The terminal portion of a branch of a woody plant may be referred to as a branchlet or twig (Figs. 6, 7). In this work these terms are used to designate, specifically, the growth of the current year and, to a lesser extent, that of the last preceding year. Twigs bear prominent distinguishing features such as buds, leaf-scars, stipule-scars, and pith, while their color, taste, and odor may also be distinctive. Various other features, such as corky ridges, thorns, and pubescence characterize certain species.
Color and hairiness of twigs are important winter characters, although they might more properly be regarded as physiological effects rather than physical features.
During the winter the color tends to become darker on the side most exposed to the sun (the upper, or southern sides). Twigs which are green in summer may become reddish in winter through the formation of anthocyanins, favored by cold weather. Thereafter the color deepens each year, for two or three or even four years. Hairs also change somewhat in color, as well as in abundance as the winter season progresses. Twigs become less and less hairy in the second and later years, under ordinary conditions. Of course some are glabrous from the beginning.
Buds. A bud, literally, is a growing point, the early undeveloped stage of a leafy shoot or a flower. In winter, of course, the growing points are dormant, and are usually covered, for protection, by bud scales, which are really modified leaves.
Buds are of two types, the terminal buds at the tips of the stem and its branches, and the lateral buds, along the sides of the branchlets (Fig. 6). In some species a true terminal bud is not formed and growth continues throughout the season until stopped by unfavorable weather. When this has happened, the young tender tip of the shoot dies back and is finally self-pruned at the highest mature lateral bud formed. This bud then appears to be terminal and is referred to as a pseudoterminal bud. Usually it continues the growth of the shoot in the next season, in much the same manner as an actual terminal bud would have done. A pseudoterminal bud may be distinguished from a true terminal by the fact that it usually has a leaf-scar immediately below it, and a small twig-scar behind it. The twig-scar, as might be expected, shows the concentric zones of bark, wood, and pith, characteristic of most woody stems. Rarely a withered twig tip persists and no clear-cut twig scar is formed.
Fig. 6 TWIG OF BUTTERNUT.
Fig. 7 TWIG OF RED MAPLE.
Lateral buds are called axillary buds if they arise in the axils of leaves, as they usually do. The axil is the distal angle formed by the petiole of the leaf with the shoot. Often more than one bud appears at a node, in which case the one directly above the leaf scar is considered the true axillary bud and the others are designated as accessory buds. Accessory buds produced to the right or left of the axillary bud are said to be collateral (Fig. 7), while those produced just above the axillary bud are said to be superposed (Fig. 6). Often the accessory buds may be flower buds, whereas the axillary bud might be a leaf bud. In other cases both flowers and leaves are borne together in mixed buds. Buds differ greatly in their size and shape, as well as in the number, arrangement, color, size, shape, and surface nature of the bud scales; all these are valuable taxonomic features. When the scales of a bud fall as spring growth begins, they leave on the twig a ring of bud scale scars (Fig. 6). A series of such scars indicates several years' growth.
Bud scales, as noted above, are actually modified leaves (or, rarely, stipules), and serve to protect the enclosed embryonic structures. In some buds the scales may be rather numerous, overlapping each other like the shingles of a house; such an arrangement is said to be imbricate. In other cases the scales (generally 2 in number) do not overlap but fit together edge to edge; these are valvate scales. Of course, in both imbricate and valvate buds, the exposed scales may not represent the total number; others maybe concealed, becoming exposed only in spring, when the bud enlarges as growth begins. In a few plants, as willows, the buds are covered by a single scale. Not all buds are scaly; some lack scales and are referred to as naked buds. These, however, have the actual growing point protected by less strongly modified, rudimentary leaves which often show veins and are generally scurfy or pubescent.
