Underground modified stems are found under the surface of soil and store food materials due to which they become quite thick and fleshy. They are usually brownish,whitish or yellowish.
Different types of Underground modified stems
1. Rhizome : This type of stem is thick, fleshy and usually horizontal. Nodes and internodes are clearly visible in such stems. Nodes bear brown scale leaves which are thin and sessile. In the axil of these scale leaves are born buds, which become aerial. Adventitious roots arise from the lower surface of nodes. At the end of each growing season the aerial branches dry and die. Rhizomes are mostly horizontal or straggling e.g., Ginger, Turmeric, Lotus, or may be vertical as in Canna, Sugarcane, Alocasia. This second type of rhizome is also called root-stock.
2. Tuber : Tuber is the swollen end of a branch developed underground. In such cases axillary branches develop underground after growing horizontally for some time and swell up at the end. These swollen portions of stem have some depressions at nodes which bear scale leaves. In the axil of these scale leaves are present some buds which give rise to aerial branches. There are no adventitious roots on the tubers. The tubers are almost spherical to slightly elongated and store reserve food e.g., Potato, Jerusalem artichoke
3. Corm : Underground stem with stored food, which is with more diameter than length. It grows in the vertical direction under the soil surface. Some horizontal circular lines are very clearly seen in corm, these are nodes with large number of scale leaves. From the base of nodes arise adventitious roots. Scale leaves bear axillary buds, which give rise to new corm. A terminal bud is always present in a corm which gives rise to aerial branch under favorable conditions. At the end of growing season aerial parts of the plant dry and die e.g., Colocasia, Crocus, Amorphophallus.
4. Bulb : It is highly reduced stem represented by a small disc like structure (stem) upon which are borne numerous fleshy scale leaves (which store food material). The disc and leaves together are called bulb. Numerous adventitious roots arise from the base of disc. At the apex of disc is borne an apical bud which gives rise to an aerial flowering shoot (Scape). Also there are some lateral axillary buds borne in the axil of the scale leaves which may form more scapes. Bulbs are of two types (a) Tunicated bulbs, (b) Scaly or imbricated bulb.
(a) Tunicated bulbs e.g., Onion (Allium cepa) : The scale leaves are arranged in a concentric fashion as seen in a cross section. The whole bulb is covered with some dry membranous scale leaves which form the tunic.
(b) Scaly or imbricated bulb : The scale leaves are not concentric but are arranged loosely like the petals of a flower, such a bulb is not a compact body and is not covered with any common tunic e.g., garlic (Allium sativum) and Lilies (Lilium species). In garlic a number of cylindrical but separate (i.e., not concentric)) fleshy scales, called cloves are arranged as in a naked bulb but a group of such cloves are enclosed in a whitish, skinny tunic.