The nature of the mechanism by which these responses are accomplished is, as yet, wholly unknown. There is accumulating a large mass of experimental evidence which goes to show that, while both temperature and light are very important factors in determining the type of changes which will take place in a living organism, the so-called "photochemical action of light" is by far the most potent of all the climatic factors which influence the course of development of a plant. But we have, as yet, no inkling of how the protoplasm of the plant adjusts or controls its responses to variations in any of these external factors. With these general considerations in mind, we may now proceed to the consideration of certain particular types of adaptations. CHROMATIC ADAPTATIONS Adaptations have been observed in both the energy-absorbing pigments of the general tissues and in the ornamental epidermis pigments of plants. The former are by far the most important from the physiological point of view; while the latter may have interesting biological significance. Under nearly all conditions of growth of land plants, the supply of the chlorophylls and their associated pigments provides for the absorption of solar energy far in excess of the amount necessary for the photosynthetic assimilation of all the carbon dioxide which is available to the plant. It has been shown that an active green leaf, on an August day, can absorb eight times as much radiant energy as would be required to assimilate all the carbon dioxide present in the air over its surface. No land plant, under normal conditions, develops supplementary pigments in order to utilize other than the parts of the spectrum which are absorbed by chlorophyll and its associated pigments.