—_ Fs 
TRANSACTIONS OF SECTION B. 675 
under identical conditions of insolation and temperature this leaf would have taken 
in an amount of carbon dioxide from the free air at a rate sufficient to produce 
0°8 gram of carbohydrate per square metre per hour. This is almost exactly equal 
to the assimilation rate of the sunflower which I deduced in 1892 from the indirect 
process of weighing equal areas of the leaf lamina before and after insolation, and 
it also agrees fairly well with some of Sachs’s original experiments of a similar 
nature, 
In another experiment made with the leaf of Catalpa bignonioides in full sun- 
light the amount of carbou dioxide in the air passing over the leaf fell from 2°80 
to 1:79 parts per 10,000, the total hourly intake for the square metre being 
344'8 c.c. When this is corrected for tension it corresponds to an assimilation in 
free air of 0:55 gram of carbohydrate per square metre per hour, 
An increase in the intensity of the daylight, as might be expected, influences to 
some extent the rate of intake of atmospheric carbon dioxide ; but providing the 
illumination has reached a certain minimum amount, a further increase in the 
radiant energy incident on the leaf does not result in anything like a proportional 
amount of assimilation. We have found, for instance, that the rate of assimilation 
of a sunflower leaf, exposed to the clear northern sky on a warm summer’s day, 
was about one-half of what it was when the leaf was turned round so as to receive 
the direct rays of the sun almost normal to its surface. Now in this latter case 
the actual radiant energy received by the leaf was at least twelve times 
greater than was received from the northern sky, but the assimilation was only 
doubled. 
These differences in the effect of great variation of illumination become still 
less marked when we use air which has been artificially enriched with carbon 
dioxide. In one instance of this kind, for example, we found the assimilation in 
the full diffuse light of the northern sky to be 87 per cent. of what it was in 
direct sunshine. 
This brings me to another interesting point on which I have already touched 
slightly—the enormous influence which slight changes in the carbon dioxide con- 
tent of the air exert on the rate of its ingress into the assimilating leaf. 
With a constant illumination, either in direct sunlight or diffuse light, the 
assimilatory process responds to the least variation in the carbon dioxide, and 
within certain limits, not yet clearly defined, the intake of that gas into the leaf 
follows the same rule as the one which holds good with regard to the absorption 
of carbon dioxide by a freely exposed surface of a solution of caustic alkali; that 
is to say, from air containing small but variable quantities of carbon dioxide the 
intake is directly proportional to the tension of that gas. 
A single experiment will be sufficient to illustrate this, 
A large sunflower leaf, still attached to the plant, and exposed to a clear 
northern sky, gave an assimilation rate equal to 0°331 gram of carbohydrate per 
square metre per hour, when air was passed containing an average amount of 2:22 
parts of carbon dioxide per 10,006. When the experiment was repeated under 
similar conditions of illumination, but with air containing 14:82 parts of CO, per 
10,000, the intake corresponded to an hourly formation of 2:409 grams of carbo- 
hydrate per square metre. The ratio of the tensions of the carbon dioxide in the 
two experiments is 1 to 6-7, and the assimilatory ratio is 1 to 7-2, so that the 
increased assimilation is practically proportional to the increase in tension of the 
carbon dioxide. 
Since an increase of carbon dioxide in the atmosphere surrounding a leaf is 
followed by increased assimilation even in diffuse daylight, it is clear that, under 
all ordinary conditions of illumination, the rays of the right degree of refrangibility 
for producing decomposition of carbon dioxide are largely in excess of the power 
of the leaf to utilise them. Under natural conditions this excess of radiant energy 
of the right wave-length must, from the point of view of the assimilatory process, 
be wasted, owing to the limitation imposed by the high degree of dilution of 
atmospheric carbon dioxide. But although the actual manufacture of new material 
within the leaf lamina is so largely infiuenced by small variations in the carbon 
dioxide of the air, we are not justified in concluding that the plant as a whole will 
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