Rhizome

Rhizome, an underground shoot, generally horizontal, often fleshy and with more or less numerous and elongated internodes. It derives its scientific name and its popular designation of rootstock from -its root-like appearance, and when the older part dies it forms what is termed a premorse root, as in the primrose and the scabious. The elongation of the rhizome may be unlimited (indefinite) or limited (definite). In the former case it continues to elongate at its apex, and bears either only foliage-leaves, as in bracken; or foliage leaves and scales in regular alternation, with annual shoots arising from their axils, as in the wood-sorrel; or only scales, with annual shoots in their axils, as in Herb Paris. The definite branching is more common, when the apex grows out into an aerial annual shoot, and one or two branches are given off from the axils of scales at its base. If the older part, persist, asympodium (q.v.) is formed, which may be either bilateral, as in ginger, or unilateral, as in Solomon's-seal. If the older part perish, as in the common buttercup (Ranunculus acris), each annual shoot forms a new individual by "innovation." The term rhizome is applied also to the rare case of vertical growth of such a sympodium, as in the water-hemlock (Cieuta). The wiry rhizomes of the sand sedge (Carex arenaria) bind together wind-blown sand-dunes. Those of many plants store up large quantities of starch, as in the arrowroots, and they may resemble a chain of corms, or when abbreviated, as in Arum, a single corm (q.v.).