Stem

Growth and Bud Formation.

Growth in length of the stem is due to elongation of the internodes; the zone of most rapid growth is at some distance below the apex; below this the rate of growth gradually diminishes until the portion is reached where growth in length no longer takes place. In some cases, as in the stems of grasses, growth in length persists for a longer time in a small region at the base of the internodes; this is known as intercalary growth. In the dwarf or short shoots, such as those of the larch, the internodes do not elongate and the leaves remain close together. Lateral buds give rise to branches, from which others, called branchlets or twigs, arise. The terminal bud, after producing leaves, sometimes dies at the end of one season, and the whole plant, as in annuals, perishes ; or part of the axis is persistent, and remains for two or more years, each of the leaves before its decay producing a bud in its axil. This bud continues the growth in spring. In ordinary trees, in which there is provision made for the for mation of numerous lateral buds, any in jury done to a few branches is easily re paired ; but in palms, which only form terminal buds, and have no provision for a lateral formation of them, destruction of the terminal bud may kill the tree. In the trees of temperate and cold climates the buds which are developed during one sea son lie dormant during the winter, ready to open in the spring. They are generally protected by external modified leaves in the form of scales, which frequently exhibit a firmer and coarser texture than the leaves themselves. They serve a temporary purpose, and usually fall off sooner or later, after the leaves are expanded. The bud is often protected by a coating of resinous matter, as in the horse chestnut and balsam poplar, or by a thick downy covering, as in the willow.

In plants of warm climates the buds are often formed by the ordinary leaves without any protecting appendages; such buds are called naked. A bud may be removed in a young state from one plant and grafted upon another by the process of budding, so as to continue to form its different parts; and it may even be made to grow in the soil, in some instances, immediately after removal. In some trees of warm climates, as papaw, palms and tree-ferns, growth by terminal buds is well seen.

When the terminal bud is injured or arrested in its growth the elongation of the main axis stops, and the lateral branches often acquire increased activity. By continually cutting off the terminal buds or branches, a woody plant is made to assume a bushy appearance, and thus pollard trees are produced. The peculiar bird-nest appearance often pre sented by the branches of the common birch depends on an arrestment in the terminal buds, a shortening of the inter nodes, and a consequent clustering or fas ciculation of the twigs. (See GALLS.) Branching.—The mode in which branches come off from the stems gives rise to various forms of trees, as pyrami dal, spreading or weeping—the angles being more or less acute or obtuse. In the Italian poplar and cypress the branches are erect, forming acute angles with the upper part of the stem; in the oak and cedar they are spreading or patent, forming nearly a right angle ; in the weeping ash and elm they come off at an obtuse angle; while in the weeping willow and birch they are pendulous from their flexibility. The comparative length of the upper and

under branches also gives rise to differences in the contour of trees, as seen in the conical form of spruce, and the umbrella-like form of the Italian or stone pine (Pinus Pinea).

Branches are sometimes long and slender, and run along the ground, producing buds with roots and leaves at their extremity. This is seen in the runner of the strawberry. In the houseleek (Sempervivum) there is a similar prostrate branch of a shorter and thicker nature, known as an offset, producing a bud at its ex tremity capable of independent existence. In many instances the branch decays, and the young plant assumes a separate ex istence. Gardeners propagate plants by the process of layering, which consists in bending a twig, fixing the central part of it into the ground, and, after the production of roots, cutting off its con nection with the parent. A stolon differs from these in being a branch which curves towards the ground, and, on reaching a moist spot, takes root and forms an upright stem, and ultimately a separate plant. This is a sort of natural layering, and the plant producing such branches is called stoloniferous. In the rose and mint a subterranean branch arises from the stem, which runs hori zontally to a certain extent, and ultimately sends up an aerial stem, which becomes an inde pendent plant. Such branches are denominated suckers, and the gardener cuts the connection be tween the sucker and the parent stem, to propagate these plants.

In the case of asparagus and other plants which have a peren nial stem below ground, subter ranean buds are annually pro duced which appear above ground as shoots or branches covered with scales at first, and ultimately with true leaves. These branches are herbaceous and perish annually, while the true stem remains below ground ready to send up fresh shoots next season. In ba nanas and plaintains the apparent aerial stem is a shoot sent up by an underground stem, and perishes after fruiting. Branches are sometimes arrested in their development, and, in place of form ing leaves, become transformed into spines or thorns, as in the hawthorn. Plants which have spines in a wild state, as the apple and pear, often lose them when cultivated, in consequence of their being changed into branches; in some cases, as in the sloe (Prunus spinosa), a branch bears leaves at its lower portion, and terminates in a spine. In some cases branches become flat and leaf-like, taking the place in the plant economy of the leaves, which are reduced to small scales or spines, as in butcher's broom ; branches showing this modification are termed cladodes or phylloclades. In Cactaceae (e.g., Opuntia, prickly pear) and fleshy euphorbias, where the leaves are reduced to spines, the fleshy stems become green and perform the func tions of leaves ; they also serve as water reservoirs for the plants, which are natives of very dry countries.