old ecclesiastical chant, when in its most simple state, and without those harmonic appendages with which it has long since been enriched by cultivated science.

PLANE, INCLINED. (See Mechanics.)
PLANETARIUM. (See Orrery.)

PLANE TREE. The Occidental plane, or buttonwood (platanus Occidentalis), is, among deciduous trees, the largest production of the American forest. It abounds most and attains the largest size along the interior waters of Pennsylvania and Virginia, and especially along the banks of the Ohio. Here stocks are sometimes found from ten to fourteen feet in diameter, often beginning only to give out their vast branches at the height of sixty or seventy feet, and near the summits of the surrounding trees. At other times, this tree divides at the base into several huge trunks, equally surpassing its neighbors in bulk. It does not appear to exist north of latitude 45°, and is inferior in size for some distance south of this point; neither is it abundant in the lower parts of the Southern States. A moist and cool soil seems indispensable, for it is never found on dry grounds. In the Western States, this tree is usually known by the name of sycamore, and in some districts is called cotton-tree. The trunk and branches are covered with a smooth, pale-green bark, the epidermis of which detaches itself in portions; the roots, when first taken from the earth, are of a beautiful red color, which disappears on exposure to light in a dry place; the leaves are alternate, palmated, or lobed; and the flowers are united in little globular, pendent balls. The wood, in seasoning, takes a dull red color, is fine-grained, and susceptible of a good polish, but speedily decays on exposure to the weather. When thoroughly seasoned, it may be used in the interior of houses, but the defect of warping is attributed to it, and cabinet-makers rarely employ it except for bedsteads, which, when coated with varnish, retain their color.-The Oriental plane, so celebrated by the ancients for the majesty of its appearance, resembles the preceding in every respect, and bears the same relation to the forests of Western Asia. The wood, in those regions where it abounds, is frequently employed in the arts, and is said to acquire great hardness by being kept under water for some years, an experiment which would be worth repeating on our own species.

PLANETS (from #davaw, to wander); moving stars, which shine by reflecting the light of the sun, around which they re

volve. Homer and Hesiod were already acquainted with Venus, but considered the morning and evening stars as two different bodies. Democritus supposed that there were several planets. Pythagoras discovered the identity of the morning and evening stars; and, in the fourth year before Christ, Eudoxus brought the knowledge of the motions of the five planets then known, from the Egyptians to the Greeks. In addition to these five planets, Mercury, Venus, Mars, Jupiter and Saturn, five others have been discovered in modern times: Herschel (Georgium Sidus, or Uranus), Ceres, Pallas, Juno, and Vesta; so that, including the earth and moon, there are now known eleven primary and eighteen secondary planets (satellites, or moons). Like the earth, many of them, if not all, have the motion of rotation on their axis, whence arise day and night, and a common motion around the sun, around which they revolve from west to east, through south, in elliptical orbits, generally making a small angle with the ecliptic, in different times, depending on their distances from the sun. The planet nearest the sun is Mercury, though thirtyseven millions of miles distant from it. It completes its revolution around the sun in eighty-eight days, moving with a velocity of 315 miles a second. It is the smallest of the six old planets, its bulk being only one eighteenth of that of the earth. Its time of rotation on its axis is twenty-four hours five and a half minutes, and its eccentricity is much greater than that of either of the other five old planets, or of Uranus. Next to Mercury is placed Venus, at a distance of sixty-eight millions of miles from the sun, around which it revolves in 225 days, having a mean velocity of 21 miles a second. It turns on its axis in twenty-three hours twenty-one minutes, as is known from observation of the spots on its surface. Mountains have also been observed in it, the height of some of which is computed to exceed eighteen miles. Seen from the earth, Venus and Mercury exhibit phases similar to those of the moon, sometimes appearing nearly full, sometimes half illuminated, or in the form of a crescent, and sometimes becoming invisible by turning to us the dark side. In size, Venus is nearly equal to the earth, and in her perigee approaches it within 27,600,000 miles, though in her apogee, she may recede 165,600,000 miles from it. We have no certain knowledge of a moon belonging to Venus; the supposed discovery of one seems to have been founded on a mistake. Mercury and Ve

