The names of Draper, Sachs and Pfeffer stand forth prominently as pioneers in this; while those of Lommel, Engelmann, Timiriazeff and Langley have been among the most active in making important contributions to the subject, and in attempting to answer the further questions connected with the mode in which the chlorophyll is concerned in utilising the energy of the solar radiations. The point is one of supreme importance, because it goes on all fours with modern questions as to the rays of light absorbed or dispersed in our atmosphere at different seasons of the year, or in special climatic conditions, to say nothing of its other scientific aspects. Unfortunately, however, we have no satisfactory explanation of the actual rôle played by the chlorophyll substance itself, in spite of much industrious work which has been done in the subject in this country and elsewhere. As regards the rays employed, it was first proved that the most effective belong to the red end of the visible spectrum, and that the effect as measured by the amounts of oxygen given off, and of starch formed in given periods of time, is more or less proportionable to the intensity of the solar light. Then it was established that no monochromatic light is so powerful as the white light from which it was obtained, though the relative numbers expressing the activity in the red and yellow regions may stand to those in the blue as something like 12:1. The latest results place the maximum assimilation in the red-orange, and this coincides with the maximum absorption in the chlorophyll. If we may accept the current views as to the distribution of energy in the spectrum of solar light, which depends on the complete absorption of all the rays by a black body, where they are estimated as heat, we have the interesting result that the agricultural or forest plant is adapted to catch and retain, broadly speaking, just those particular rays which possess most energy. The probability is increasing that the protoplasmic machinery is the really effective mechanism in the process, and we may figure this machinery as so holding or presenting the molecules of carbon-dioxide and water to the impact of the light-vibrations, that the latter are enabled to undo the molecular structure; the atomic combinations thereby liberated may then be supposed to form a body like formic-aldehyde, which by polymerisation becomes a carbohydrate of the nature of a sugar such as glucose, which the protoplasm then builds up into its substance and subsequently deposits as starch, and stores temporarily in the form of grains or as amorphous material.