The use of tracer carbon, as carbon-14, has made possible considerable progress in the mapping of the routes taken by the carbon atom from CO/sub 2/ into plant substances. The techniques of separation and identification that have made the progress possible lie largely in the region of chromatography and radioautography involving fractional-gamma amounts of material. Most of the earlier steps of carbon incorporation are now known. In addition, a number of the later steps on the routes to amino acids and proteins and other plant substances are now under investigation. As a result of the recognition of the earlier stages of carbon incorporation, a number of proposals have been made about the photochemical act itself. These proposals have led to the development of direct physical tests of their validity, and some results of these will be described. The remaining principal area of investigation involving the route of oxygen atoms from water to molecular oxygen is largely unexplored, but the use of new methods of analyzing for the heavy isotopes of oxygen may make possible more progress in this area. 44 references. (auth)