680 REPORT—1599, 
This corresponds to an actual average rate of passage of the molecules of the 
atmospheric carbon dioxide of about 266 centimetres per minute. 
Now, we have already seen, in the case of a Catalpa leaf, that if the gaseous 
exchange during assimilation goes on only through the stomatic openings, we 
require a minimum velocity of something like 380 centimetres per minute, a velocity 
which we are sensibly approaching in our experiments with apertures of about 
1 mm. in diameter. But the effective area of a stomatic opening of the Catalpa 
leaf is equal to that of a circle with a diameter of less than ;45 mm., and since our 
experiments indicate a very rapid increase in the velocity of diffusion as the aperture 
is diminished, it is clear that no difficulty, as regards the physics of the question, can 
be raised against the idea that atmospheric carbon dioxide reaches the active 
centres of assimilation by a process of free diffusion through the leaf stomata. 
One of the most interesting problems connected with plant assimilation relates 
to the efficiency of a green leaf as an absorber and transformer of tne radiant 
energy incident upon it. 
It is already well known that the actual amount of energy stored up in the 
products of assimilation bears a very small proportion to the total amount reaching 
the leaf: in other words, the leaf, regarded from a thermo-dynamic point of view, 
is a machine with a very low ‘economic coefficient.’ We require, however, to 
know much more than this, and to ascertain, amongst other things, how the 
efficiency of the machine varies under different conditions of insolation, and in 
atmospheres containing varying amounts of carbon dioxide. 
The measure of the two principal forms of work done within the leaf, the 
vaporisation of the transpiration water on the one hand, and the reduction of 
carbon dioxide and water to the form of carbohydrates on the other, can be ascer- 
tained by modifying our experiments in such a manner as to allow the transpiration 
water to be determined, as well as the intake of carbon dioxide. 
For the actual measurement of the total energy incident on the leaf under | 
various conditions we are now using one of Professor Callendar’s recording 
radiometers of specially delicate construction, which will be ultimately calibrated 
in calories. This instrument gives promise of excellent results, but up to the 
present time the work we have done with it is not sufficiently advanced for me to 
describe. We may, however, obtain a very fair idea of the variation in the 
efficiency of a leaf from one or two examples in which the amount of incident 
energy was deduced from other considerations. 
In the case of a sunflower leaf exposed to the strong sunlight of a brilliant day 
in August the average amount of radiant energy falling on the leaf during the five 
hours occupied by the experiment was estimated at 600,000 calories per square 
metre per hour. The average hourly transpiration of water during that time was 
at the rate of 275 c.c. per square metre, and the assimilated carbohydrate, estimated 
by the intake of carbon dioxide, was at the rate of 0°8 gram per square metre per 
hour. 
The vaporisation of 275 c.c. of water must have required the expenditure of 
166,800 calories, and the endothermic production of 0:8 gram of carbohydrate 
(taking the heat of combustion at 4,000 gram calories) corresponds to the absorp- 
tion of 3,200 calories. Thus, as the final result under these particular conditions 
of experiment, we find that the leaf has absorbed and converted into internal work 
about 28 per cent. of the total radiant energy incident on it, 27:5 per cent. being 
used up in the vaporisation of water, and only one-half per cent. in the actual 
work of assimilation. 
In strong diffure light, such as that from a northern sky on a clear summer's 
day, the leaf has a higher ‘economic coefficient,’ using that term in rela-~ 
tion to the permanent storage of energy in the assimilatory products. In one 
instance of this kind in which the total energy received by the leaf was approxi- 
mately 60,000 calories per square metre per hour, it was found that 96 c.c. of 
water were evaporated and 0°41 gram of carbohydrate was formed for the same 
area and time. This indicates an absorption and utilisation by the leaf of some- 
thing like 95 per cent. of the incident energy, of which 2°7 per cent. has been made 
