
1905.| On the Physiological Processes of Green Leaves. 53 
From the physical considerations adduced in a previous paper it has been 
shown that gaseous diffusion through stomata must vary, other things being 
equal, with the linear dimensions of the openings, and it appears to be a fair 
deduction that the linear dimensions of the stomata in a leaf still attached to 
the plant are about 45 per cent. less than those of the stomata of the same 
leaf after separation from the plant and immersion of the petiole in water. 
That the artificial conditions under which such a detached leaf is placed 
should have an influence on the delicate self-regulating mechanism of the 
guard-cells of the stomata is perhaps scarcely to be wondered at, especially 
when we bear in mind that a leaf which is still attached to its plant is 
receiving water more or less highly charged with salts derived from the soil, 
which must exert a distinct osmotic effect on the leaf-cells, whereas only pure 
water is supplied to the cut leaf. 
Whether or not this is the true explanation of the observed differences 
the fact remains that for equal areas, and under similar conditions, the rate 
of assimilation of a leaf still attached to the plant is about 45 per cent. less 
than that of a detached leaf, and this must in future be taken into considera- 
tion in interpreting the experiments of Sachs, in which equality in 
assimilatery power in the attached and detached leaves has hitherto been 
assumed. 
It would appear therefore that Sachs was not justified in making the 
correction for diurnal depletion which he did; but even if we neglect this 
supposed depletion and assume that none of the products migrate from the 
leaf to the stem during the insolation, the apparent increase in weight of an 
attached leaf, both in the experiments of Sachs and those of Brown and 
Morris, still indicates a rate of assimilation about twice that indicated by the 
direct measurement of the intake of carbon dioxide, as given in Table I. 
The first explanation of the discrepancy which suggests itself is that the 
conditions for assimilation in our leaf-cases are not so favourable as those 
which exist when the leaf is freely exposed to the air. We have already 
seen that when cut leaves are employed the advantage in one respect 
(provisionally ascribed to the wider opening of the stomata) is in the 
direction of obtaining more rapid photosynthesis than in leaves attached 
to the plant. There is another aspect of the subject, however, which 
requires careful consideration. Owing to the practical difficulty of 
keeping down the temperature of the leaf enclosed in its case it is 
impossible to carry out such experiments successfully in full sunshine, and 
they must be made either on cloudy days, or by moderating the sunshine by 
an artificial “cloud” in the nature of a thin canvas screen. Unless it can be 
