The underground stems, by being situated below the surface of the soil, protect themselves against unfavorable conditions of weather and the attack of animals and serve as storehouses for reserve food, and in vegetative propagation. Their stem nature can be distinguished by the presence of nodes and internodes, scale leaves at the nodes, axillary buds in axils of scale leaves and a terminal bud. Further, the anatomy of the underground stem resembles that of an aerial stem. The underground stems are of four types namely rhizome, tuber, bulb, and corm.
A rhizome is a thick horizontally growing stem which usually stores food material. It has nodes and internodes, scale leaves, axillary buds, adventitious roots and a terminal bud. Scale leaves enclosing the axillary buds are seen arising from the nodal points of the stem. Some of the axillary buds develop into branches which grow upwards into the air and then produce normal green foliage leaves. Usually, the growing points of the rhizome continue to remain underground causing an elongation of the rhizome. Roots develop from the lower surface of the rhizome. Eg. Ginger, Turmeric.
fig. 27.17 Rhizome of Ginger
fig. 27.18 Root Stock (Vertical Rhizome) of Alocasia
Tuber is a swollen end of an underground branch which arises from the axil of a lower leaf. These underground branches grow horizontally outwards in the soil. Each tuber is irregular in shape due to the deposition of food materials (starch). On the surface of each tuber, many leaf scars are seen. These leaf scars are the impressions of fallen scale leaves. Each such leaf scar encloses an axillary bud. A leaf scar with an axillary bud is called an eye. These eyes of a potato are capable of producing new plants by vegetative propagation. E.g., Potato.
fig. 27.19 Tuber of Potato
Here, the stem is reduced and represented by a short disc. The lower surface of the stem produces many adventitious roots. E.g., Onion, Garlic.
In bulbs of onion, garlic, etc. the inner leaves are fleshy while the outer ones are dry. This is called as tunicated bulb since the concentric leaf bases form a complete covering or tunic. The apical bud of the bulb produces the shoot. The axillary buds sometimes produce daughter bulbs, as in garlic.
fig. 27.20 Bulb of Onion
fig. 27.20 Bulb of garlic
A corm is a greatly swollen underground basal portion of an erect stem. The swelling is due to the storage of reserve food material. It bears scale leaves and axillary buds. At the end of the growing season, the aerial parts die. With the return of favorable conditions usually one axillary bud (rarely more than one) near the apex develops into a new shoot utilizing the food reserve material in the old corm. The new plant produces a new corm at its base. The earlier corm shrivels off. E.g. Amorphophallus, Colocasia.
fig. 27.21 Corm of Colocasia