In 1750, Bonnet, a Genevese naturalist, remarked that leaves immersed
in water became covered in the sun with small bubbles of a gas that he
compared to small pearls. In 1772, Priestley, after discovering that
the sojourn of animals in a confined atmosphere renders it
irrespirable, investigated the influence of plants placed in the same
conditions, and he relates, in these words, the discovery that he made
on the subject:
"I put a sprig of mint in a quantity of air in which a candle had
ceased to burn, and I found that, ten days later, another candle was
able to burn therein perfectly well." It is to him, therefore, that is
due the honor of having ascertained that plants exert an action upon
the atmosphere contrary to that exerted by animals. Priestley,
however, was not completely master of his fine experiment; he was
ignorant of the fact, notably, that the oxygen is disengaged by plants
only as long as they are under the influence of light.
This important discovery is due to Ingenhouse. Finally, it was
Sennebier who showed that oxygen is obtained from leaves only when
carbonic acid has been introduced into the atmosphere where they
remain. Later on, T. De Saussure and Boussingault inquired into the
conditions most favorable to assimilation. Boussingault demonstrated,
in addition, that the volume of carbonic acid absorbed was equal to
that of the oxygen emitted. Now we know, through a common chemical
experiment, that carbonic acid contains its own volume of oxygen. It
was supposed, then, that carbonic acid was decomposed by sunlight into
carbon and oxygen. Things, however, do not proceed so simply. In fact,
it is certain that, before the complete decomposition into carbon and
oxygen, there comes a moment in which there is oxygen on the one hand
and oxide of carbon (CO_{2} = O + CO) on the other.
The decomposition, having reached this point, can go no further, for
the oxide of carbon is indecomposable by leaves, as the following
experiment proves.
If we put phosphorus and some leaves into an inert gas, such as
hydrogen, we in the first place observe the formation of the white
fumes of phosphoric acid due to the oxidation of the phosphorus by the
oxygen contained in the leaves. This phosphoric acid dissolves in the
water of the test glass and the latter becomes transparent again. If,
now, we introduce some oxide of carbon, we remark in the sun no
formation of phosphoric acid, and this proves that there is no
emission of oxygen.
[Illustration: DEMONSTRATION THAT STARCH IS FORMED IN LEAVES ONLY AT
THE POINTS TOUCHED BY LIGHT.]
This latter hypothesis of the decomposition of carbonic acid into a
half volume of vapor of carbon and one volume of oxygen being
rejected, the idea occurred to consider the carbonic acid in a
hydrated state and to write it CO_{2}HO.
In this case, we should have by the action of chlorophyl: 2CO_{2}HO
(carbonic acid) = 4O (oxygen) + C_{2}H_{2}O_{2} (methylic aldehyde).
This aldehyde is a body that can be polymerized, that is to say, is
capable of combining with itself a certain number of times to form
complexer bodies, especially glucose. This formation of a sugar by
means of methylic aldehyde is not a simple hypothesis, since, on the
one hand, Mr. Loew has executed it by starting from methylic aldehyde,
and, on the other, we find this glucose in leaves by using Fehling's
solution.
The glucose formed, it is admissible that a new polymerization with
elimination of water produces starch. The latter, in fact, through the
action of an acid, is capable of regenerating glucose.
It may, therefore, be supposed that the decomposition of carbonic acid
by leaves brings about the formation of starch through the following
transformations: (1) The decomposition of the carbonic acid with
emission of oxygen and production of methylic aldehyde; (2)
polymerization of methylic aldehyde and formation of glucose; (3)
combination of several molecules of glucose with elimination of water;
formation of starch.
Starch is thus the first stable product of chlorophylian activity. Is
there, in fact, starch in leaves? It is easy to reveal its presence by
the blue coloration that it assumes in contact with iodine in a leaf
bleached by boiling alcohol.
Mr. Deherain has devised a nice method of demonstrating that this
formation of starch, and consequently the decomposition of carbonic
acid, can occur only under the influence of sunlight. He pointed it
out to us in his course of lectures at the School of Grignon, and
asked us to repeat the experiment. We succeeded, and now make the
_modus operandi_ known to our readers.
The leaf that gave the best result was that of the _Aristolochia
Sipho_. The leaf, adherent to the plant, is entirely inclosed between
two pieces of perfectly opaque black paper. That which corresponds to
the upper surface of the limb bears cut-out characters, which are here
the initials of Mr. Deherain. The two screens are fastened to the leaf
by means of a mucilage of gum arabic that will easily cede to the
action of warm water at the end of the experiment.
The exposure is made in the morning, before sunrise. At this moment,
the leaf contains no starch; that which was formed during the
preceding day has emigrated during the night toward the interior of
the plant.
After a few hours of a good insolation, the leaf is picked off. Then
the gum which holds the papers together is dissolved by immersion in
warm water. The decolorizing is easily effected through boiling
alcohol, which dissolves the chlorophyl and leaves the leaf slightly
yellowish and perfectly translucent.
There is nothing more to do then but dip the leaf in tincture of
iodine. If the insolation has been good, and if the screens have been
well gummed so that no penumbra has been produced upon the edge of the
letters, a perfectly sharp image will be instantly obtained. The
excess of iodine is removed by washing with alcohol and water, and the
leaf is then dried and preserved between the leaves of a book.
It is well before decolorizing the leaf to immerse it in a solution of
potassa; the chlorophylian starch then swells and success is rendered
easier.--_Lartigue and Malpeaux, in La Nature_.
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