Question

Give any two reasons to justify that onion bulb is a modified stem.

Answer 1

Hint: Stem is an aerial part of a plant that is developed by plumule. Stem is negatively geotropic and positively phototropic. It also aids in keeping the plant upright, in addition to it performs the storage function. Bulb is a type of stem modification.

Complete answer:
Stems are modified morphologically, in order to perform various functions that are necessary for survival in a particular environment or habitat. Stem is also modified for the storage of food, vegetative propagation, and to provide support to the plant body. The modification of stem can be categorized into three types namely, underground stem, aerial stem, and sub aerial.

The underground stem is a modified part of the plant that is derived from the tissues of the stem and it generally exists under the surface of soil. This type of modifications aids as storage tissue for nutrition and food, perennation and also for the propagation of new clones.

Bulb is a type of underground modification of stem and can also be said as a highly reduced disc shaped stem. The leaf bases are edible and fleshy. The bulb can be scaly or tunicate.

Onion bulb is a modified stem due to following reasons-
1. It consists of a large number of adventitious roots at the posterior end.
2. It consists of a large number of fleshy scales.
3. Both axillary and terminal buds are present.

Note: The underground modifications of stem are bulb, rhizome, corm, and tuber. The aerial modifications of the stem are stolon, offset, sucker, and runner. The aerial modifications of stem are bulbils, thorns, phylloclade, tendrils, and cladode.