
= 
1905.] On the Physiological Processes of Green Leaves. 67 
through the side, and since a fairly rapid stream of air was passing through 
the apparatus, and the leaf was under favourable conditions for transpiration, 
it is almost certain that the temperature of the leaf cannot have been far 
removed from that of the air surrounding it. 
Table XIV.—Experiments on Leaves illuminated by Welsbach Light. 







CO, 
CO, 
: evolved 
Distance Area of : evolved 
Duration : per square 
of source leaf : durin . 
. Tempera-| . of experi- : decimetre 
Name of plant. of light in square - | experiment ; 
ture, C . ment in . : per hour 
from leaf 3 centi- in cubic : . 
. hours. ; in cubic 
in feet. metres. centi- : 
centi- 
metres. 
metres. 
December 25— 
Dioscorea cayennensis, 
(1) A. In darkness... — 19771 200 ‘0 2°2 40 °30 9°16 
B. Illuminated... 1 22° °3 — 2°3 33 °58 7°30 
January 10— 
Dioscorea cayennensis. 
(2) A. In darkness... _ 19° ‘9 169 *4 2°8 36 °35 7 662 
B. Illuminated... it 22° °7 — 2°55 22°17 5 129 

Partial re-assimilation of the carbon dioxide of respiration has evidently 
taken place in both instances under the influence of the artificial illumination, 
in (1) to the extent of about 1°86 ¢c.c. per sq. decimetre per hour, and in 
(2) to the extent of about 2°53 cc. These it is true are amounts which 
compare favourably with the rate of assimilation of leaves exposed to sunlight 
in ordinary air containing the normal amount of carbon dioxide, but we are 
certainly not justified in deducing that the Welsbach light is rich in photo- 
synthesizing rays, since the conditions are widely different. In ordinary air 
the maximal partial pressure of the carbon dioxide surrounding the assimilating 
centres must fall considerably short of 3/10,000 of an atmosphere, whereas 
during active respiration the air of the intercellular spaces may be highly 
charged with that gas, thus increasing the assimilatory effect in proportion. 
In the former case the limiting factor is the small partial pressure of the 
carbon dioxide, the photosynthetic rays being in excess; whilst in the latter 
case the limitation is due to the comparative want of a sufficient amount of 
radiant energy of the right kind, and not to paucity of carbon dioxide. 
That this is the true explanation was shown by another experiment on one 
of these very actively respiring leaves of Dioscorea by illuminating it in the 
F 2 
