Page:The New International Encyclopædia 1st ed. v. 16.djvu/159

PLtJCKER. 12T PLUM. PLTJCKER, pluk'er, Julius (1801-68). A Geinian mathematician and physicist, born at Elberfeld. After holding positions at the uni- versities of Halle and Berlin he became pro- fessor of mathematics at Bonn (1830). Plileker produced orks of very great importance in the field of analytic geometry. In his Analy- thch-geometrische EnlivickuUinijcn (2 vols., 1828, ls:U) he developed the dvial relation between modern analysis and pure geometry. The System (ler analytlschen Geometric (1835) deals exten- sively with the classification of cubic curves. HisTheorie der algehraischen Curven (1839) contains the famous 'six equations' relating to the singularities of higher plane curves. (See Curve.) This was followed bj- his System der Geometrie des Raiimes in neiter analytischer Be- handluiigsiceise (1840; 3d ed. 18.52). From 1847 onward Pliicker devoted his attention to physics, and he became professor of this science at Bonn. He devoted his attention especially to the magnetic properties of gases and fluids, to the electric luminosity of rarefied space, and to the phenomena of the spectroscope and the Geissler tubes. In 1805 Pliicker once more turned his at- tention to mathematics and. invented wjiat is now called line geometry. He died before he had finished the publication of a work on this subject, containing his theory of 'complexes' and 'congruences' (see Coxcruexce). but it was com- pleted by his pupil and assistant. Klein, under- the title Xeue Geometrie des Raumes geyriindet aiif die Betrachtung der geraden Linie als Rauin- element (1868-69). His scientific memoirs were published in the Proceedings of the Koyal Society of Science at Giittingen. He w'as also a contributor to the Philosophical Transac- tions of the Royal Society. xmong the papers contributed are: "On a Xew Geometry of Space" (1865, vol. 155); "On the Mag- netic Induction of Crystals" (1858, vol, 148); "On the Spectra of Ignited Gases" ( 1865, vol, 155). Consult: Clebsch, Zum Ged'dcMnis an J. Pliicker (Giittingen, 1872) ; Dronke, Julius Pliicker (Bonn, 1871). Hh (Jesaminelte Abhand- lungeii have been published in two parts by the Royal Society of GiJttingen (Leipzig, 1895-96). PLTJM: (AS. plume, plyme, OHG. pflumo, pfruma, Ger. Pfaume. from Lat. prunum, plum, pruiui. plum-tree, from Gk. irpovmv, prounon, TfxiSfivoy, pronmnon. plum, Trpoi^i^, proune, ■trpovtt.vri, proumne. plum-tree). Small trees or shrubs be- longing . to the genus Prunus. and extensively cultivated as orchard fruits in temperate regions. The fruit is distinguished from the peach, its near relative, by its smooth skin and unwrinkled stone. It is larger than the cherry and is further distinguished from it by the bloom covering it. There are three main groups of widely cultivated plums : ( 1 ) Domestica or European plum (Prunus domestica) ; (2) Japanese plum {Pru- nus trifoliata) ; and (3) native American plums, cultivated only in America, hayini; been derived largely by selection during the closing years of the nineteenth century' from niuucrous native species, the most important of which are; Americana types, from Prunus Americana, com- monly fovmd from the Atlantic west to the Rocky ^Mountains and south to the Gulf; the Wild Goose or Hortulana types, from Primus hortulana; and Chickasaw types, from Prunus angustifolia. The domestica varieties were the first plums cultivated in the United States. They thrive in the regions north of Pennsylvania and west to the Great Lakes and on the Pacific Slope, where the American prune industry has reached its greatest develop- ment. The European plum {Prunus domestica) is native to Eastern Europe and West-Central Asia. Another plum native to Southern Europe and extensively grown as a .stock is the mj-ro- bolan or cherry plum (Prunus cerasifcra) . There are but very few cultivated varieties of this species. The .Japanese plums are native to China. They stand next in commercial impor- tance to the European plums. JIany varieties ripen much earlier than the European plums. WILD PLUM {Prunus Americana). The fruit is very firm and stands shipment well, and the trees are not so susceptible to black knot and the eurculio. They were first intro- duced in America about 1870, and have rapidly become popular in the United States, where they can be grown much farther south than the Euro- pean plums. There are many varieties of most of tlie aliove species of plums now in cultivation, and in addition a large number of named hybrids. Plums true to variety are propagated usually by budding the seedlings in August or September. Whip grafting is used to some extent and has proved especially valuable in working plums on peach roots. They may also be top-worked in the same manner as apples. Plums, as a rule, do best in a deep, rather heavy soil with an open subsoil. The domestieas prefer a hea"y clay soil, while the Japanese varieties and some of the hybrids may be grown on light sandy soils. Most varieties may be set in the orchard when one year from the bud or graft, but with the Euro- pean plums two-year-old trees are preferred. Waugh and other investigators have shown that for all practical purposes native American plums and .Tapanesp varieties are sterile to their own pollen and will not produce fruit unless the trees are mixed in the orchard with other varieties. Plums are set in the orchard about 15 feet apart each way. A few varieties rei|uire 20 feet each way. Pruning the plum is confined largely to the formation of the head during the first two or three years' growth in the orchard, and after- wards to removing dead and crossed branches. The plum orchard in America is ustially plowed in early spring and kept well cultivated until the middle of summer, when it is seeded to some cover crop. In some of the Northwestern States mulching with straw, marsh hay, etc., has been found preferable to cultivation. A moderate amount of barnyard manure applied every second year has been found beneficial in the plum or-