PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
271 
and in ordinary circumstances, exhalation continues during’ the night, though in 
much less quantity than during the day. 
I have attempted to determine what difference could be perceived in the exhala- 
tion of leaves when the one or the other of their surfaces was exposed to light. I 
began by ascertaining that, when the leaves of a great many different plants are 
thus exposed in the air or in the sun’s rays at the same temperature, the loss of weight 
they experienced in the two cases is proportional during the same time ; at first, 
however, rather greater when their under surface is exposed to light than when it 
is the upper or varnished surface. Thus, in two hours, horse-chestnut leaves lost 
water in the ratio of 13*6 per cent., when the under surface was exposed to light, to 
11*2 per cent, when it was the upper one ; with pear-tree leaves, the ratio was 8’45 to 
7'73 per cent. But the more the leaf dries up, the quicker the difference diminishes ; 
and when no more real exhalation exists, but there is only deperdition of water, it be- 
comes nothing, and the leaves lose, during an equal time and at the same temperature, 
a proportional weight of water, whatever be the surface exposed to light. 
To ascertain the variations in the exhalation properly so called, I weighed the 
leaves recently gathered, either with their footstalks only, or with the branch on which 
they grew ; I immersed them afterwards by the footstalk or branch in a bottle filled 
with water of which the weight had been accurately taken. One of the surfaces of the 
leaf was then exposed to diffused light and the other covered with a screen. After a 
given time, I weighed accurately the leaf and the bottle, and the loss gave the amount 
of exhalation. The same leaf was then inverted and placed in a contrary direction, so 
that its other surface was exposed to light during a space of time precisely equal to 
the first. The loss in weight disclosed to me the quantity of the exhaled water. 
The temperature was carefully observed, though I had ascertained by direct experi- 
ment, so far agreeing with Senebier’s, that heat had very little influence on the exha- 
lation itself. I could besides reverse many times during the day the experiment on 
the same leaf, and thus obtain alternately for its two surfaces the same circumstances 
of heat and light. For leaves that have a great tendency to turn over, those of the 
raspberry-bush, for instance, it was somewhat difficult to maintain them without con- 
torsion of the flat part of the leaf with their under surface exposed to light. With 
leaves of this description the experiment on that surface could not be continued above 
two hours. 
I have made numerous experiments on a great many species of leaves, in all tem- 
peratures and in all weathers, but it would be tedious to give the particulars of them. 
I shall only report the general results. 
1st. A leaf immersed in water by its footstalk increases at first in weight a little 
more during the same time, when its under surface is exposed to light than when it 
is the upper one that is so. This result is the consequence of the absorption being 
a little greater in the first case ; but at the end of the experiments the leaves have 
pretty nearly the same weight they had at the beginning. 
