Many crude drugs are derived from the underground or subterranean organs or structures of plants. These include some roots and modified underground stems, such as rhizomes, corms, and bulbs.
Roots, which constitute drugs, are usually the strongly developed primary (tap) roots, although many secondary roots and root system sometimes produce some important root drugs. Roots bear no leaves, scales or buds. The only appendage present on the roots is the lateral branch roots, which are similar in construction to the main root. Thus there are no external scar of leaf or scale on the roots. The outer surface of the thick or main roots sometimes possesses remains of the bases of the branch roots or their scars or scattered rootlets. Barks of the old thick roots bear cracks and fissures like those of the aerial stem barks.
Internal Characters of Roots
The transversely cut surface of a root reveals characteristic features by which a monocotyledonous root can be distinguished from a dicotyledonous one. In the dicotyledons, to which most root drugs belong, there is a central woody core surrounded by the cambium, .a cylinder of secondary phloem and an external layer of cork. In monocotyledons, a central pith is usually present and is often composed of thick-walled lignified cells; the xylem is porous and a cortex is present. However, all roots are externally covered by a piliferous layer. The major part of the root is made up of the parenchymatous cortex. Internal to the cortex is a well-developed endodermis. The stele is surrounded by a single layer of pericycle and has alternating bundles of xylem and phloem on different radii, arranged in a circle. The xylem bundles frequently develop until they meet at the center of the root, forming a rod-shaped mass of xylem.
Aconite, Belladonna, Calumba, Dandelion, Ipecacuanha, Jalap, Rauwolfia, Sarsaparilla and Senega are some of the important root drugs. Some of them are described below in the form of Monographs as representative examples for the students so that they can also describe other root drugs.