Light is the most important of all the external agencies which influence the vegetal organism, and the sun's rays have been the most potent force in shaping the development of existent plant forms. The sunbeam stands in a manifold relation to the plant. First and foremost, light is the universal source of energy, by the aid of which the chlorophyll apparatus in green leaves builds up complex food substances from simple compounds obtained from the soil and air, a process necessary for the nutrition of the entire living world. Some obscure organisms, such as the "nitrosomonas," soil bacteria, are able to accomplish the construction of complex substances, by means of energy derived from other chemical compounds, which were, however, formed originally by green plants. These food-building processes are designated as photosynthesis, chemosynthesis, electrosynthesis, thermosynthesis, etc., according to the source of energy used. By photosynthesis, carbon dioxide from the air and water from the cell are combined in the green cells of leaves, forming sugar and possibly other substances. During this process an amount of oxygen approximately equal to that of the carbon dioxide taken up is exhaled. It will be of interest to note the relation of the living world to the atmosphere. Eight hundred to nine hundred grammes of carbon dioxide are produced in the respiration of a single person for a day, and the entire product of the human race for this period is twelve hundred million kilogrammes. In addition, large quantities of the gas result from the combustion of the four hundred and sixty millions of kilogrammes of coal and wood burned yearly. The lower animals, fungi, and green plants themselves contribute an amount which must bring the total to twice the immense sum named above. The atmosphere contains three or four hundredths of one per cent of carbon dioxide, or an amount of about two to three thousand billions of kilogrammes. No especial variation in this proportion has been detected since observations upon this point were first made. The fact that no increase takes place is partly due to the absorption of the gas by plants, and its replacement by oxygen, and also to certain geological processes in constant operation. Absorption takes place at the rate of about two and a half grammes for every square metre of leaf surface per hour, or about twenty-five to thirty grammes daily, since the process goes on only in daylight. It is to be seen that a single human being exhales as much carbon dioxide as may be removed from the air by thirty or forty square metres of leaf surface. According to Ebermayer, a hectare (2.47 acres) of forest would use eleven thousand kilogrammes of carbon dioxide yearly, and the amount used by plants is generally much in excess of that furnished by the activity of the inhabitants of any given area. Plants thrive and show increasing vigor as the amount of carbon dioxide in the air rises until two hundred times the present