Inasmuch as light must be absorbed in order to give rise to photochemical action, we find that such action is characterized by high absorption power on the part of the substance that under goes the chemical transformation. If all the incident light were absorbed the substance would reflect nothing and would, therefore, appear black. If the absorption were only par tial and if it took place in such a way that the absorbed energy constituted the same fraction of the incident white light at every wave-length, then the reflected light would contain all the colors that exist in sunlight and in the same proportion in which these colors must be present to produce the sensation of whiteness; and hence in this case the substance would appear white or gray. In the actual case as we find it in nature, the incident light is only partially absorbed, but the absorption is quite marked (and may even be almost complete) between certain wave-lengths, while in other parts of the spectrum there is little or no absorption. The absorbing substance, therefore, appears colored, since it absorbs certain wave-lengths with marked efficiency and gives off the others by re flection. In studying the absorption of light scientifically the spectroscope is employed, and the particular wave-lengths that are absorbed are determined by allowing the spectrum to fall upon a uniformly thick layer of the substance under investigation and examining the trans mitted light to see what particular colors have been absorbed— absorption being indicated by the presence of dark bands (or *absorption bands") in the spectrum of the light that has passed through the test layer. in the case of the green coloring matter of plant leaves, the absorption band that corresponds to the photo synthetic activity is in the red part of the spectrum.
It is not possible to determine with precision the efficiency wjth which the absorbed vibratory energy of the light is converted into potential chemical energy in the leaf of a growing plant, but it is known that this efficiency is quite high. According to the best available estimates it may range from 40 per cent upward, and some authorities have even estimated it at 98 per cent in special cases.
The synthesis of highly organized com pounds from carbon dioxide and water is per formed by living organisms in various ways. Certain of the bacteria, for example, perform syntheses of this kind by utilizing the energy obtained by the simultaneous oxidation of hy drogen or other substances that may be present. Most of the plant life of the world, however, depends upon photosynthesis, effected through the agency of the green "chlorophyll" that the leaves contain. The coloring matter extracted from green leaves contains four recognizably distinct constituents— two of which are green and two yellow. The green substances appear to be the ones that effect the photosynthesis. One of them ("a-chlorophyll") is bluish green in color and the other (lb-chlorophyll") is yellowish-green, and appears to be an oxide of a-chlorophyll. These constituents are probably
identical in all plants, and they contain mag nesium and nitrogen, but no phosphorus or iron. The yellow pigments that are present in the leaves are carotin (C401-1.) and its oxide xanthophyll (C01-15.02). Carotin is identical with the ulutein" that occurs in the corpora lutea of mammals, and xanthophyll is isomeric with the ulutein" of fowl's eggs. The view that xanthophyll is related to cholesterol is now known to be erroneous. Carotin may perhaps assist in the decomposition of carbon dioxide.
There is a close and suggestive structural similarity between chlorophyll and the hwmo globin of the human blood, and this fact may lead to a further understanding of the way in which chlorophyll acts. We do not yet know how it affects the synthesis of starch and sugar from carbon dioxide and water. In the living leaf the chlorophyll occurs in the form of con centrated layers upon (or near) the surfaces of certain tiny structural granules called which appear to perform an important function of some kind or other. It has been shown, for example, that when a seedling is grown in the dark and subsequently placed in the light it does not necessarily pos sess photosynthetic power as soon as chloro phyll appears in it; and this shows that other elements, structural or chemical, are needed be fore photosynthesis can occur. It is also inter esting to note that a mere trace of chloroform vapor in the air stops photosynthetic activity, even in a healthy and vigorous leaf.
The fascinating suggestion has been made that formaldehyde is first formed in the leaf in accordance with the simple equation CO2 + + 02. If it could be demonstrated that this action really occurs, we should prob ably be justified in the expectation that the mysteries of the green leaf will soon be cleared up. Formaldehyde easily polymerizes with the formation of higher compounds, and there would not be any great difficulty, apparently, in accounting for the formation of starch, sugar and other related substances after formaldehyde had once been obtained. The fact that for maldehyde has a strong toxic effect on living matter would not necessarily constitute an ob jection to the theory, because the formaldehyde might become polymerized, in the growing leaf, almost as rapidly as it was formed, so that it would never be present in sufficient quantity to exert a harmful influence. Considerable doubt has been thrown upon the formaldehyde hy pothesis by the work of Schryver, Wager and Warner, who find that although an aldehyde of some kind appears to be formed in the leaf, it probably is not formaldehyde. Moreover, the presence of carbon dioxide is not essential to its production.
The carbon dioxide absorbed by the leaf may combine, first, with the chlorophyll and then split off in a different form, bnt we have no proof as yet that an action of this kind