Question

What are underground stems? Describe the various types, giving one example.

Answer 1

Hint: Stem is the ascending part of the plant which bears branches, leaves, fruits and flowers. Stem doesn’t always have the typical structure. In few plants, in order to perform necessary functions, stems get modified into different structures, such as underground stem modification, aerial stem modification and sub-aerial stem modification.

Complete answer:
Underground stems
Underground stems don’t possess a typical green coloured body. Typical stems are negatively geotropic, whereas underground stems are positively geotropic in nature. But still, they are identified as stems because they possess nodes, internodes, and buds. Based on the kind of growth (transverse or oblique or vertical) and specific part which stores food, underground stems can be classified as Bulb, Stem tuber, rhizome and Corn.

Classification of underground stems
(a) Bulb – It is a specialized underground stem in which the lower base bears root and the upper part bears a fleshy rosette of leaves. The stem is found to be highly reduced and is discoid in shape. They are of two types,
(i) Tunicated Bulb – The whole bulb is covered by dry membranous leaves called tunics, and hence the name, tunicated bulb. Example – Allium cepa (onion).
(ii) Scaly Bulb – Bulbs are not covered by any tunics, and hence called scaly bulbs. Example – Lilium bulbifera (lily).
(b) Corm – It is the underground modification of the main stem that grows vertically at specific depth of the soil. Example – Amorphophallus.
(c) Rhizome – Brown coloured underground stems that grow dorsoventrally up to a certain depth is called rhizome. The ventral side of the rhizome has adventitious roots. Example – Zingiber officinale (Ginger).
(d) Stem tuber – The axillary branches of the stem which grows near the soil become swollen due to accumulation of starch. The tip of the branch becomes tuberous in nature. Example – Solanum tuberosum (potato).

Note: It is important to remember the different kinds of underground stem modification, the part of the stem which undergoes modification and its specific example. The typical stem undergoes such specific modification according to the plant species, and to perform specialized functions. Most of the underground stems are consumed as food by man.