TWIGS 7 



nomic 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 8,nd 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 composi- 

 tion (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. 