nus appear, at times, like black spots passing across the face of the sun, whenever, in their motion in their orbits, like the moon in solar eclipses, they enter the plane of the ecliptic within a few hours of their inferior conjunction. This phenomenon is called a transit of Mercury or Venus. A transit of the latter planet is of rare occurrence, two only taking place in about 120 years. Those of Mercury are much more frequent. The next transit of Venus will take place in 1874; the next of Mercury, May 5 of the present year, 1832, and May 7, 1835, both of which will be visible in the U. States. These two planets, which are nearer to the sun than the earth, are called the inferior planets, and those more distant are called the superior. Next in order to the earth (q. v.) and its moon (q. v.) is Mars, 143 million miles distant from the sun. In its orbit, which it accomplishes in one year and 322 days, it moves with a velocity of fifteen miles a second. It is flattened at the poles about one sixteenth of its diameter, and turns once in twenty-four hours thirty-nine minutes on its axis, which is inclined to the plane of its orbit at an angle of sixty-one degrees. The surface of Mars is about one fourth that of the earth, and, his density being less, the quantity of matter is only one seventh. Spots and belts are often observed on Mars; from which it is conjectured that it has a dense atmosphere. Between Mars and Jupiter there is a great distance, which led to the supposition that there was some body between them; and this conjecture was verified, in the beginning of this century, by the discovery of four new planets. January 1, 1801, Piazzi (q. v.), at Palerino, discovered Ceres, which, at a distance of 263 million miles from the sun, completes its revolution in four years seven months, moving with a mean velocity of 114 miles a second. On account of its small size, it is not visible to the naked eye, and, viewed through a telescope, has the appearance of a star of the seventh magnitude. This discovery was followed, March 28, 1802, by that of Pallas by Olbers, at Bremen. It is about the same distance from the sun, and accomplishes its revolution in about the same time as Ceres. It is supposed to be rather larger than either Vesta, Juno or Ceres. This planet is distinguished from every other by the great inclination of its orbit to the ecliptic. Juno, which revolves around the sun in four years and four months, commonly appears like a star of the eighth magnitude, and was discovered September 1, 1804, by Harding, at

Lilienthal. Finally, March 29, 1807, Olbers discovered Vesta, which appears of the fifth to the seventh magnitude, is 225 million of miles from the sun, and completes its revolution around the sun in three years and eight months. Jupiter, the largest of the known planets, at a distance of 490 million miles from the sun, accomplishes its revolution, at the rate of seven miles a second, in eleven years and 314 days, and is attended by four moons, which were discovered by Galilei, at Florence, January 7, 1610, and the largest of which has a diameter nearly equal to the semi-diameter of the earth. The diameter of Jupiter itself is 11 times greater than the diameter of the earth; its surface is 118 times, and its bulk 1281 times greater than that of the earth. In nine hours fifty-six minutes it revolves on its axis, which is inclined at an angle of eightyseven degrees to its orbit, and at the poles it is flattened one fourteenth of its diameter. On the surface of this planet belts parallel to the equator are usually observed. At nearly twice the distance of Jupiter, or 900 million miles from the sun, Saturn passes through its orbit, 5760 million miles in length, in twenty-nine years and 169 days, accompanied by seven moons (of which five were discovered in the seventeenth century by Huygens and Cassini, two in 1789 by Herschel), and by a very remarkable double ring, which is 21,000 miles from the surface of the planet, and 27,000 miles in breadth; and the interval between them is about 3000 miles. According to Herschel, this ring completes its rotation in ten hours thirty minutes, while that of the planet itself is ten hours eighteen minutes. Finally, the knowledge of our solar system was enlarged, March 13, 1781, by Herschel's discovery of the Georgium Sidus (Herschel, Uranus), which is 1800 million miles distant from the sun, and, accompanied by six satellites, accomplishes its revolution in eighty-four years nine days, at the rate of about four miles a second. Its surface is nineteen times larger than the earth's, but so much less solid, that its quantity of matter is only 77} times greater. To render the vast distances from the planets to the sun more comprehensible, an illustration, addressed to the senses, is often drawn from the velocity of a cannon ball, moving at the rate of eight miles a minute. With this velocity a cannon ball would go from the sun to Mercury in nine and a half years, to Venus in eighteen, to the earth in twentyfive, to Mars in thirty-eight, to Vesta in sixty, to Juno in sixty-six, to Ceres and

