilliant discoveries of recent years on the constitution and synthesis of 
the carbohydrates have not brought us sensibly nearer to an explanation of the 
first processes of the reduction of carbon dioxide in the living plant. The hypo- 
thesis of Baeyer still occupies the position it did when it was first put forward 
nearly thirty years ago, although it has, it is true, received a certain amount of 
support from the observations of Bokorny, who found that formaldehyde can, 
under certain conditions, contribute to the building up of carbohydrates in the 
chloroplasts. 
The changes which go on in the living cell are so rapid, and are of such a com- 
plex kind, that there seems little or no hope of ascertaining the nature of the first 
steps in the process unless we can artificially induce them under much simpler 
conditions. 
The analogy which exists between the action of chlorophyll in the living plant 
and that of a chromatic sensitiser in a photographic plate, was, I believe, first 
pointed out by Captain Abney, and was more fully elaborated by Timiriazeff, who 
was inclined to regard chlorophyll as the sensitiser par excellence, since it absorbs 
and utilises for the assimilatory process the radiations corresponding approxi- 
mately to the point of maximum energy in the normal spectrum. The view 
which Timiriazeff has put forward, that there is a mere physical transference 
of vibrations of the right period from the absorbing chlorophyll to the reacting 
carbon dioxide and water, is, I think, far too simple an explanation of the 
facts. Chromatic sensitisers have been shown to act by reason of their antece- 
dent decomposition and not by direct transference of energy, and the same 
probably holds good with regard to chlorophyll, which is also decomposed by the 
rays which it absorbs. We must probably seek for the first and simplest stages 
of the assimilatory process in the interaction of the reduced constituents of the 
chlorophyll and the elements of carbon dioxide and water, the combinations so 
formed being again split up in another direction by access of energy from 
without. 
The failure of all attempts to produce such a reaction under artificial condi- 
tions is, I think, to be accounted for by the neglect of one very important factor. 
We are dealing with a reaction of a highly endothermic nature, which is 
probably also highly reversible, and on this account we cannot expect any sensible 
