Heliotropism

( Bot.) The tendency of plants to turn toward the sun. If a seedling plant be placed in a gla ve el filled with water in a window, the stem and leaves may be seen gradually to bend towards the outer light, while the roots turn in the opposite direction. Nearly all plants manifest this tendency in the shoots and leaves, a familiar example in a floral organ being the tendency of a sunflower to turn toward the sun. Study of this phenomenon has shown that the growth of the side next to the sun is retarded and that of the opposite side increased, the one side becoming concave, and the other convex, thus causing a curvature toward the light. It is found that the cells on the concave side become le turgid, and those on the opposite side more so, thus forcing the part to bend. The cause of this condition of the cells is unknown. He'liotype, п. A method of printing on an ordinary lithographic pre , from a gelatine surface hardened with alum, to which pictures from photographic negatives have been transferred. See ENGRAVING, PHOTO-. Heliozo/a, n. ( Biol . ) A cla of Protozoa, familiarly known as "sun animalcules," and belonging to the rhizopod type, or that of those which move by protruding proce es of living matter. These proce es differ from those of the Amœbæ in being slender and radiant; and in other particulars from the For a men if era. The Heliozoon cell is globular in form, with one or more nuclei and vacuoles, and is generally provided with an outer skeleton, gelatinous or silicious in substance. In the latter case it is sometimes composed of loose spicules, through which the proce es extend outward. Reproduction is achieved by the division of the cell into two, by budding off particles, or by the formation of interior spores, the young being occasionally provided with flagellate or whip-like cilia, and being active in motion, in contrast to the sluggish character of the adults. Η . dwell usually in fresh water, but are in some instances marine. They are microscopic in dimensions.