Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/106

96 of leaves and branches, nodes, or points whence leaf-buds are produced, being placed at regular intervals. No such provision occurs in roots. The intervals between nodes are called internodes. The stem, although it has a tendency to rise upwards when first developed, in many instances becomes prostrate, and either lies along the ground partially covered by the soil, or runs completely underneath its surface, giving off roots from one side and buds from the other. Some stems are therefore subterranean, and are distinguished from roots by the provision made for regular leaf-buds. The first rudiment of the young stem in the embryo appears outside the seed or spore after the radicle has been protruded. It is termed the plumule (fig. 58), and differs from the radicle in the absence of a root-cap and in its tendency to ascend. The apical growing portion of the young stem constitutes the terminal bud of the plant, and by its development the stem increases in height ; but in addition there is a provision for the production of lateral buds, which develop into lateral shoots more or less resembling the parent stem, and by these the branching of the plant is determined. These buds are found in the axil of previously-formed leaves; or, in other words, in the angle formed between the stem and leaf. They are hence called axillary. They are produced always from the outer portion of the stem except in the case of Equisetaceae (Horsetails), where they have a deep-seated origin. At first they consist entirely of cellular tissue, but in the progress of growth vas cular bundles are formed in them continuous with those of the stem, and ultimately branches are produced, which in every respect resemble the axis whence the buds first sprang. As the axis of the bud increases in length, cellular projections appear at regular intervals upon the primary meristem, which are the rudimentary leaves. Buds, as has been stated, are either terminal or lateral. By the production of the former, stems increase in length, while the latter give rise to branches (rami), from which others, called branchlets or twigs (ramidi), arise, and add to the diameter of the stem. 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 formation of numerous lateral buds, any injury done to a few branches is easily repaired ; but in Palms, which only form terminal buds, and have no provision for a lateral formation of them, an injury inflicted on the terminal bud is more likely to have a prejudicial effect on the future life of the plant. In the trees of temperate and cold climates the buds which are developed during one season lie dormant during the winter, ready to burst out under the genial warmth of spring. They are generally protected by external modified leaves in the form of scales (tegmenta or peruke), 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. Linuasus called leaf-buds hibernacula, or the winter quarters of the young branch. In some plants, as in Platanus, the buds destined to live through the winter are so completely surrounded by the base of the petiole as not to be visible until the leaf has fallen off. These are said to be intrapetiolar.

In the bud of a common tree, as the Sycamore (fig. G3), e is seen the cicatrix left by the leaf of the previous year c, with the pulvinus or swelling p, then the scales e, e, arranged alternately in a spiral, and overlying each other in what is called an imbricated manner. On making a transverse section of the bud (fig. 64), the overlying scales e, e, e, e, are distinctly seen surrounding the leaves /, which are plaited or folded round the axis or growing-point. 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 Cycas, Papaw-tree, Palms, and Tree-ferns, growth by terminal buds is well seen. In these plants the elongation of the stem is generally regular and uniform, so that the age of the plant may be estimated by its height ; owing to this mode of growth they do not attain a great diameter. Although provision is made for the regular formation of buds, there are often great irregularities in consequence of many being abortive, or remaining in a dormant state. Such buds are called latent, and are capable of being developed in cases where the terminal bud, or any of the branches, have been injured or destroyed. In some instances, as in Firs, the latent buds follow a regular system of alternation ; and in plants with opposite leaves, it frequently happens that the bud in the axil of one of the leaves only is developed, and the different buds so produced are situated alternately on opposite sides of the stem. Occasionally, after a partial development as branches, buds are arrested and form knots or nodules. The embryo-buds or nodules of the Beech, Cedar, and Olive are apparently of this nature. Fig. 64. FIG. G3. Leaf-bud of Sycamore (Acer Pseudo-platawu) covered with scales FIG. C4. Transverse section of the same leaf-bud. 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 con tinually cutting off the terminal buds, a woody plant is made to assume a bushy appearance, and thus pollard trees are produced. Pruning has the effect of checking the growth of terminal buds, and of causing lateral ones to push forth. The peculiar bird-nest appearance, often presented by the branches of the common Birch, depends on an arrestment in the terminal buds, a shortening of the internodes, and a consequent clustering or fasciculation of the twigs. In some plants there is a natural arrestment of the main axis after a certain time, giving rise to peculiar shortened steins. Thus the crown of the root is a stem of this nature, forming buds and roots. Such is also the case in the stem of Cyclamen, Testudinaria Elephantipes, and in the tuber of the Potato. The production of lateral in place of terminal buds sometimes gives the stem a remark able zigzag aspect. Branches are sometimes arrested in their progress at an early stage of their development, and do not appear beyond the surface of the stem ; at other times, after having grown to a considerable- size, they undergo decay. In both instances the lower part of the 