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. [Illustration: 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.] [Illustration: 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_. [Illustration: Fig. 97. Rootstocks, or creeping subterranean branches, of the Peppermint.] 104. =The Rootstock, or Rhizoma=, in its simplest form, is merely a creeping stem or branch growing beneath the surface of the soil, or partly covered by it. Of this kind are the so-called _creeping_, _running_, or _scaly roots_, such as those by which the Mint (Fig. 97), the Couch-grass, or Quick-grass, and many other plants, spread so rapidly and widely,--"by the root," as it is said. That these are really _stems_, and not roots, is evident from the way in which they grow; from their consisting of a succession of joints; and from the leaves which they bear on each _node_, in the form of small scales, just like the lowest ones on the upright stem next the ground. They also produce buds in the axils of these scales, showing the scales to be leaves; whereas real roots bear neither leaves nor axillary buds. Placed as they are in the damp and dark soil, such stems naturally produce roots, just as the creeping stem does where it lies on the surface of the ground.