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 photosynthesis, 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