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 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.