. Explain the System of Underground Modification of Stem


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UNDERGROUND MODIFICATIONS OF STEM

After modification such stems become subterranean and are meant for storage of food, perennation and vegetative reproduction. Underground modifications of stem are of following types:

i. Rhizome: It is a modified main stem, which grows horizontally under the surface of the earth exhibiting diageotropic mode of growth. It is thick, prostrate, dorsiventral and branched and has distinct nodes and internodes. The nodes bear small scaly leaves in the axils of which, there are axillary buds. Adventitious roots develop on the lower surface of the nodes. Food materials are stored in both nodes and internodes, e.g., banana (Musa sapientum), mankand (Alocasia indica), Dryopteris (a fern), ginger (Zingiber (officinale), Canna, lotus (Nelumbium spaciosum), etc.

stem

ii. Tuber: It is the swollen end of a special underground branch. It is round or oval or cylindrical shape growing horizontally under the ground. The stem tuber is covered by a corky skin. Each tuber possesses a number of spirally arranged depressions called eyes, each representing a node. At each node there is a scaly leaf in the axil of which, a bud is present and adventitious roots are absent. There is a heavy deposit of food material, e.g., potato (Solanum tuberosum), Jerusalem artichoke (Helianthus tuberosus). 

scale

iii. Bulb: It is highly reduced underground-modified main stem. It assumes a disc or convex-lens or cone-shape. Nodes are very closely situated so internodes are very small. From upper surface of nodes scaly leaves are borne and from their lower surface numerous adventitious roots arise. Scaly leaves are of two types: outer dry, brown and inner fleshy, white or red scaly leaves. Fleshy scaly leaves store food at their base in the form of carbohydrates. At the apex of bulb there is an apical bud which gives rise to an aerial shoot, known as scape. In the axils of fleshy scaly leaves, axillary buds develop, e.g., onion (Allium cepa), tulip (Tulipa), tuberose (Polyanthes), lily (Lilium sp.) garlic (Allium sativum) etc.

base

iv. Corm: It is the underground modification of the basal part of main stem. It is a short, thick, swollen, unbranched, solid and fleshy stem growing vertically in the soil. It bears a number of circular nodes, which are situated very close so that internodes are very small. There is a heavy deposit of food materials in both nodes and internodes. From the base of nodes many contractile adventitious roots arise. Scale leaves bear axillary buds, which give rise to daughter corms. A terminal bud is always present in a corm which gives rise to aerial branch under favourable conditions, e.g., elephant-foot, ver. zamikand (Amorphophallus campanulatus), Colocasia, Gladiolus, Freesia, etc.

node

 

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