of light in the plant. Potassium and calcium nitrate, for example, yield nitrites and later ammonia in such a process, and acetic, glycollic, propionic, malic, tartaric and citric acid are hroken up, yielding formaldehyde, carbon dioxide and other substances.
The greatest addition to the potential energy of the plant, however, is that in which carbon dioxide from the air enters leaves and in the ensuing process carbohydrates result and oxygen escapes. This photosynthesis is perhaps the most fundamental of all processes in the world of living things, since it is with this action initially that the construction of nearly all organic material is begun. Various theories have been proposed to account for the procedure from the entrance of carbon dioxide into the plant to the formation of sugars, but none of these will stand the test of our critical experiments. Their inadequacy may be ascribed to the fact that the function of the light in the matter is not yet clear. At present we may only say that upon the entrance of carbon dioxide into a leaf it probably unites with potassium to form the bicarbonate, in which salt it is more easily broken up than as if it remained a free acid. Here are then bicarbonate and water in the presence of chlorophyll, the green coloring substance of the plant. The spectrograph of this substance reveals the fact that rays of certain wave-lengths are absorbed by it. In other words, these rays impinging on the chlorophyll change the nature of its electronic movement and cause some disintegration of its structure. The disturbance, whatever it may be, is communicated to the bicarbonates, and to the water, which are reduced, the free oxygen escaping and some simple carbonhydrate resulting. So far we may proceed in complete harmony with known facts. Between this and the appearance of hexoses in the leaf is a wide gap. Once bridged and the full effect of light in the entire course of photosynthesis made out, it may be possible to simulate a process which now takes place only in living tissue, and make available to the race a source of energy all but limitless in its potentialities.
The formulation of plans for this work has necessitated a large number of measurements of intensity and of the reducing effect of light under various conditions and in various places in which experimentation might be carried on, by Dr. Spoehr. Among the noteworthy results of such calibration is the demonstration that the blue-violet rays, direct and skylight, is greater at the Desert Laboratory on a shoulder of the Tucson mountains (2,700 feet) sheltered from the prevailing winds and resultant dust, than on the summit of the Santa Catalina mountains (9,250 feet) in which the skylight is less, but the direct light passes through a layer of air a mile less in thickness than that which reaches the laboratory. Watery vapor and dust particles may account for some of the absorption of light on the summit of the forested mountain slopes. Our concern with light as a factor of environment however by no means ends with the part it plays in the reduction and combination
