The underground stem is also known as subterranean stem. The underground stem usually stores food material and it also acts as a means of vegetative propagation. Under unfavourable condition only the subterranean stem persists rest of the aerial parts die. On the onset of the favorable climate the subterranean stem give rise to aerial shoot system. The subterranean stems are perennial. They usually differ from the normal roots because of these characteristics such as; A. underground stem possess nodes and internodes. B. It is covered with scale leaves. C. Buds arise from the axils of the scale leaves. The under ground stem is divided into four type.
They are (i) Rhizome (ii) Corm (iii) Stem tuber (iv) Bulb.
(i) Rhizome: These are thick and prostrate under ground stem bearing nodes and internodes. The stem stores food materials so occurs as thick and fleshy. The brown scaly leaves protect the axillary bud. When favourable condition approaches the terminal and axillary buds will grows into aerial shoot system. Example: Ginger
(ii) Corm: This is the swollen underground stem and has prominent apical bud. The apical bud grows vertically and will develop aerial shoot system. The buds are protected by scale leaves. Example: Colocasia
(iii) Stem tuber: This the swollen end of the special underground stem. It is arising from the axil of the lower leaf. Adventitious roots are absent. Example: Potato In Potato the small depressions present on the tubers are called as the eye of the potato. The bud is a means of vegetative propagations in Potato.
(iv) Bulb: It the most condensed underground stem. It appears as a disc structure bearing root and scale leaves. There are number of adventitious roots arising from lower part of the stem. The terminal bud is positioned at the middle of the bulb. Example: Onion.