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 _formative_ and _tonic_ effect of light. Otherwise expressed, the influence of variations of light upon plants causes adaptive reactions, and disturbances of the nutritive processes and growth. In consequence of these facts the reaction of any given organ to changes in the intensity of the illumination will depend upon its specific functions and relation to the remainder of the organism. The stems formed by seedlings and awakening underground organs are usually surrounded by plants or other objects which cut off more or less sunlight. The developing shoot can not spread its leaves to the light advantageously until it has outstripped or grown beyond the objects intervening between it and the light. This necessity is one of the most important conditions in the struggle for existence. To meet it, a very great majority of seed-forming plants have acquired the power of accelerated elongation of the stems when deprived of their normal amount of light. Very striking examples of this reaction are offered by the awakening corms of the Jack-in-the-pulpit (_Arisæma triphyllum_). The corms usually lie at a distance of five or six centimetres below the surface of the soil, and when the growth of the large bud begins in the spring the heavy sheathing scales elongate and pierce the soil, opening when the surface is reached at the distance of a few centimetres. If the corm should have been buried deeper in the substratum by floods or drifts of leaves, the growth of the bud scales will continue until the light is reached, though it may be a distance of twenty centimetres. Such growth may be seen if the corms are grown in a deep layer of sphagnum moss, or in a dark room.