is particularly observable in the corolla, but also, in a lower degree, in the leaves. The disposition of plants to turn towards the light is easily seen in such as have light from one side ouly, as all the stalks, branches, leaves and blossoms turn in that direction. Another important point in the physiology of plants is their breathing. This consists in an absorption and exhalation, especially observed in the case of the leaves. If a fresh leaf is put in a tumbler filled with spring-water, and exposed to the rays of the sun, it soon appears covered with small air-bubbles, which, by degrees, rise to the surface of the water, where they burst. If they are caught, it is found that they contain oxygen. The light of the sun is necessary to this phenomenon; mere heat is insufficient to produce it. Experiments respecting the breathing of plants have led to very different opinions. Ingenhouss thinks that plants exhale oxygen only in the light of the sun, but during the night azote and carbonic acid gas. According to Senebier, healthy plants and their leaves do not exhale any air whatever during the night; the same was maintained by Spallanzani. Ackermann, on the other hand, maintains that plants, like animals, must continually inhale the basis of vital air (oxygen), and exhale carbonic acid. But plants exhale not only gaseous matter; fluids are evap orated from them, the amount of which is considerable. It is asserted that a tree of middling size evaporates daily about thirty pounds of moisture.-As to the odor of plants, the recent progress of chemistry shows that the basis of it does not (as might have been supposed of so fleeting, diffusible, almost imponderable, entirely invisible a substance, affecting only the olfactory nerves) consist of a gaseous matter. Fourcroy showed that there does not exist a separate principle of scent. This property is as essential to bodies as gravity, but is proportionate to their volatility: the most volatile bodies have the strongest odor. The taste of plants seems to depend on the proportious of their elementary ingredients, and on the degree of heat to which the plant is exposed. The rays of the sun, also, have a powerful influence on it. Of the colors of plants the same is true that has been said of their scent. Even Aristotle observed that plants are colored by the sun. Ray, Bonnet, Senebier, and others, made various experiments connected with this point. Senebier found that when plants were put in a dark place, their green leaves become first yellow on the surface and then white;

whilst young plants whica nad grown up in the dark, when brought by him gradually to the light, exchanged their white color for yellow, which, after a while, became darker, and showed by degrees green spots, continually increasing in number and size, so that, after some time, the parts before white acquired a perfectly green color. With blossoms raised in the dark the change of color is but slight. Bonnet asserts the cooperation of heat in this process; but, according to the experiments of Van Mons and Vasalli, the light of lamps and of the moon operates in the same way. The cause of this remarkable phenomenon is at present known. Plants become lighter in consequence of combination with the oxygen which they inhale, darker if they lose it. The different proportion of oxygen to its other component parts gives the various gradations and shades. Saturation with oxygen gives the yellow and white color. But if a plant saturated with oxygen is exposed to the rays of the sun, the substance of the light unites with the oxygen, the latter escapes, and the plant reassumes its green color. For the rest, the color seems to have its seat in the cellular substance; the epidermis, however, is without color. The chemical analysis of plants shows that all vegetable matter consists chiefly of hydrogen, carbon and oxygen. Their different proportions produce the variety of vegetable substances. Of these substances chemistry has distinguished gum, fecula or starch, sugar, gluten, albumen, gelatin, caoutchouc or Indian rubber, wax, fixed oil, volatile oil, camphor, resin, gum-resin, balsam, extract, tannin, acids, aroma, the bitter, the acrid and the narcotic principles, and ligneous fibre. Several of these substances are capable of transformation into each other. Thus the tasteless mucilage passes into sugar or acid. These changes are produced by heat, moisture, air, alkalies, which change more or less the proportion of the original constituents. The formation, therefore, of the various substances in vegetables is the consequence of truly chemical operations, which may be traced from the germ to the ripe fruit. To determine how the original constituents are absorbed by light and heat, and united to each other by the vegetable organization in such a manner that they produce the various substances of which plants are composed, and which again, in their last analysis, are resolved into those original constituents-this is the problem of vegetation. The way in which plants grow, i. e. in which the nutritious

