108 THE STEM. 



their tissue (chiefly in the form of starch, 81), such as the so- 

 called roots of Ginger, of the Iris or Flower-de-luce, of the Cala- 

 mus or Sweet Flag, and of the Blood-root. They grow after the 

 manner of ordinary stems, advancing from year to year by the an- 

 nual development of a bud at the apex, and emitting roots from the 

 under side or the whole surface ; thus established, the most ancient 

 portions die and decay, as corresponding additions are made to the 

 opposite growing extremity. Each year's growth is marked in the 

 rootstock of the Iris, &c., by a set of annular leaf-scars, left by the 

 decay of the foliage of that year. In the Solomon's Seal and the 

 Diphylleia (Fig. 138) it is more indelibly recorded by the series of 

 broad and rounded scars on the upper surface, not unlike the im- 

 pression of a seal (whence the popular name of Solomon's Seal), 

 which is left by the separation in autumn of the herbaceous stalk 

 of the season. The rootstock of Diphylleia is merely a string of 

 such thickened and extremely abbreviated axes, formed by the 

 annual development of a bud which, without elongation, sends up 

 at once the single herbaceous stalk that bears the foliage and flow- 

 ers. In our common Dentaria or Tooth wort, and in Hydrophyllum, 

 the base of this annual stalk or of the leaf-stalks partakes in the 

 thickening and persists as a part of the rhizoma, in the form of 

 fleshy scales or tooth-shaped processes. In other scaly rootstocks, 

 these persistent bases of the leaves are thin and more like bud- 

 scales, and slowly decay after a year or two. All such markings 

 are vestiges of leaves, &c., and indicate the nodes : they show 

 that the body that bears them belongs to the stem ; not to the root, 

 which is wholly leafless. Rootstocks branch, like other stems, by 

 the development of lateral buds from the axils of their scales or 

 leaves. Thickened rootstocks serve as a reservoir of nourishing 

 matter, for the maintenance of the annual growth, in the same 

 manner as thickened roots (128). When such subterranean stems 

 are thickened interruptedly, they produce 



175. A Tuber, This is usually formed by the enlargement of 

 the apex, or growing bud, of a subterranean branch, the elongation 

 of which is arrested, and the whole excessively thickened, by the 

 deposition of starch, &c., in its tissue. This accumulation serves 

 for the nourishment of the buds (eyes) which it involves, when 

 they develope the following year. The common Potato offers the 

 most familiar example ; and it is very evident on inspection of the 

 growing plant, that the tubers belong to branches, and not to the 



