A woody twig consists of an axis with attached leaves (Fig. 6.1). If the leaves are attached to the twig alternately or in a spiral around the stem, they are said to be alternate, or alternately arranged. If the leaves are attached in pairs, they are said to be opposite, or oppositely arranged, or if they are in whorls (groups of three or more), their arrangement is whorled. The area, or region (not structure), of a stem where a leaf or leaves are attached is called a node, and a stem region between nodes is called an internode. A leaf usually has a flattened blade, and in most cases is attached to the twig by a stalk called the petiole.
Each angle between a petiole and the stem contains a bud. The angle is called an axil, and the bud located in the axil is an axillary bud. Axillary buds may become branches, or they may contain tissues that will develop into the next season's flowers. Most buds are protected by one to several bud scales, which fall off when the bud tissue starts to grow.
There often (but not always) is a terminal bud present at the tip of each twig. A terminal bud usually resembles an axillary bud, although it is often a little larger. Unlike axillary buds, terminal buds do not become separate branches, but, instead, the meristems within them normally produce tissues that make the twig grow longer during the growing season. The bud scales of a terminal bud leave tiny scars around the twig when they fall off in the spring. Counting the number of groups of bud scale scars on a twig can tell one how old the twig is.
Sometimes other scars of different origin also occur on a twig. These scars come from a leaf that has stipules at the
base of the petiole. Stipules are paired, often somewhat leaflike, appendages that may remain throughout the life of the leaf. In some plants, they fall off as the buds expand in the spring, leaving tiny stipule scars. The stipule scars may resemble a fine line encircling the twig, or they may be very inconspicuous small scars on either side of the petiole base.
Deciduous trees and shrubs (those that lose all their leaves annually) characteristically have dormant axillary buds with leaf scars left below them after the leaves fall. Tiny bundle scars, which mark the location of the water-conducting and food-conducting tissues, are usually visible within the leaf scars. There may be one to many bundle scars present, but more often than not, there are three. The shape and size of the leaf scars and the arrangement and numbers of the bundle scars are characteristic for each species. One can often identify a woody plant in its winter condition by means of scars and buds.
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