the Respiratory Exchanges of Leaves . 
7*3 
doses was like that produced in the other leaves. As in the case of 
Helianthus the absorption of oxygen was more affected than the production 
of C 0 2 ; when the dose was big enough to cause a marked stimulation, the 
respiratory quotient fell from about 0-9 before exposure to 0*7 after chloro- 
form ; but after a few hours the original ratio was reached again. Also, as 
in the case of Helianthtis , the maximum rates were observed in most 
experiments in the first hour, the rates then falling quickly to the normal 
level. A repetition of the same dose after three hours produced a second 
stimulation, the maxima being, however, rather less than after the first 
dose. 
After exposure to doses of o*ac.c. or more no stimulatory effect was 
detected ; but both C 0 2 production and 0 2 absorption diminished rapidly 
to a low level. Here again the absorption of oxygen was the more affected, 
and its rate fell below the rate of production of C 0 2 , the ratio 
co 2 . . 
of nsing 
much above unity, instead of falling far below unity as it did in the other 
leaves. 
It is to be noticed that Tropaeolum lies midway between Cherry Laurel 
and Helianthus in the degree of susceptibility, or (more probably) penetra- 
bility, of its leaves to chloroform vapour. The stimulation produced by 
ten minutes’ exposure to o-i c.c. chloroform per litre is similar to that after 
a dose of 0.05 c.c. for ten minutes in the case of Helianthus , and 0.2 c.c. for 
twenty minutes in the case of Cherry Laurel. Gaseous exchanges take 
place normally less rapidly in Tropaeolum than in Helianthus , and the 
distribution and character of the stomata (including, for instance, their very 
ready tendency to close) may in part account for the less rapid penetration 
of chloroform. It is probable, however, that the wax which covers the 
surface of the leaf takes up the chloroform and protects the epidermal cells 
from being affected so soon as in leaves of Helianthus , where they have 
neither the waxy covering of Tropaeolum nor the thick cuticle of Cherry 
Laurel. 1 
Discussion. 
The fact which stands out most prominently in the foregoing experi- 
ments is the large absorption of oxygen which accompanies the disorganiza- 
tion of leaves of Cherry Laurel, Portugal Laurel, and Helianthus exposed 
to chloroform vapour in sufficient concentration. This result was only 
observed when visible signs of disorganization appeared, and only when one 
of these signs was the appearance of a brown or black coloration ; in leaves 
of Tropaeolum which do not show any such marked change of colour during 
1 Weevers (loc. cit., p. 255) attributes the differences which he observed in the minimum time of 
exposure that was followed by fatal results in great part to differences of water content. This view will 
clearly not explain the different behaviour of leaves of Helianthus and Tropaeolum. 
