
1905.] On the Physiological Processes of Green Leaves. 61 
as in experiments on assimilation. After six hours’ exposure the leaves were 
again printed, and the areas determined afresh. 
It will be seen on examining Tables IX and X that the errors which may 
be introduced into the Sachs’ weighing method from the causes there noted 
may be of considerable magnitude. The average error observed due to 
differences in symmetry amount to 2-2 per cent. and that due to change of 
area to 111 per cent. Assuming that the accumulated errors from all sources 
in a Sachs’ experiment amounted to 2°0 per cent., with a leaf having a dry 
weight of 0°50 gramme per square decimetre, this would lead to an over or 
under-estimate of the matter assimilated of 0-010 gramme per square 
decimetre for the total time of experiment, and if the duration of the 
experiment were five hours, to an apparent assimilation or depletion at the 
rate of 0°002 gramme per decimetre of leaf per hour. But this is about the 
average amount of true assimilation observed for the leaf of Catalpa 
bignonioides by the direct method of determining the carbon dioxide assimilated, 
so that an under-estimate of the area of only 2 per cent. in the insolated half- 
leaves would on the Sachs’ weighing method give an over-estimate of the 
assimilation of 100 per cent. of the true value, whereas it would only affect 
the results obtained from our method of carbon dioxide absorption to the 
extent of 2 per cent. 
In the light of these experiments we cannot avoid the conclusion that the 
Sachs’ method cannot be trusted for anything like exact quantitative 
estimation of the photosynthetic work which is going on in an assimilating 
leaf. As ordinarily applied its general tendency is to give far too high an 
estimate of the rate of assimilation, which can only be measured with any 
approach to exactness by a determination of the actual intake of carbon 
dioxide into the leaf from an atmosphere containing that gas in small and 
determinate amounts. 
Section (3).—The Relation of the Distribution of Stomata to the Rate of Gaseous 
Hachange in the Leaf. 
This is a subject which has been very fully and satisfactorily investigated 
by F. F. Blackman,* who demonstrated for the first time that the exchange 
of carbon dioxide between the leaf and the surrounding air closely follows the 
stomatic distribution on the two surfaces of the leaf, and that the experiments 
can only be interpreted on the supposition that the gaseous exchanges take 
place by free diffusion through the open stomates and not by diffusion across 
the imperforate portions of the leaf-cuticle. 
* ‘Phil, Trans.,’ vol. 186 (1895), p. 502. 
