Page:Collier's New Encyclopedia v. 07.djvu/334

PLANE TREE 268 PLANT BREEDING PLANE TREE, any species of the genus Platanus of which five or six exist. They are tall trees with ponderous trunks, the bark of which peels off an- nually, leaving the surface smooth and bare. The Oriental plane tree, P- orientalis, an umbrageous tree, 70 to 90 feet high, has palmate leaves like those of the sycamore. It is a native of -west- ern Asia and Cashmere. Its smooth- grained wood is used in the East for cabinet making. In India its bruised leaves are applied to the eyes in ophthal- mia, and its bark, boiled in vinegar, given in diarrhoea. The Occidental or American plane tree, P. occidentalis, has less deeply divided and indented leaves, and no membranous bracts along the female flowers. On the banks of the Ohio and the Mississippi there are trees 10 to 16 feet in diameter. Called, also, in the United States, buttonwood and water beech, and sycamore, and _ in Canada, cotton tree. A third species, often confounded with this one, ^ is the maple-leaved plane, P. acerifolm, the species sometimes with giant trunk, cul- tivated in some London squares. The Scotch or mock plane tree is Acer pseudo-platanus. PLANET, a heavenly body which, to old-world observers, seemed to wander about aimlessly in the sky, thus marked- ly contrasting with the orderly move- ments of the fixed stars. Subsequently it was discovered that the seemingly er- ratic bodies were as regular in their movements as the others, revolving, like the earth, around the sun. Planets are primary or secondary, the former re- volving around the sun, the latter around the primaries. The primary planets knovpn to the ancients were five: Mer- cury, Venus, Mars, Jupiter, and Saturn. Omitting asteroids, comets, and meteoric rings, eight are now known. Mercury, Venus, the Earth, Mars, Jupiter, Satuni, Uranus, and Neptune. Twenty-one sec- ondary planets are known, the Moon, two satellites of Mars, five of Jupiter, eight of Saturn, four of Uranus, and one of Neptune. The planets Mercury and Venus, being nearer than the Earth to the Sun, are called inferior planets; the others, being more distant, are termed superior. Another classification is sometimes adopted, that into intra- and extra-asteroidal planets; that is, those nearer and those more remote from the sun than the asteroids. Under the first are included, Mercury, Venus, the Earth, and Mars, all of which are comparatively small, while the others, Jupiter, Saturn, Uranus, and Neptune, are the giants of the system. For in- stance, the Earth is 7,918 miles in diame- ter, and Mars 4,200, but Jupiter is 8.5,- 000. The intra-asteroidal planets com- plete the annual revolution in short periods, the Earth, for example, in 365.26 days, while Neptune takes to do so 60,127 days, or about 165 years. The minor planets, planetoids, or asteroids are between Mars and Jupiter. PLANETOIDS, the name given to a great group of minute planets placed to- gether between Mars and Jupiter. Professor Titius, of Wittenberg, having drawn attention in 1772 to the fact that, with the exception of Jupiter, each planet has an orbit just about double that near- est to it on the side of the sun, Professor Bode, of Berlin, drew the natural infer- ence that the one exception to the rule would probably be removed by the dis- covery of a planet less remote from the sun than Jupiter, and more distant than Mars. On Jan. 1, 1801, a planetary body, afterward called Ceres, was found by Piazzi in the part of the solar system theoretically indicated; it was, however, far more diminutive in size than had been expected. Within the next six years three more asteroids (Pallas, Juno, and Vesta) were found in proximity to Ceres. Up to October, 1903, 542 small planets had been discovered, 70 by Americans. All are of minute size, and some angular in place of spherical. The term asteroid, applied to these small bodies, is now becoming obsolete, the appellation minor planets taking its place. They are sometimes also called extra-zodiacal planets, from their orbits stretching outside the zodiac, which is not the case with those of the normal type. Authorities differ respecting some minute points in the list of asteroids. Melete, when discovered on Sept. 9, 1857, was mistaken for Daphne, an error not detected till January, 1859. Herschel, Proctor, etc., number it 56, and place the date 1857, where it offends the eye, in 1859; others, with Mr. G. F. Chambers, transfer it to 1857, which alters the numbering of all the minor planets from 47 to 56. There are other minute differ- ences between lists of asteroids by lead- ing authorities. PLANT BREEDING, the science of producing new or improved species of plants by a process of crossing or selec- tion. The tendency of all plants is to reproduce plants possessing all their own characteristics, but changes and improve- ments are brought about by various methods. The simplest method of the selection process is to save the seeds of only those plants which possess in a marked degree the desired characteristics. This process is usually slow and uncertain,