
1905.] On the Physiological Processes of Green Leaves. 57 
of this comparison of methods rests so far on the assumption that the leaves 
enclosed in their cases are under conditions which, at any rate, are not less 
favourable to assimilation than those experienced by leaves assimilating in 
free air. The reasons in favour of the correctness of this assumption have 
been already given, but in order to meet any possible objections on this point 
we instituted another series of experiments in which the increase in dry weight 
and the simultaneous intake of carbon dioxide could be determined on the same 
leaves. 
These experiments were made on leaves of Catalpa bignonioides in the 
following manner. Four selected symmetrical leaves, whilst still attached to 
the plant, were covered with tin-foil in the evening in order to ensure their 
effective depletion by the morning of the following day. These leaves were then 
carefully detached by cutting through the petioles under water,and were brought 
into the laboratory. From each pair of leaves the alternate right and left halves 
were then cut away along the mid-rib, and the halves still attached to the 
mid-rib were arranged in a large glazed experimental case of the usual pattern, 
the petioles of each pair of halves dipping into test-tubes of water fixed 
outside the case. These halves were then placed in strong light of sufficient 
intensity to cause the leaves to assimilate freely, and a fairly rapid stream of 
air was drawn through the vase, the carbon dioxide taken in by the leaves 
being estimated in the usual manner. 
At the close of the experiment, which lasted some hours, the half-leaves 
. were removed from the case and the mid-ribs were sliced off, the respective 
areas of the separated laminz and mid-ribs being determined separately 
by the application of the planimeter to the photographic prints. From the 
total amount of. carbon dioxide assimilated during the experiment a pro- 
portional deduction was made for the area of the mid-rib portions which had 
taken part in the assimilation. The dry weight of the separated laminz was 
then determined, the final drying taking place in a current of dry hydrogen 
at 100° C. 
‘Meanwhile, the area and dry weight of the other halves separated at the 
commencement of the experiment were determined, when we had all the 
data for determining (1) the increased weight of the leaf-lamina per square 
decimetre per hour (Sachs’ method), and (2) the intake of carbon dioxide and 
the corresponding amount of carbohydrate produced per square decimetre 
per hour. The final results of four such experiments are given in the follow- 
ing Table VIII. The areas of leaf employed varied from about 550 to 750 
Sq. cm. 
