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Rh ground up and mixed with meal; the gum secreted by the buds was employed by the old herbalists for various medicinal purposes, but is probably nearly inert; the cotton-like down of the seed has been converted into a kind of vegetable felt, and has also been used in paper-making. A closely related form is the well-known Lombardy poplar, P. fastigiata, remarkable for its tall, cypress-like shape, caused by the nearly vertical growth of the branches. Probably a mere variety of the black poplar, its native land appears to have been Persia or some neighbouring country; it was unknown in Italy in the days of Pliny, while from remote times it has been an inhabitant of Kashmir, the Punjab, and Persia, where it is often planted along roadsides for the purpose of shade; it was probably brought from these countries to southern Europe, and derives its popular name from its abundance along the banks of the Po and other rivers of Lombardy, where it is said now to spring up naturally from seed, like the indigenous black poplar. It was introduced into France in 1749, and appears to have been grown in Germany and Britain soon after the middle of the last century, if not earlier. The Lombardy poplar is valuable chiefly as an ornamental tree, its timber being of very inferior quality; its tall, erect growth renders it useful to the landscape-gardener as a relief to the rounded forms of other trees, or in contrast to the horizontal lines of the lake or river-bank where it delights to grow. In Lombardy and France tall hedges are sometimes formed of this poplar for shelter or shade, while in the suburban parks of Britain it is serviceable as a screen for hiding buildings or other unsightly objects from view; its growth is extremely rapid, and it often attains a height of 100 ft. and upwards, while from 70 to 80 ft. is an ordinary size in favourable situations.

P. canadensis, the “ cotton-wood ” of the western prairies, and its varieties are perhaps the most useful trees of the genus, often forming almost the only arborescent vegetation on the great American plains. It is a tree of rather large growth, sometimes 100 ft. high, with rugged grey trunk 7 or 8 ft. in diameter, and with the shoots or young branches more or less angular; the glossy deltoid leaves are sharply pointed, somewhat cordate at the base, and with flattened petioles; the fertile catkins ripen about the middle of June, when their opening capsules discharge the cottony seeds which have given the tree its common western name; in New England it is sometimes called the “ river poplar." The cotton-wood timber, though soft and perishable, is of value in its prairie habitats, where it is frequently the only available wood either for carpentry or fuel; it has been planted to a considerable extent in some parts of Europe, but in England a form of this species known as P. monilifera is generally preferred from its larger and more rapid growth. In this well-known variety the young shoots are but slightly angled, and the branches in the second year become round; the deltoid short-pointed leaves are usually straight or even rounded at the base, but sometimes are slightly cordate; the capsules ripen in Britain about the middle of May. This tree is of extremely rapid growth, and has been known to attain a height of 70 ft. in sixteen years; it succeeds best in deep loamy soil, but will flourish in nearly any moist but well-drained situation. The timber is much used in some rural districts for flooring, and is durable for indoor purposes when protected from dry-rot; it has, like most poplar woods, the property of resisting fire better than other timber. The native country of this form has been much disputed; but, though still known in many British nurseries as the “ black Italian poplar,” it is now well ascertained to be an indigenous tree in many parts of Canada and the States, and is a mere variety of P. canadensis; it seems to have been first brought to England from Canada in 1772. In America it seldom attains the large size it often acquires in England, and it is there of less rapid growth than the prevailing form of the western plains; the name of “ cotton-wood " is locally given to other species. P. macrophylla or candicans, commonly known as the Ontario poplar, is remarkable for its very large heart-shaped leaves, sometimes 10 in. long; it is found in New England and the milder parts of Canada, and is frequently planted in Britain; its growth is extremely rapid in moist land; the buds are covered with a balsamic secretion. The true balsam (poplar, or tacamahac, P. balsamifera, abundant in most parts of Canada and the northern States, is a tree of rather large growth, often of somewhat fastigiate habit, with round shoots and oblong-ovate sharp-pointed leaves, the base never cordate, the petioles round, and the disk deep glossy green above but somewhat downy below. This tree, the “ liard ” of the Canadian voyageur, abounds on many of the river sides of the north-western plains; it occurs in the neighbourhood of the Great Slave Lake and along the Mackenzie River, and forms much of the driftwood of the Arctic coast. In these northern habitats it attains a large size; the wood is very soft; the buds yield a gum-like balsam, from which the common name is derived; considered valuable as an antiscorbutic, this is said also to have diuretic properties; it was formerly imported into Europe in small quantities under the name of “ baume focot," being scraped off in the spring and put into shells. This balsam gives the tree a fragrant odour when the leaves are unfolding. The tree grows well in Britain, and acquires occasionally a considerable size. Its fragrant shoots and the fine yellow green of the young leaves recommend it to the ornamental planter. It is said by Aiton to have been introduced into Britain about the end of the 17th century.

