PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 273 
solar light on the green parts of vegetables. Senebier has advanced that it is in- 
dependent of the {cuticle) green organs, and takes place in the parenchymatous 
matter. This is true as long as the plant is not disorganized. Thus, for instance, 
a slice cut in the parenchyma of a leaf of Rochaea falcata, weighing ten grains, 
gave out in the sun, when immersed in water containing carbonic acid in solution, a 
certain number of bubbles of oxygenated air. The bubbles appeared to form them- 
selves in the exterior cells, and seemed as if they could not disengage themselves 
without trouble ; they remained attached to the fragment, and it was necessary to 
shake the vessel in order to make them come up. An equal weight of the parenchyma 
of the Rochaea having been pounded, so as to destroy all the cells, was exposed to 
the sun in an equal quantity of the carbonic acid solution and during the same time. 
No oxygenated air was produced. The same negative result was obtained when the 
expressed juice of this leaf was put in the sun with water, although it contained a 
great deal of the green matter that fell to the bottom of the tube. When I put into 
the water a slice of the leaf of Rochsea of the same dimensions as the first, but in 
which the cuticle had remained with a small layer of green matter, although this 
fragment weighed only five grains, or the half of the preceding one, I saw a regular 
series of bubbles disengage themselves from the stomata. I could measure in an 
equal length of time with the two tubes plunged in the same solution of carbonic 
acid, three times as much oxygenated air from the slice covered by its cuticle than 
from the one that was not so covered. 
The same experiments repeated with a great many other plants gave similar results, 
and show, — first, that the green chromule alone is not endowed with the property of 
decomposing carbonic acid, and that this faculty is the consequence of a physiological 
action of the cells ; secondly, that this decomposition is increased by the agency of 
the vessels and pores that exist on the cuticles of leaves. 
In order to appreciate the influence of light on the production of oxygen gas when 
it acted on one or the other of the surfaces of leaves, I took two leaves of the same 
plant as equal as possible in weight and surface, and placed them in two similar bell- 
glasses, inverted over the same weak solution of carbonic acid or over spring- water. 
One of the leaves was placed in such a manner as to present to the sun its under 
surface, and the other its upper one. The other side of the bells was covered with 
blaek paper. The gases produced were carefully measured after the experiment, and 
the temperature was noted. 
I have made a great many experiments on a great variety of leaves ; and without 
giving the details in this place, I will only add, that when the leaves are in their 
natural position, that is to say, with their varnished surface exposed to the light, the 
bubbles of oxygen gas are produced much quicker and in far greater quantity 
than when the under surface is exposed to light. During the same length of time, two 
or three times as much gas is formed in the first case as in the second ; and the dif- 
ference is the more marked the longer the experiment is continued, for the produc- 
