osynthesis,
proceeding through formaldehyde as an intermediate product, can be
successfully duplicated _in vitro_ in the laboratory.

Assuming, then, that formaldehyde is the first photosynthetic product in
the process of the production of carbohydrates from water and carbon
dioxide, the simple empirical equation for this transformation would be

H_{2}O + CO_{2} = CH_{2}O + O_{2}.

It is apparent, however, that the process is not so simple as this
hypothetical reaction would indicate, as water and carbon dioxide can
hardly be conceived to react together in any such simple way as this.
Various theories as to the exact nature of the steps through which the
chemical combinations proceed have been advanced. A discussion of the
experimental evidence upon which these are based and of the conclusions
which seem to be justified from these experimental studies is presented
below. The only value which may be attached to the empirical equation just
presented is that it does accurately represent the facts that a volume of
oxygen, equal to that of the carbon dioxide consumed in the process, is
liberated and that formaldehyde is the synthetical product of the reactions
involved.

It should be noted, in this connection, that formaldehyde is a powerful
plant poison and that few, if any, plant tissues can withstand the toxic
effect of this substance when it is present in any considerable
concentration. Hence, it is necessary to this whole conception of the
relation of formaldehyde to the photosynthetic process, to assume that,
however rapidly the formaldehyde may be produced in the cell, it is
immediately converted into harmless carbohydrate forms.

THE CONDENSATION OF FORMALDEHYDE INTO SUGARS

As has been mentioned, it is easily possible to cause either formaldehyde,
or trioxymethylene, to condense into C_{6}H_{12}O_{6}, using milk of lime
as a catalyst. Of course, no such condition as this prevails in the plant
cell, and the mechanics of the protoplasmic process may be altogether
different from those of the artificial syntheses. Furthermore, the hexose
produced by the artificial condensation of these simpler compounds is, in
every case, a non-optically active compound, while all natural sugars are
optically active (see Chapter IV). Emil Fischer has succeeded, however, by
a long and round-about process which need not be discussed in detail here,
in converting the artificial hexose into glucose a