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 anaesthetic 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.

Proceeding upon this assumption, many investigators have studied the
question as to whether formaldehyde actually is present in green leaves.
Several workers have reported successful identification of formaldehyde in
the distillate from green leaves; while others have criticized these
results and have maintained that formaldehyde can likewise be obtained by
distilling decoctions of dry hay, etc., in which the photosynthetic process
could not possibly be conceived to be at work. Other investigators, notably
Bach and Palacci, reported that they had succeeded in artificially
producing formaldehyde from water and carbon dioxide, in the presence of a
suitable catalyzer or sensitizer. Euler, however, later showed
conclusively that under the conditions described by these investigators,
formaldehyde can be obtained even if no carbon dioxide is present, being
apparently produced by the action of water upon the organic sensitizer
which was used.

These conflicting rep