This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
(Rhizomes, Corms, Bulbs, and Roots)
The subterranean organs of perennial plants serve as storage-rooms for the reserve material destined for the subsequent use of the plant. These organs often contain accumulated in them alkaloids, glucosides, and other medicinally valuable constituents, in addition to, such carbohydrates as starch, sugar, inulin, etc.; hence the utilisation of such subterranean organs for medicinal purposes. They are generally collected in the autumn after the summer foliage has filled them with reserve material and before the development of the stem in the spring has partially exhausted them. Whether, however, they are in all cases richest in active constituents at this particular period has not yet been satisfactorily demonstrated.
In many herbaceous plants, such as dandelion, pellitory, etc, the aerial stem dies down in the autumn, leaving the lower modified portion attached to the root into which it passes more or less imperceptibly. This lower portion of the stem bears the buds destined to develop into new stems; being a subterranean stem it is to be regarded as an erect rhizome (often called rootstock), and is collected together with the root. Such drugs as dandelion and pellitory root consist therefore of both root and rhizome.
In other cases the transition from root to rhizome is more abrupt, as it is in valerian, serpentary, etc, but here also both are collected and dried together. In other cases, again, the rhizomes are separated, as they are in ginger and turmeric, the roots being rejected; or both roots and rhizome occur simultaneously in the drug, as with liquorice, gelsemium, etc. Comparatively few drugs consist either of root or of rhizome (corm or bulb) alone. This being the case it is evidently impracticable in classifying the drugs satisfactorily to separate roots from rhizomes.
Rhizomes may be defined as stout or slender, prostrate, oblique, or erect, hypogaeic or epigaeic stems. They may be distinguished from roots, which in many cases they closely resemble, by the following characters:
(i) They bear more or less evident cataphyllary leaves in the axils of which buds are present, (ii) In many instances they exhibit a more or less evident pith, though frequently this is not easily discernible in organs as old as these are when collected. (iii) The transverse section exhibits under the microscope leaf-traces in the cortex.
Bulbs may be regarded as subterranean buds, modified by the enlargement of the leaves to receive reserve material.
Corms are also modified buds, the basal stem portion of which serves as a reserve organ.
Roots may be distinguished from rhizomes
(i) By the absence of cataphyllary leaves (or their scars) and of buds, (ii) By the absence of pith, (iii) By the absence of leaf traces in the cortex.
The treatment that these organs receive after their removal from the ground varies with their nature. Small and slender rhizomes and roots are usually dried entire; larger ones are often peeled, cut, or sliced in various ways to facilitate rapid drying. In some cases slow drying is preferred in order to induce certain changes resulting in the production of aromatic or other bodies (e.g. valerian, gentian, orris).