Leaf scars. Most woody plants of the northeastern United States are deciduous, i. e., the leaves fail as the growing season comes to a close. The fall of the leaf is associated with the development of a corky abscission layer and after the leaf has fallen there remains at the point of its attachment a portion of this layer, known as the leaf scar, sealing off the living tissues beneath (Fig. 8). Since the petioles vary greatly in appearance in cross section, the leaf scars are also quite variable, and are of further taxonomic value because of the variation in number and arrangement of the bundle scars (or traces), which indicate the broken ends of the vascular bundles passing from the stem into leaves. Common numbers of traces are one or three (Figs. 9,10,11,12), although the usual number for a given species may be increased through compounding or branching of the bundles before they reach the abscission layer. Tiny stipule scars may also be present, one on each side of the leaf scars, marking the points where the stipules were attached (Fig. 15), or the modified stipules themselves may be present in the form of paired prickles (black locust, Fig. 17) bud scales (magnolia), or tendrils (greenbrier). Of course, stipules or stipule scars are not found on all twigs, since some species of plants are exstipulate (without stipules). The scars, when present, are generally slit-like in shape and inconspicuous. In a few species, as sycamore and tuliptree, (Fig. 16), they encircle the twig from one edge of the leaf scar to the other.
Branch and Fruit Scars. As noted above, some species do not form a true terminal bud, but the withered tip of the shoot may slough off, leaving a branch scar. In a few species, e. g. buffalo-nut, short lateral branches bearing several leaves may drop at the close of the season; in such species branch scars may sometimes be more numerous than leaf scars.
Fruit scars are similar in appearance to branch scars, but are often found in a terminal position (Fig. 13). They represent the point of abscission of the shoot bearing the inflorescence and the fruits. In species which normally have a true terminal bud, as buckeye, the presence of fruit scars may be mystifying until their real nature is learned.
Pith. The central portion of a twig is composed of a cylinder of parenchyma cells called pith. It is usually a different color from the xylem (wood) surrounding it and is readily recognizable in transverse or longitudinal sections of twigs. In most species the pith is circular in cross section but it may be star-shaped (oaks), 5-sided (cottonwoods), or more or less triangular (alders). In color it is usually white but may be various shades of pink, yellow, brown, or green.
Usually the pith is continuous and homogeneous in composition (Fig. 18). A modification of this type is diaphragmed, having plates of heavier-walled horizontally-elongated cells at more or less regularly spaced intervals (Fig. 19). Examples are tuliptree and black gum. In a few species the pith disappears between the diaphragms, resulting in small empty spaces; such pith is called chambered (Fig. 20). Pith may also be spongy, i. e., filled with small irregular cavities, or partially or entirely excavated, i.e., "dug out" or lacking.
Fig. 18 DIAPHRAGMED PITH.
Fig. 19 CHAMBERED PITH.
Studies of the pith may be facilitated by application of a small drop of phloroglucin, followed by a drop of hydrochloric acid. This results in the wood turning a bright red in color, presenting the outline of the pith in sharp focus.
Lenticels. Lenticels are small, often wart-like prominences scattered over the surface of twigs; they serve to admit air to the living tissues beneath (Figs. 6, 7). They may be circular in shape or quite irregular. In cherries and birches they are elongated horizontally. In some cases they are relatively conspicuous, as in elder, in other cases quite inconspicuous. In general they are of little value in identification of twigs.
Spur Shoots. In some species (e.g.,larch, birch) certain twigs grow very slowly and appear as dwarf branches, even though they may bear a normal number of leaves. A spur shoot is short, usually stocky, and with crowded leaf scars. Often the flower buds may be produced on spurs, as in apple and pear (Fig. 14).
Prickles and thorns. Small spines or prickles occur on the twigs of numerous species of woody plants and they are often diagnostic in character. In some cases they represent modified leaves, as in barberry. In other cases they are modified stipules, as in black locust (Fig. 17), while in still other cases they are cortical emergences (outgrowths of the cortex), scattered over the surface of the twig, as in rose and gooseberry.
Thorns, on the other hand, are modified sharp-pointed twigs, and have the vascular bundles characteristic of other twigs. In some cases careful examination will reveal the presence of tiny buds and leaf scars. They may be branched, as in honey-locust, or unbranched, as in hawthorn, In some instances (e.g., crabapple), a twig may be sharp-pointed, without actually appearing to be a thorn and might represent a structure from the evolutionary standpoint on the way to becoming a thorn.