Further, Usher and Priestley's later results showed that radium emanations, acting upon a solution of carbon dioxide in water, produce hydrogen peroxide and formaldehyde, and the latter polymerizes but not up to the point represented by the hexose sugars; also, that the ultra-violet rays from a mercury vapor lamp are very effective in bringing about the production of hydrogen peroxide and formaldehyde from a saturated aqueous solution of carbon dioxide, the reaction taking place even in the absence of any "sensitizer," but much more readily if some "optical" or "chemical" sensitizer is present. Finally, these investigators were able to duplicate all their results, using green plant tissues, and to show that the temperature changes which take place in a film of chlorophyll when it is exposed to an atmosphere of moist carbon dioxide in the sunlight are such as would be required by the formation of formaldehyde and hydrogen peroxide from carbonic acid. More recently, Ewart has showed that formaldehyde can combine chemically with chlorophyll; from which fact, Schryver deduces the theory that if for any reason the condensation of formaldehyde into carbohydrates by the cell protoplasm does not proceed as rapidly as the formaldehyde is produced by photosynthesis, the excess of the latter enters into combination with the chlorophyll, and that if condensation into sugar uses up all the free formaldehyde which is present in the active protoplasm, the compound of formaldehyde with chlorophyll is broken down setting free an additional supply for further sugar manufacture. According to this conception there are, in the chlorophyll-bearing protoplasm, not only the agencies for the production of formaldehyde from carbon dioxide and water and for the condensation of this into carbohydrates, but also a chemical mechanism by means of which the amount of free formaldehyde in the reacting mass may be regulated so that at no time will it reach the concentration which would be injurious to the cell protoplasm or fall below the proper proportions for sugar-formation. This explanation affords a satisfactory solution of the difficulty which formerly confronted the students of photosynthesis, namely, the fact that free formaldehyde is powerfully toxic to cell protoplasm. Without some such conception, it was difficult to imagine how the presence of formaldehyde in the cell contents, even as a transitory intermediate product, could be otherwise than injurious. As a result of these studies, the nature of the chemical changes which result in the production of formaldehyde as the first product of photosynthesis, with the liberation of a volume of oxygen equal to that of the carbon dioxide consumed, seems to be fairly well established. THE PRODUCTION OF SUGARS AND STARCHES