parts pass into the plants, is thus stated :Water and carbon resolve themselves into their constituent parts, enter into new unions, and thus form the solid portions of plants. Hydrogen separates from the oxygen in order to unite with carbon, and thus oils, resin and the like, are forined. At the same time, oxygen is formed from the water and carbonic acid, and passes off, in union with caloric, as oxygen gas. By means of these substances, the increase of the vegetable fibres, or the proper growth, is produced, though we are not able to see clearly the way in which it is effected. As to the fructification of plants, the same general theories exist as in regard to the fructification of animals; i. e. the theory of evolution, which considers the germ of all creatures as already existing, and only waiting for the process which is to call them into life, and the more philosophical theory of actual generation by a wonderful cooperation in the two sexes. This process in plants takes place in the following way, very similar to that in the case of animals :-Plants have male and female organs of generation, which may be observed by the naked eye; yet these parts are generally not permanent, as in the case of animals, but change after fructification has taken place. The pollen or farina is prepared and preserved in certain vessels destined for this purpose, called anthers. Its finest part penetrates through the stigma, an opening in the female part, through the pistil to the ovary,and fructifies the germs or ovules lying there. With most plants both sexes are united in one flower; with a few they are separated. The former are called perfect flowers, the latter male or female. The two latter either stand on one stem or belong to different plants. With the (so called) perfect flowers fructification is effected most easily; and also, where the same stem has male and female blossoms, no particular difficulty exists; but where the two sexes are entirely separated, fructification takes place only when the two plants of different sexes stand near enough for the pollen of the male plant to be carried to the female by the wind or by insects. If this or an artificial fructification does not take place, the germ either falls off, or it forms a fruit, which, however, is incapable of germinating. Wonderful, indeed, are the ineans by which nature effects the fructification of these plants! Within the flower of the plants are generally glands, which exude a honey, by which insects are attracted; but, in order to obtain this, they must powder themselves in the male

flowers with the pollen. Visiting afterwards a female flower with the same view, they must deposit the pollen on the pistil. In some other plants, where the male and female parts in perfect flowers are placed so as not to be able to reach each other, little flies are attracted by the honey, but immediately upon their entrance the flower closes, and thus the insects, who crawl in all directions to find a way of escape, are forced to fructify it. Grasses are generally fructified by the wind. Linnæus founded his system (sexual system) on the generating organs of plants. (See the article Botany, for other systems.) He divided the whole vegetable world into twenty-four classes. The twenty-three first comprise the plants with visible blossoms, the phanerogamous. Of these, the thirteen first receive their names from the number of their stamens, or male organs of generation: their names are, 1. monandria, with one stamen; 2. diandria, with two; 3. triandria, with three; 4. tetrandria, with four; 5. pentandria, with five; 6. hexandria, with six; 7. heptandria, with seven; 8. octandria, with eight; 9. enneandria, with nine; 10. decandria, with ten; 11. dodecandria, with twelve to nineteen; 12. icosandria, with twenty; 13. polyandria, with more than twenty stamens. In all these classes, the orders, or first divisions of classes, are determined by the number of female parts of fructification; i. e. the pistils; for instance, monogynia, with one pistil; digynia, with two; trigynia, tetragynia, &c. The fourteenth and fifteenth classes are determined rather by the situation of the filaments. They are called, 14. didynamia, in whose blossoms are always four stamens, of which two are longer than the rest-hence the name; 15. tetradynamia, in whose blossoms are always six stamens, of which four have longer filaments than the others. Each of these classes contains but two orders. Those in the fourteenth are determined by the circumstance of the seed lying naked in the calyx (gymnospermia), or being covered (angiospermia). In the fifteenth class, the orders are determined by the comparative length of the pod or silique, the first being termed siliculosa, the second siliquosa. In the 16th, 17th and 18th classes, the number of bundles in which the filaments are united, determines the class; 16. monadelphia (one brotherhood), when the filaments are united in one bundle; 17. diadelphia (two brotherhoods), when they are united in two; 18. polyadelphia (many brotherhoods). The orders in these classes are determin