DE VRIES, HUGO, a Dutch botanist, born in 1848 at Harlem. He was edu cated at Leyden and German universi ties, and in 1871 joined the staff of the University of Amsterdam as a lecturer. He afterward became professor of botany at that institution. He devoted special attention to the development of the theory of mutation and made important contributions to that branch of evolution ary science. His researches resulted in a change of the method of studying evolu tion from observation to experimental work. He wrote several books, including "Plant Breeding" (1907).
DEW, a deposition of water from the atmosphere on the surface of the earth in the form of minute globules. During the day the earth both absorbs and emits heat, but after sunset its supply of warmth is cut off, while it still continues to radiate heat into the surrounding space. Grass, flowers, and foliage being good radiators, lose after sunset the heat which has previously been absorbed by them, without receiving any in return, and their temperature consequently falls considerably below that of the atmos phere. From the proximity of these cold substances the particles of vapor in the adjoining air are condensed and deposited on their surfaces in the form of dew, or of hoar-frost where the temperature of the earth is below 32°. When the sky
is clouded the heat abstracted from the earth's surface by radiation is restored by the clouds, which, being good radia tors, send back an amount of heat equal to what they receive; and a balance of temperature being thus maintained be tween the earth and the surrounding at mosphere, no dew is formed.
Horizontal surfaces, and those which are exposed to a wide expanse of sky, re ceive a greater supply of dew than shel tered or oblique surfaces, where circum stances diminish the amount of radia tion. The radiation from the earth's sur face is one of these happy provisions for the necessities of living beings with which nature everywhere abounds. The heavy dews which fall in tropical regions are in the highest degree beneficial to vegetation, which, but for this supply of moisture, would, in countries where scarcely any rain falls for months, be soon scorched and withered. In cold climates the earth, being cold and suf ficiently moist, requires little dew; ac cordingly the clouds, which are so com mon in damp and chilly regions, prevent the radiation of heat; the surface is thus preserved warm, and the deposition of dew is, in a great measure, prevented.