Von Baeyer, in 1870, advanced the hypothesis that the first step in the process is the breaking down of carbon dioxide into carbon monoxide and oxygen and of water into hydrogen and oxygen; that the carbon monoxide and hydrogen then unite to produce formaldehyde, which is immediately polymerized to form a hexose. These theoretical changes may be represented by the following equations: { CO_{2} = CO + O 1. { { H_{2}O = H_{2} + O 2. H_{2} + CO = CH_{2}O 3. 6(CH_{2}O) = C_{6}H_{12}O_{6} In the investigations and discussions of this hypothesis, it has been ascertained: first, that carbon monoxide has never been found in the free form in plant tissues; second, that when _Tropaeolum_ plants were surrounded with an atmosphere in which there was no carbon dioxide, but which contained sufficient carbon monoxide to give a concentration of this gas in the cell-sap equivalent to that in which CO_{2} is normally present, the plants grew normally and apparently elaborated starch; third, other and more extensive experiments indicated, however, that green plants in general cannot make use of carbon monoxide gas for photosynthesis, although this does not prove that von Baeyer's idea that CO is a step in the process is necessarily erroneous; and finally it was shown that carbon monoxide, in sufficient concentration to produce the results with _Tropaeolum_ mentioned above, usually acts as a powerful anæsthetic towards most other plants. While these considerations do not positively prove that von Baeyer's hypothesis is incorrect, they render it so improbable that it has generally been abandoned in favor of others which are described below. Erlenmeyer, even before the experimental work mentioned in the preceding paragraph had been reported, suggested that instead of assuming a separate breaking down of the carbon dioxide and water, it is easier to conceive that they are united in the cell-sap into carbonic acid and that this is reduced by the chlorophyll-containing protoplasm into formic acid and then to formaldehyde, as indicated by the following equations: 1. H_{2}CO_{3} = H_{2}CO_{2} + O 2. H_{2}CO_{2} = CH_{2}O + O Like von Baeyer's hypothesis, this assumes that formaldehyde and oxygen are the first products of photosynthesis.