100. Some tendrils are leaves or parts of leaves, as those of the Pea (Fig. [35]). The nature of the tendril is known by its position. A tendril from the axil of a leaf, like that of Passion-flowers (Fig. [92]) is of course a stem, i. e. a branch. So is one which terminates a stem, as in the Grape-Vine.
[101.] Spines or Thorns (Fig. [95, 96]) are commonly stunted and hardened branches or tips of stems or branches, as are those of Hawthorn, Honey-Locust, etc. In the Pear and Sloe all gradations occur between spines and spine-like (spinescent) branches. Spines may be reduced and indurated leaves; as in the Barberry, where their nature is revealed by their situation, underneath an axillary bud. But prickles, such as those of Blackberry and Roses, are only excrescences of the bark, and not branches.
Fig. 95. A branching thorn of Honey-Locust, being an indurated leafless branch developed from an accessory bud far above the axil: at the cut portion below, three other buds (a) are concealed under the petiole.
Fig. 96. Spine of Cockspur Thorn, developed from an axillary bud, as the leaf-scar below witnesses: an accessory leaf-bud is seen at its base.
102. Equally strange forms of stems are characteristic of the Cactus family (Fig. [111]). These may be better understood by comparison with
[103.] Subterranean Stems and Branches. These are very numerous and various; but they are commonly overlooked, or else are confounded with roots. From their situation they are out of ordinary sight; but they will well repay examination. For the vegetation that is carried on under ground is hardly less varied or important than that above ground. All their forms may be referred to four principal kinds: namely, the Rhizoma (Rhizome) or Rootstock, the Tuber, the Corm or solid bulb, and the true Bulb.
