The following points highlight the four main types of underground modified stems. The types are: 1. Rhizome 2. Tuber 3. Corm 4. Bulb.
Type # 1. Rhizome:
It is a fleshy modified stem which grows horizontally beneath the surface of the soil. It has distinct nodes and internodes and minute scale leaves at the nodes. Lateral buds come out as usual from the axils of scale leaves and so, it becomes branched. (Figs. 56 & 57, F)
The terminal bud produces the green aerial shoot in favourable season which dies after flowering, leaving a scar on the rhizome. Rhizome bears many adventitious roots. Examples—ginger, turmeric, mango ginger, fern, etc. In Alocasia (B. Man- kachu) the rhizome is vertical and is called root stock (Fig. 57, G).
Type # 2. Tuber:
In potato plant slender branches develop from the underground part of the stem and grow more or less horizontally beneath the surface of the soil. The growth of the branch is arrested and the tips begin to enlarge due to accumulation of food matters.
These swollen ends of the underground branches are the tubers. They have so many buds, commonly called eyes, situated at slightly depressed pits at the axils of scale leaves. Adventitious roots are usually absent, e.g. potato.
Students often commit mistake about the morphology of potato and sweet potato. Potato is a modified stem and sweet potato is a modified adventitious root. Condensed nodes and internodes, scale leaves and eyes are clear characters to establish that potato is a modified stem.
It is interesting to note that the potato plant bears three types of stem,— first, green aerial stems; second, underground stem growing beneath surface of the soil which may be called a stolon; and third, the swollen tip—the tuber. (Figs. 56 & 58)
Type # 3. Corm:
It is a very fleshy stout modified stem. It is usually vertical and produces green aerial portion from the terminal bud. Scale leaves are present and daughter corms are formed from the axils of the scale leaves. Corm is really a much condensed rhizome. Adventitious roots develop from the body of the corm. Example—Amorphophallus. (Fig. 57, A & B)
Type # 4. Bulb:
It is a short underground stem. Here the stem is reduced to a flattened disc with compressed nodes and internodes. Fleshy scale leaves develop from the disc, practically enveloping it. The scale leaves may bear axillary buds which, in course of time produce daughter bulbs. The terminal bud grows into the aerial shoot during favourable season.
A cluster of adventitious roots are given off from the disc. The scale leaves are fleshy due to the storage of food matters. Bulbs which are covered by some brown papery scales are called tunicated bulbs, e.g. onion. In lily the scale leaves overlap each other and have no papery covering or tunic. They are known as scaly bulbs. (Fig. 57, C, D & E)