haled. 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 proportion is reached. An increase of the gas in the atmosphere would therefore be partly corrected by the absorption and by the stronger vegetation induced. Nothing short of a comprehensive cataclysm could work such disturbance to the composition of the air as to endanger the well-being of the animal inhabitants of the earth.
The activity of a square metre of leaf surface results in the formation of one and a half to two grammes of solid substance per hour in sunlight. A vigorous sunflower with one hundred and forty-five leaves constructed thirty-six grammes of solid matter in a day, and a squash with one hundred and sixteen leaves one hundred and sixteen grammes in the same length of time. The amounts formed by such trees as the beech, maple, oak, poplar, elm, and horse-chestnut, with leaf surfaces aggregating three hundred to one thousand square metres, must be correspondingly large.
A comparison of plants grown in strong sunlight, diffuse light, and darkness will reveal many differences in stature and internal structure. These differences are for the most part due to the forma-