TWIGS ^ 



minal bud may be distinguished from a true terminal by the fact 

 that it usually has a leaf-scar inamediately 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. 



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 super - 

 posed (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, arrange- 

 ment, 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, over- 

 lapping each other like the shingles of a house; such an arrange- 

 ment 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 cover- 

 ed 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 pubes- 

 cent. 



Leaf scars. Most woody plants of the northeastern United 

 States are deciduous, i. e. , the leaves fall as the growing season 

 comes to a close. The fall of the leaf is associated with the devel- 

 opment 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 taxo- 



