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.