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 and 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.