nd fructose, the
optically-active sugars which occur naturally in plant tissues. The
condensation of formaldehyde directly into glucose and fructose in the
plant cell is brought about by some process the nature of which is not yet
understood. Probably synthetic enzymes (see Chapter XIV), whose nature and
action have not yet been discovered, come into play. It is a noteworthy
fact, however, that the mechanics of this apparently simple chemical
change, upon which the whole nutrition of the plant depends, and which
furnishes the whole animal kingdom, including the human race, with so large
a proportion of its food supplies, is as yet wholly unknown.

It is the common practice to represent the whole results of the
photosynthetic action by the empirical equation

6H_{2}O + 6CO_{2} = C_{6}H_{12}O_{6} + 6O_{2};

but here again the only value to be attached to such an algebraic
expression is that it accurately represents the gaseous exchange of carbon
dioxide and oxygen involved in the process. Certainly, it throws no light
upon the nature of the process itself.

THEORIES CONCERNING PHOTOSYNTHESIS

The many theories which have been advanced concerning the nature of the
chemical changes which are involved in photosynthesis have served as the
basis for much experimental study of the problem. The following brief
summary will serve to point out the general trend of these investigations
and the present state of knowledge concerning the chemistry of
photosynthesis.

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