274 
PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
tion of oxygen by the leaf whose under surface is exposed to the sun goes on con- 
stantly diminishing. 
I devised another rather striking mode of making the same experiment. In one 
of my trials, I had been surprised to observe that the leaves of Camellia japonica 
did not, when exposed to the sun in spring-water, disengage oxygen gas by their 
stomata, but that bubbles of this gas went off through the footstalks. I ascertained 
afterwards that this fact had already been mentioned by Dutrochet. It afforded me 
the means of showing the difference of the action of light on the two surfaces of the 
same leaf. 
To make this experiment, I place in two large tubes filled with spring-water, two 
Camellia leaves having an equal surface, and their footstalk directed upwards. 
One of the leaves has its upper, the other its lower surface exposed to light, and the 
opposite side is shaded. I leave the apparatus for at least five or six hours in 
the dark, after which I expose it to the diffused light at the temperature of 20° C. 
(68° F.) ; the direct rays of the sun not being necessary to the production of the phe- 
nomenon. After twenty minutes of exposure, there appear on the leaf, the upper sur- 
face of which is exposed to the light, numerous bubbles of a gas containing 85 to 90 
per cent, of oxygen, disengaging themselves from the aerial vessels placed in the 
centre of the footstalk, and whose apertures are clearly visible. The bubbles are 
very small, and so numerous and so rapidly emitted, that it is absolutely impossible 
to count them ; they chase one another, and are all gathered up at the top of the 
tube. Begun at half-past nine in the morning, for instance, the bubbling lasts with 
the same activity for an hour, after which it ceases. Towards the end, the current 
of bubbles is a little less rapid ; but the smallest number I was able to count at 
twenty minutes past ten o’clock, was 120 bubbles per minute. In the leaf whose 
lower surface is exposed to light, the gas begins to disengage itself only after thirty 
minutes of exposure to light; the current of bubbles is much less rapid, and during 
the whole time of duration I could always easily count them. The greatest number 
produced was 145 in a minute; and while the other leaf still gave 120 of them per 
minute, this one gave only thirty. The bubbles ceased at the same moment in both 
leaves, and consequently lasted only three quarters of an hour in the leaf whose under 
surface was illuminated. I was not surprised, therefore, in measuring the quantities 
of disengaged gases, to find that the leaf of Camellia exposed to light in its natural 
position had given three times as much oxygen gas as the other whose position was 
inverted, all other circumstances being similar, and both being plunged in the same 
liquid. When the apparatus is again placed in darkness, a new accumulation of 
carbonic acid takes place in the cells of the leaves ; and if they remain in it a sufficient 
time, the disengagement of bubbles of oxygen gas by exposure to light begins again, 
with the same differences as to time and proportion, according as the leaf is illumi- 
nated on its upper or lower surface. I have seen the same leaves producing the same 
phenomenon for seven or eight days in succession, by keeping them alternately in 
