272 
PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
2nd. As soon as sufficient time has elapsed to allow water to penetrate into the leaf, 
the loss by exhalation is, in all temperatures and in every atmospherical state, much 
greater when the under surface of leaves is exposed to light than when it is the 
varnished side. The proportion has varied a little in the experiments, and according 
to the nature of leaves, but in general the loss has been three times more considerable; 
for instance in the maple, the horse-chestnut, the pear-tree, the plane, &c. In some 
cases, the difference has been still greater ; but sometimes it has been only double. It 
is obvious that to this enormous increase of the loss of water, which takes place in 
the inverted leaves, a loss already so considerable in the ordinary state, is to be mainly 
attributed the state of uneasiness, followed by withering and death, which is the con- 
sequence of this position when it is forced upon them ; and we thus see one of the 
ends of nature in giving them the means of delivering themselves from this evil. 
By means of the coloured glasses, I endeavoured to estimate the influence of each 
of the rays of the spectrum in the production of this phenomenon. Out of a great 
many series of experiments I shall select only one. 
In two hours, a leaf of raspberry-bush, weighing twenty-three grains, placed in 300 
grains of water, exhaled, with its under surface exposed to the light, in a tempera- 
ture of 20° C. (68° F.), — 
grs. 
In diffused light ......... 4*3 water. 
In the blue rays 6*3 water. 
In the yellow rays 2’0 water. 
In the green rays 2'0 water. 
In the red rays TO water. 
In the dark .0*4 water. 
This leaf, the next day, during an equal space of time, and in the same tempera- 
ture, exhaled, with its upper surface exposed to the light, — 
In diffused light .... 
grs. 
2'2 water. 
Under blue glass . . . 
2’8 water. 
Under yellow glass . . . 
. . . . . . 0*5 water. 
Under red glass .... 
0’4 water. 
In the dark 
0‘0 water. 
In all the experiments the exhalation was greater in the blue rays than in the 
others, whether it was the upper or the under surface of the leaf that was ex- 
posed to the influence of light. The blue rays excite a greater exhalation than 
the diffused light ; but this light has more influence than the other rays. The red 
is that in which the exhalation is the smallest. 
^ 5. Action of Light on the Decomposition of Carbonic Acid. 
It is known that this phenomenon, which in plants effects the assimilation of 
carbon, and perhaps also of oxygen gas, takes place only under the influence of the 
