Scientific American Supplement No. 822, October 3, 1891

by Various

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 CO₂HO.

In this case, we should have by the action of chlorophyl: 2CO₂HO (carbonic acid) = 4O (oxygen) + C₂H₂O₂ (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.