ARBORICULTURE 315 exposed to the sea-breeze, the common and the Norway maple may be substituted; in the warmer parts of the island, the evergreen oak (Quercus Ilex] ; and for the marshes of the warmer parts, Taxodium distichum or deciduous cypress. For moist soils which cannot be drained, the white, trembling, and Ontario poplars have the property of sending their roots along the surface of the ground. Some species of willow and some poplars will grow near water in situations where their roots can enter into it, but will not grow in itndrained soil All the known species of trees and shrubs may be successfully grown in almost any kind of soil not beyond average moisture, dryness, or tenacity. In some soils, however, they thrive better than in others, and the timber produced generally varies in quality according to the soil. Thus, a rich soil, while it contributes to the rapid growth of the coniferous tribe, renders their timber less durable ; and the same law holds, more or less, with every other species of tree. The influence of climate on trees is much greater than that of soil ; for, whilst many trees grow on any soil, every tree may be said to have its particular climate ; that is, a climate in which, the soil and other circumstances being suitable, it will produce the largest and most enduring timber. Hence, when we take the geographical range of any species, we find what may be called a central climate, where it attains its largest size ; and as it recedes from this climate, by latitude or elevation, into one either colder or hotter, it gradually diminishes in size, till it at last appears in the form of a shrub. Thus the common oak, which in Britain attains its largest size in Sussex and Hampshire, dwindles into a shrub on the mountains of Scotland and in the north of Africa ; its degeneracy being occasioned in the one case by extreme cold, and in the other by extreme heat. Even within Britain the absolute character of trees, relatively to climate, is obvious. The English or narrow-leaved elm, supposed to be a native of Asia Minor and of China, attains a large size near London, producing a great bulk of timber in a short period, and ripening its seeds; while in Scotland it is considered only an ornamental tree. The Lombardy poplar, which in the central counties of England attains 125 feet in height in fifty years, is seldom seen of timber size in Scot land. The sweet chestnut and walnut, cultivated both for fruit and timber in many parts of England, can rarely be grown with profit for either purpose north of Newcastle. Climate may be considered in regard to the average yearly and monthly temperature, and the degree of atmo spheric moisture. A high annual temperature is no proof that a climate is suitable for trees ; but a high summer temperature is suitable for many kinds, though the winter temperature may be very low. Thus the oaks and other trees of North America, attaining there a prodigious size, survive a winter as cold as that of St Petersburg, where no native oaks are found ; but North America enjoys a very high temperature during summer, which rapidly develops the foliage, and matures the young shoots, enabling them to withstand the most rigorous winter. The much larger rainfall of that portion of America, as compared with St Petersburg, has doubtless important bearings on the ques tion. In England the average temperature of the year is as great as that of the oak countries of the United States ; but summers in the former country are comparatively cold, moist, and more cloudy ; and though its winters are much milder than those of the latter, the spongy, unripened, young shoots are always more or less injured by frost. Again, in a mild climate, the trees of those countries which have a severe winter come into leaf earlier in the spring than the indigenous trees, and frosts often occurring at that season, they are liable to injury. Evergreen trees form an important division of the vegetable kingdom; and of these there are two classes, distinct relatively to climate and temperature. The first comprises the conifers, which endure a degree of cold as great as that in which any deciduous tree thrives; and the second, the broad-leaved evergreen trees, such as the holly, box, laurustinus, laurel, evergreen oak, cork-tree, and the evergreen magnolia, trees of comparatively mild climates, and always indigenous on islands or on continents at low elevations, and at no great distance from the sea : hence the large number of evergreen trees which grow well in Britain, compared with those which survive the winter in the same latitude on the Continent. A small proportion only of the trees cultivated in Britain are indigenous. Some are natives of other parts of Europe, and about two -thirds of the whole are from North America. Of these North American trees there are scarcely any worth cultivating in Britain for their timber, the summer not being sufficiently hot and light to bring the timber to maturity. The most useful trees of Britain are those which are indigenous, such as the oak, ash, broad-leaved elm, Scotch pine, &c. ; or those found in the same hemisphere and in the same parallels of latitude, such as the larch, spruce, silver fir, &c. Of all trees cultivated in Europe, the most valued for the strength and durability of its timber is the common oak ; and next perhaps to it, the larch. The trunk of the oak, when freed from the soft or outer wood, and thoroughly seasoned by exposure to the air, will last an unknown period of time in buildings and machines. The common European oak is more durable as timber than any of the American oaks, even when grown in America, unless we except the live oak (Quercus virens); and no timber equals it for ship-building, except the teak of India. The most generally useful timber grown in Britain is the Scotch fir; but as this is imported from the north of Europe, and a substitute for it from North America, it is not planted in Britain so extensively as it otherwise would be. The timber of the larch is more durable than that of the Scotch fir ; but being apt to warp, and not so easily worked, it is less convenient for house-carpentry and joinery. The timber of the common ash is valuable in the construction of agricultural instruments and machines, and it is one of the few woods which are almost as valuable when young as when mature. The wood of the broad-leaved elm is strong and durable, but that of the English and Dutch elms is less so. The wood of the poplar and of the willow, when exposed to constant atmospheric changes, speedily decays ; but when thoroughly seasoned, and kept perfectly dry, it is very durable. TREES SUITED FOE PARTICULAR PURPOSES. 1. Conifers or resinous trees; 2. Hard- wood trees and 3. Soft-wood trees. 1 . Conifers. Coniferous trees are characterised by straight erect trunks, with branches in regular tiers, which do not acquire the size of timber. They never send up shoots from the stool when cut over at the ground. The leaves are generally linear or needle-shaped, without veins, and evergreen. The seeds are produced in cones, and the whole plant is more or less resinous. They belong to the Gymnosperms, a family of the Dicotyledons. The LARCH (Larix europcea, D.C.) produces the most valuable timber, which is of great durability. Young larches, G or 8 feet high, are useful for sheep stakes, rustic palings, or dead fences. Young trees from 10 to 15 feet in height are found to form excellent hop-poles. 1 The 1 For the durability of the mature timber, both in civil and naval architecture, we may refer to the Transactions of the Highland Society of Scotland, vol. xi. p. 165, and London s Arboretum JSritannicurr^
vol. iv. p. 2387.