426 THE POPULAE SCIENCE MONTHLY 



of light in the plant. Potassium and calcium nitrate, for example, yield 

 nitrites and later ammonia in such a process, and acetic, glycollic, pro- 

 pionic, malic, tartaric and citric acid are hroken up, yielding formalde- 

 hyde, 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 photo- 

 synthesis is perhaps the most fundamental of all processes in the world 

 of living things, since it is with this action initially that the construc- 

 tion of nearly all organic material is begun. Various theories have been 

 proposed to account for the procedure from the entrance of carbon diox- 

 ide 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 bicar- 

 bonate, 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 spectro- 

 graph of this substance reveals the fact that rays of certain wave-lengths 

 are absorbed by it. In other words, these rays impinging on the chloro- 

 phyll change the nature of its electronic movement and cause some dis- 

 integration of its structure. The disturbance, whatever it may be, is 

 communicated to the biearbonates, 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 num- 

 ber of measurements of intensity and of the reducing effect of light 

 under various conditions and in various places in which experimenta- 

 tion 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 