P. euphratica, believed to be the weeping willow of the Scriptures, is a large tree remarkable for the variability in the shape of its leaves, which are linear in young trees and vigorous shoots, and broad and ovate on older branches. It is a native of North Africa and Western and Central Asia, including North-West India. With the date palm it is believed to have furnished the rafters for the buildings of Nineveh.

POPLIN, or, a mixed textile fabric consisting of a silk warp with a weft of worsted yarn. As the weft is in the form of a stout cord the fabric has a ridged structure, like rep, which gives depth and softness to the lustre of the silky surface. Poplins are used for dress purposes, and for rich upholstery work. The manufacture is of French origin; but it was brought to England by the Huguenots, and has long been specially associated with Ireland. The French manufacturers distinguish between popelines unies or plain poplins and popelines à dispositions or Écossaises, equivalent to Scotch tartans, in both of which a large trade is done with the United States from Lyons.

POPOCATEPETL (Aztec popoca “ to smoke,” tepetl “ mountain ”), a dormant volcano in Mexico in lat. 18° 59' 47” N., long. 98° 33' 1” W., which with the neighboring Ixtaccihuatl (Aztec “ white woman ”) forms the south-eastern limit of the great basin known as the “ Valley of Mexico.” As it lies in the state of Puebla and is the dominating feature in the views from the city of that name, it is sometimes called the Puebla volcano. It is the second highest summit in Mexico, its shapely, snow-covered cone rising to a height of 17,876 ft., or 438 ft. short of that of Orizaba. This elevation was reported by the Mexican geological survey in 1895, and as the Mexican Geographical Society calculated the elevation at 17,888 ft., it may be accepted as nearly correct. The bulk of the mountain consists of andesite, but porphyry, obsidian, trachyte, basalt, and other similar rocks are also represented. It has a stratified cone showing a long period of activity. At the foot of the eastern slope stretches a vast lava field—the “malpays ” (malapais) of Atlachayacatl—which, according to Humboldt, lies 60 to 80 ft. above the plain and extends 18,000 ft. east to west with a breadth of 6000 ft. Its formation must be of great antiquity. The ascent of Popocatepetl is made on the north-eastern slope, where rough roads are kept open by sulphur carriers and timber cutters. Describing his ascent in 1904, Hans Gadow states that the forested region begins in the foothills a little above 8000 ft., and continues up the slope to an elevation of over 13,000 ft. On the lower slopes the forest is composed in great part of the long-leaved Pinus liophylla, accompanied by deciduous oaks and a variety of other trees and shrubs. From about 9500 ft. to 11,500 ft. the Mexican “ oyamel,” or fir (Abies religiosa) becomes the principal species, interspersed with evergreen oak, arbutus and elder. Above this belt the firs gradually disappear and are succeeded by the short-leaved Pinus montezumae, or Mexican “ ocote ”—one of the largest species of pine in the republic. These continue to the upper tree-line, accompanied by red and purple Pentstemon and light blue lupins in the open spaces, some ferns, and occasional masses of alpine flowers. Above the tree line the vegetation continues only a comparatively short distance, consisting chiefly of tussocks of coarse grass, and occasional flowering plants, the highest noted being a little Draba. At about 14,500 ft. horses are left behind, though they could be forced farther up through the loose lava and ashes. On the snow-covered cone the heat of the sun is intense, though the thermometer recorded a temperature of 34° in September. The reflection of light from the snow is blinding. The rim of the crater is reached at an elevation of about 17,500 ft. Another description places the snow-line at 14,268 ft., and the upper tree-line