Question

How many types of transformed semi-gaseous stems are there ?​

Answer 1

A. Underground Modifications for Perennation, Vegetative Propagation and Storage:

A number of herbaceous plants find it impossible to survive above ground during unfavourable seasons of the year, usually the winter. Thus, they are short living annuals. Many such plants store up food material in the underground parts.

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When the aerial’ shoot withers away during the unfavourable season the underground parts remain safe. Next spring, as the season again becomes favourable, aerial shoots come out which ulti­mately flower and again die next year after storing up food underground.

Perennation is ensured this way by vegetative reproduction. These underground structures, although not green, are recognised to be stem structures because of (1) the presence of nodes and internodes; (2) scale leaves and adventitious roots coming out from the nodes; (3) buds at the axils of scale leaves, at the apices of these underground structures and on the apices of their branches; and (4) their anatomical structures which resemble stems.

There are several very common types of them:

(1) Rhizome:

Rhizomes are underground dorsiventral stems or branches growing horizontally- under the surface of the soil. These are brownish in colour so that they are easily mistaken for roots. But, they are divided into nodes and internodes, there are brown scale leaves at the nodes and there are axillary as well as apical buds. Adventitious roots develop on the lower surfaces of the nodes.

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The more common types of rhizomes are rather fleshy due to the storage of much, food material. An apical bud gives rise to the annual shoot which dies at the close of the season leaving a scar while the rhizome continues its growth by a lateral bud .

The rhizome is, therefore, usually of a sympodial nature. Common examples of such rhi­zomes are ginger (Zingiber officinale ) and turmeric (Curcuma domestica) which are much branched. The rhizome of Canna is not branched so profusely. Rarely, monopodial rhizomes may be found as in the case of ferns like Pteris. Here, the internodes are rather long and the rhizome grows horizontally by the apical bud which does not terminate in a shoot. Aerial leaves are developed laterally from the nodes.

A special type of rhizome is the rootstock which is a rhizome growing up vertically instead of horizontally. Very common examples of rootstock are found in bananas and many aroids (e.g., Alocasia indica).

The sobole may be considered either as a thin rhizome or as a runner. They are described later under runners.

Rhizomes and other kinds of underground stems sometimes bear a special type of roots called contractile or pull roots . These roots contract after formation and pull the growing rhizome or other organ deeper down into the soil. These are illus­trated in connection with the root modifications.

Rhizome

(2) Stem-Tuber:

Tuber is a general term applied to any fleshy part of the plant which may store food. These may be stems (underground as in the stem-tubers under discussion and aerial in some bulbils and pseudobulbs) or roots (root-tubers).

They are largely used as food and the most common of them is potato (Solanum tuberosum ) . When potato is grown from eye buds, after the plant is rooted to the soil, the lower part of its stem is covered by earth. Axillary or adventitious branches grow from this underground part of the stem . These branches swell up at their apices into tubers due to the accumulation of food substances (mostly starch) and the arrest of growth.

Each tuber develops slowly. It is a swollen bit of stem on which many internodes are pre­sent but not distinct because of the great swelling. The nodal points are noted by the pre­sence of scale leaves at an early stage and eyes or rudimentary buds at the axils (marked by small depressions) of these scale leaves

These eyes are arranged spirally on the tuber and are more crowded towards the distal end (called ‘rose end’) which is terminated by an apical bud.

The point of attachment of the tuber to the stem is marked by a scar which is opposite the apical bud. The tuber is protected by a corky skin. Adventitious roots are absent on the tuber at this stage.

Stem-Tuber

Tubers are also found in some other common plants as the weed Cyperus rotundas and the vegetable artichokes (Helianthus tuberosus or Jerusalem artichoke storing inulin and Stachys tuberifera or Chinese artichoke storing stachyose). In Helianthus tuberosus the scale leaves and nodes are much more prominent.