sunlight, although
photosynthesis can be brought about by the energy from certain forms of
artificial light.

(3) Carbon dioxide and water must be decomposed by the energy thus
absorbed, and the nascent gases thus produced combined into some synthetic
organic compound, with a resultant storage of potential energy.

(4) This first organic synthate must be condensed into some carbohydrate
suitable for translocation and storage as reserve food.

(5) The oxygen, which is a by-product from the decomposition of the water
and carbon dioxide and the resultant synthetic process, must be returned to
the air by a gas exchange.

Of the five steps in this process, the first two and the last are
essentially purely physical phenomena, the chemical changes involved being
those of the third and fourth steps. Hence, it is only these two parts of
the process which need be taken into account in a consideration of the
chemistry of photosynthesis.

FORMALDEHYDE, THE SIMPLEST CARBOHYDRATE STRUCTURE

The simplest carbohydrates known to occur commonly in plant tissues are the
hexoses (see Chapter IV) having the formula C_{6}H_{12}O_{6}, which is just
six times that of formaldehyde, CH_{2}O. Also, it is known that
formaldehyde easily, and even spontaneously, polymerizes into more complex
forms having the general formula (CH_{2}O)_n_; trioxymethylene,
C_{3}H_{6}O_{3}, being a well-known example. Further, both trioxymethylene
and formaldehyde itself can easily be condensed into hexoses, by simple
treatment with lime water as a catalytic agent. Hence, it is commonly
believed that formaldehyde is the first synthetic product resulting from
photosynthesis, that this is immediately condensed into hexose sugars, and
that these in turn are united into the more complex carbohydrate groups
which are commonly found in plants (see Chapter IV).

There is considerable experimental confirmation of the soundness
of this view. The whole photosynthetic process takes place in
chlorophyll-containing plant tissues with astonishing rapidity, sugars,
and even starch, appearing in the tissues almost immediately after their
exposure to light in the presence of carbon dioxide. Hence, any
intermediate product, such as formaldehyde, is present in the cell for
only very brief periods and in very small amounts. But small amounts of
formaldehyde can often be detected in fresh green plant tissues and, as
will be pointed out below, the whole process of phot