
—1905.] On the Physiological Processes of Green Leaves. 49 
The rate of assimilation of leaves in air which has been artificially 
enriched with carbon dioxide is seen to be considerably greater than it is in 
normal air. In a previous communication direct proof has been given * that 
up to five or six times the normal amount of CQ,, assimilation, ceteris 
paribus, is approximately proportional to the amount of that gas present. A 
further general proof of this proposition may be obtained by reducing the 
values given in the last column of Table II in the proportion of the amounts 
of carbon dioxide in the leaf-cases (as given by Column (3)) to that of 
average air of from 2°8 to 3 parts per 10,000. A series of values is then 
obtained of the same order of magnitude as those of the last column of 
Table I. 
Section (2).—A Comparison of the Direct Method of Determining the Rate of 
Assimilation in Ordinary Air with the Weighing Method of Sachs. 
Up to about twenty years ago the only known methods for determining 
the rate of assimilation under free air conditions were based on an estimate, 
more or less exact, of the increase in dry weight of a plant during a 
considerable period of its growth. In 1884, Sachs, in his classical paper 
“Hin Beitrag zur Kenntniss der Ernahrungsthatigkeit der Blatter,” t asserted 
that with due precautions the varying dry weights of known areas of leaf- 
lamina could be used as measures of the amount of carbohydrate produced 
in the leaf. 
In the case of a detached leaf of Helianthus annuus. he found that there 
was an increase in weight during several hours of favourable isolation 
_ corresponding to 0°01648 gramme per square decimetre of leaf area per hour. 
Under similar conditions detached leaves gained more in weight than leaves 
still attached to the plant, a result which Sachs attributed to the simul- 
taneous depletion of the leaf in the latter instance, owing to the migration of 
some of the products of assimilation into the stem of the plant. By adding 
the rate of loss during the hours of darkness to the rate of gain during 
insolation, Sachs obtained certain values which he believed to represent 
approximately the rate of assimilation during the day time. For Helianthus 
and Cucurbita his final results are given below. For purposes of 
comparison with our own results we have calculated the corresponding 
amounts of carbon dioxide equivalent to the carbohydrate which this increase 
is supposed to indicate. 
* © Roy. Soc. Proc.,’ vol. 70, p. 399. 
t ‘ Arbeit. d. Bot. Instit. Wurzburg,’ vol. 8, p. 28. 
VOL. LXXVI.—B. E 
