Explain the underground stem modifications.

Explanation

Perennial and some biennial herbs have underground stems, which are generally known as root stocks. Root stock functions as a storage and protective organ. They are classified as follows:
1. Bulb: It is a condensed conical or convex stem surrounded by fleshy scale leaves. They are of two types:
(i) Tunicated (coated) bulb: In which the stem is much condensed and surrounded by several concentric layers of scale leaves. The inner scales commonly fleshy, the outer ones dry. These are two types:
(a) Simple Tunicated bulb - Eg: Allium cepa
(b) Compound Tunicated bulb - Eg: Allium sativum.
(ii) Scaly bulb: They are narrow, partially overlap each other by their margins only. Eg: Tulipa spp.
Pseudobulb is a short erect aerial storage or propagatmg stem of certain epiphytic and terrestrial sympodial orchids. Eg: Bulbophyllum.
2. Corm: Succulent underground stem with an erect growing tip. Surrounded by scale leaves and exhibit nodes and internodes. Eg: Amorphophallus.
3. Rhizome: Underground stem grows horizontally with several lateral growing tips. Posses conspiquous nodes and internodes covered by scale leaves. Eg: Zingiber officinale, Canna.
4. Tuber: Succulent underground spherical or globose stem with many embedded axillary buds called "eyes". Eg: Solanum tuberosum Helianthus tuberosus.