furnished by sunlight. It is the power which drives the plant, just as the electric current is the power which drives the motor. Deprive the plant of light, and the sugar reserves will soon be consumed and nectar secretion will cease. Kenoyer placed buckwheat flowers under both light and dark jars, and after two days the amount of sugar in the nectar of the flowers under the dark jar began to decrease. After four days approximately only one-fourth as much sugar was secreted per flower. He obtained, also, the same results by covering the leaves of the plant with black paper. The flowers of cucumber, snapdragon, and sage all secreted less sugar in darkness. If, however, the flowers were placed in the dark, and the leaves remained in the light, the amount of sugar in the nectar did not decrease. The secretion of nectar is thus dependent on the food reserves made by the leaves. Darwin observed that the extra-floral nectaries of the common vetch (Vicia sativa) ceased to secrete nectar when the sun was hidden by clouds, and the hive-bees left the field, but as soon as the sun broke out again they returned to the feast.
Experiments in illuminating plants with electric lights show that the rate of growth increases with the intensity of the illumination. Clover blooms much earlier in constant light than in shorter daily illumination, though this is not true of all plants. The long days of northern regions promote the manufacture of more sugar, and the same result is produced by the more intense light of alpine heights. The long warm days and cool nights of the north are particularly favorable to bee culture; for example, at Roberval, on Lake St. John, Quebec, an average annual colony yield of 200 pounds is obtained from alsike and white clover. Light is also a direct stimulus to the growth of flowers. In the shade, the colors of many flowers are paler than in the light. The flowers of the chickweed will not open except in light, and some other flowers develop imperfectly in darkness. But too intense light is injurious. Leaves commonly arrange themselves in such a way that they will receive as much light as possible, but if the light is too intense they change their position, so that a smaller amount strikes their surface.
THE INFLUENCE OF ALTITUDE
Flowers at high altitudes have brighter colors and tend to secrete more nectar than in the lowlands. Mueller states that in the lowlands the spur of an orchid (Platanthera solstitialis) was only about a third filled with nectar, but that in the Alps it was over half full. According to Bonnier and Flahault, Silene inflata, one of the pink family, was much richer in nectar at an elevation of 5895 feet than at an elevation of 1300 feet. They also cite statistics gathered by the French Department of the Pyrenees, from which it would appear that the average honey production per colony was, from sea-level to 1000 feet, 6 pounds 10 ounces, and that with every rise of 1000 feet it increased from 2 to 4 pounds per colony, until at 4000 to 5000 feet it was 19 pounds, 13 ounces. According to Richter, “There is a great deal of evidence pointing to a more profuse nectar secretion of alfalfa at higher elevations.”
The increase in nectar secretion is apparently due to the more intense light and the wide range of temperature between day and night. At an altitude of 8510 feet, the chemical activity of the sun’s rays is 11 per cent, greater than at sea level. The greater light intensity drives the machinery of the leaf more rapidly, and more sugar is made. It has already been shown that the alternation of warm days and cold nights promotes nectar secretion. In the Alps and in all mountainous regions the nights are much colder than the days. As we ascend a mountain the air grows thinner, or less dense, and at a height of 3.5 miles the pressure per square inch is only one-half as much as on the earth’s surface. It also contains much less water vapor. Water vapor acts as a great blanket to the earth’s surface and vegetation. If the amount of vapor is small, the temperature will fall rapidly. Hence the cold