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Describe different types of modifications of the underground stem.

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Modifications of stem:

(A) underground (subterranean) Stern Modifications:
In some plants. the main stem or its branches lie below the surface of soil (underground). store food material and help in perennation(i.c.they tied over the unfavourable season). These modified stems also helps in vegetative propagation During unfavourable conditions, the aerial shoots die but the underground stem survives which gives rise to new aerial shoots on the approach of next favourable season Underground sterns are non-green and resemble roots in their general appearance. However. they can be distinguished b the presence of-

  1. nodes and internodes.
  2. scale leaves
  3. adventitious roots,
  4. axillarv and terminal buds,
  5. exogenous branches.

1. Sucker.

It is a modified under – ground branch of stem that arises from the axillary7 bud of the underground part of aerial stem. It grows obliquely below the surface of soil to some distance and then emerges out in the fonn of aerial shoot. Each sucker is differentiated into nodes and inter nodes, bears scale leaves and develops adventitious roots at the base of nodes. Examples- Chrysanthemum, Mentha an’ensis (Mint, vem. Podina).

2. Rhizome:

It is a perennial underground main axis of the plant which grows indefinitely below the soil surface producing aerial leaves or shoots above ground and adventitious roots on lower,side in favourable season. They perennate during unfavourable season. Most of the rhizomes are fleshy and store reserv e food materials. They are differentiated into nodes and intemodes. On the basis of their position in the soil, rhizomes may be of two types-

  • Rootstock: They are upright or oblique with their tips reaching the soil surface, e.g., Alocasict indicct;
  • Straggling: They are horizontal in position and generally branched, e.g.. Nelumbo nucifera (Lotus, vern. Kamal), Ginger (Zingiber officinale), Turmeric (Curcuma domestica), Saccharum, etc.

3. Corm:

It is very much condensed, fleshy solid and vertically growing root stock with a large apical bud. It is usually unbranched. The shape of corm is almost spherical or somewhat It bears adventitious roots which arise from its basal portion. Each corm bears a number of circular nodes with scales and one or more axillary buds in the axil of scale leaves.

In favourable growing season, some of these buds develop adventitious roots and grow into new aerial shoots They store food material in the basal portion and form new conns. Examples, Amorphophallus (Elephant foot, ver. Zamin- kand or Suran) , Colocasia (vern. Kachalu), Fressia, Crocus sativus (Saffron, vern. Kesar),Alocasia, Glodiolus, etc. The corm of Amorphophallus campanulatus is a huge condensed single intemode with several axillary buds.

4. Tuber:

The stem tubers are swollen ends of specialized underground stem branches which arise as suckers or stolons. They are oval or spherical in shape which do not bear adventitious roots. They are usually protected by corky skin having lenticels for aeration. Each tuber bears spirally arranged depressions called eyes. These represent nodes. The eyes are more crowded towards the apical end (rose end) and less towards the stolon end (heel end).

The common example of stem tuber is potato (Solanum tuberosum). It stores starch as reserve food material. The potato tubers are used for vegetative propagation. When they are placed in the soil under favourable conditions, the axillary buds grow out in the form of branches at the expense of stored food material. Some of these branches become green aerial shoots

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