
1905.] On the Physiological Processes of Green Leaves. 51 
discuss how far Sachs was justified in adding the hourly gain of weight 
of the leaf during the day to the hourly /oss during the night in order to arrive at 
his result. He was led to this correction by observing that in placing two 
leaves under similar external conditions, but having one detached with its 
petiole in water, whilst the other was still attached to the plant, the gain in 
weight of the detached leaf per unit area was greater than that of the 
attached leaf. Whilst, for instance, the detached leaf gained in weight at the 
rate of 0:01648 gramme per decimetre per hour, the leaf on the plant only gained 
at the rate of 0:00914 for the same area and time, thus showing a falling-off of 
44°5 per cent. 
The experiments of Brown and Morris also showed the same falling off in 
weight-increase in the case of the attached as compared with the detached 
leaves as shown in Table IV, this falling-off being 53 per cent. in Experi- 
ment (1) and 30 per cent. in Experiment (2), or an average of 41°5 per cent., 
i.¢., almost exactly Sachs’ value. 
There can be no doubt about the different behaviour of attached and 
detached leaves under the above conditions, but we believe the fact has 
received an erroneous explanation which has led to a considerable over- 
estimate of the rate of assimilation as deduced from the Sachs weighing 
method. 
When the rate of assimilation of two similar leaves placed side by side is 
determined by the actual amount of carbon dioxide taken into the leaf, we 
have found that, contrary to what might have been expected, the leaf attached 
to the plant assimilates at a /ess rate than the detached leaf with its petiole 
in water. This is shown by the two experiments following, exactly similar 
leaves being enclosed in similar leaf-cases and exposed under identical 
conditions of insolation and air-current. | 
The falling-off in the assimilation of the attached leaf as compared with the 
detached leaf is 34°3 per cent. in Experiment (1) and 54°8 per cent. in Experi- 
ment (2), the mean being 44°5 per cent., which corresponds almost exactly 
with the differences in weight of the attached and detached leaves of Helianthus 
in the previously-recorded experiments of Sachs. But in this instance we can- 
not fall back upon leaf depletion of the attached leaf as the explanation, since 
our method, unlike that of Sachs’ is quite independent of any migration 
of the products of assimilation during the experiment, for it is based on the 
actual intake of carbon dioxide into the leaf. We are forced to conclude 
therefore that the differences recorded in Table V are due to differences in the 
stomatal openings in the two cases, the stomata of the detached leaves being 
more widely opened than those of the leaf still attached to the plant. 
E 2 
