Page:The New International Encyclopædia 1st ed. v. 09.djvu/796

* HELIOTROPE. 736 HELIOZOA. rymbosiim is a large-trussed, large-flowered, nar- cissus-scented species. Many hybrid heliotropes are now to be seen in flower-gardens and green- houses, whieli exhibit great variety in the size HELIOTROPE. and color of their Howers. They delight in a light, rich soil. The shrubby kinds are generally propagated by cuttings. HELIOTROPE, or Bloodstone. A variety of chalcedony that is of a green color, with small spots of red jasper. It is found in Siberia, in the Hebrides on the west coast of Scotland, and in the United States — in Chatham County, Ga., Orange County, N. Y., and at various localities in Oregon and Colorado. It was known to the ancients, who valued it for gem purposes. During the early ages of the Christian Church it was used for the engraving of sacred subjects, the figures being so arranged that the red spots were made to represent drops of blood. It is still fre- quently used as a seal. HE'LIOT'ROPISM, or PHOTOT'ROPISM (from Gk. tjXios, helios, sun + Tpoiri), trope, a turning). The sensitiveness of plant organs to the direction of light-rays incident upon them. i.e. they bend so as to direct their axes toward the source of illumination (Fig. 1). Thus, when- ever plants are grown in front of a window, the stems bend toward the light. Some organs are negatively heliotropic. This is seen best in the case of certain roots (especially those of plants belonging to the mustard family) when grown in water. If such roots are laterally il- luminated after having been grown in the dark, they bend away from the light. Another mode of response to the stimulus of one-sided illumination is known as 'diaheliotro- pism.' Most leaves are diaheliotropic; they bend so as to bring their blades into a plane at right angles to the direction of the incident rays, the normally upper side of the leaf always facing the light (Fig. 2). This reaction is well shown by many house plants, such as the geranium ( Pelargonium ), when subjected to one-sided lighting. The form of heliotropic response may be very dificrent when the intensity of the il- lumination varies. Many leaves, when exposed to a light of medium intensity, are diahelio- tropic; but when the intensity of the light in- creases beyond a certain limit, they bend so as to direct their tips either toward or away from the source of the stimulating rays. They thus present less surface for the light to fall upon. O.xalis leaves show this response on bright summer days. The leaflets fold along the midrib and drop downward as though wilted. Clover (Trifolium), the sensitive plant (Mi- mosa), cassia, the bean (Phaseolus), and many a Fig. 1. POSITIVE heliotropism. PeedlinpTS of oata grown in darkness : a. two exposed to Ituht trom direction o( arrow, but with sensitive tips capped witti tin foil ; h. fonr. wliose tips were not shaded, sicetched at intervals as the bending toward the lig:ht increased. Growth is accelerated on the side less illuminated. The organ responds to lateral illumination by bending in such a way as to bring its axis into a certain fixed position with reference to the di- rection of the rays of light. The main shoots of most higher plants are positively heliotropic — Fig. 2. TRANSVERSE HELIOTROPISM OF LEAVES. Diagrams showing the plane of the leaves of tropffiolum with light successively from the direction of the arrows. others exhibit this phenomenon al.so. The final position assumed by any plant organ is never due, in nature, to the heliotropic response alone, but results from the combined action of many factors, of w-hich light and gravity are the most important. See Physiology of Plants. HEXIOZO'A (NeoLat. nom. pi., from Gk. tJXios, hcUos. sun + fijoi', zoon, animal). An order of rhizopods, comprising forms naked, or clothed with a siliceous skeleton, and with fine, more or less stiff pseudopodia radiating from all sides. Contraction vacuoles are generally pres- ent. They are fresh-water forms, and are fre- quently called 'sun-animalcules.' They are some- times supplied with a stalk, and occasionally form colonies, but many of them are free-swim- mina. They take their food into the body by means of the currents of protoplasm on the