104 



THE STEM. 



usually shortens itself by coiling spirally, thus drawing up the 

 climbing shoot closer to the supporting object. 



186. A Spine or Thorn is an imperfectly developed, indurated, leaf- 

 less branch of a woody plant, attenuated to a point. The nature of 



spines is manifest in the Plaw- 

 thorn (Fig. 16-5), not only by 

 their position in the axil of a 

 leaf, but often by producing im- 

 perfect leaves and buds. And 

 in the Sloe, Pear, &c., many 

 of the stinted branches become 

 spinose or spinescent at the 

 apex, tapering otF gradually in- 

 to a rigid, leafless point, thus 

 exhibiting every gradation be- 

 tween a spine and an ordinary 

 branch. These spinose branch- 

 es are less liable to appear on 

 the cultivated tree, when duly 

 cared for, such branches being 

 thrown mostly into more vigor- 

 ous growth. In the Hawthorn, 

 the spines spring from the main 

 axiUary bud, while accessory 

 buds (169), one on each side, ap- 

 pear, and one or both grow the 

 next season into an ordinary 

 branch. In the Honey Locust, it is the uppermost of several ac- 

 cessory buds, placed far above the axil, that develops into the thorn 

 (Fig. 164). And here the spine itself branches, and sometimes be- 

 comes extremely compound. Sometimes the stipules of the leaves 

 develop into spines, as in the Prickly Ash. Sometimes the leaf it- 

 self is developed as a spine ; of wliich the Barberry affords a familiar 

 example. When the spine is situated in the axil of a leaf or a leaf- 

 scar, it is necessarily of the nature of a branch. Wlaen it bears a 



FIG. 164. Branching thorn of the Honey Locust (Gleditschia), an indurated branch devel- 

 oped from an accessory bud produced above the axil. o. Three buds under the base of the 

 leafstalk, brought to view in a section of the stem and leafstalk below. 



FIG. 165. Thorn of the Cockspur Thorn, developed from the central of three axillary buds ; 

 one of the lateral buds is seen at its base. 



