Stem

The stem is the main axis of the shoot system that bears nodes, internodes, buds and holds up the leaves, branches, flowers and fruits.

1.0Characteristics of the Stem

1. The stem develops from the plumule of the seed.

2. Young stems are green while the older, woody stems are non-green.

3. Stems grow towards sunlight.

4. Stems grow away from soil and water.

5. Stems may be branched or unbranched.

6. Stems have nodes and internodes.

7. Buds occur at the apex (tip) of the stem (terminal bud) and in the axil of leaves (axillary bud).

2.0Modifications of the Stem

(1) Aerial modifications

(a) For support (Tendril, Twiners).

(b) For defense (spines)

(c) For preparation of food (Cladode, phylloclade).

(d) For vegetative propagation.

(e) For Support

(i) Tendril: The plants having weak stem, their axillary buds get modified into thin, coiled and leafless structure called tendril which coils around neighbouring support, e.g. Bitter gourd, grapevine, passion flower, cucumber etc.

(ii) Twiner: Morning glory has a weak stem and does not have any special organs for climbing therefore, it twines around other plants with the help of a sensitive stem tip and climbs towards sunlight. Such plants are classified as twiner.

(b) For defense: The stem of some plants gets modified into sharp pointed organs called thorns or prickles (outgrowths on the surface of stem) which protect the plants from grazing animals. Thorns may vary in shape and size from plant to plant and sometimes also help in climbing, e.g. Lemon, bougainvillea, duranta, rose, etc.

(c) For manufacturing food

In the cactus, the stem becomes green, leaf-like, flat to perform photosynthesis and to prevent loss of water, leaves get modified into spines.

(i) Cladode: In Asparagus, leaves become modified into spines and the branches become modified to form cylindrical, green leaf-like structures. These photosynthetic leaf-like structures are called cladodes.

(ii) Phylloclade: In Opuntia, the stem becomes flat and succulent to conserve water and develops green cells, i.e., chlorenchyma (for photosynthesis). The leaves become reduced to small spines. These fleshy photosynthetic stems are called phylloclades.

(d) For Vegetative Propagation

In the case of sugarcane, part of the aerial stem containing bud can be used for vegetative propagation. 

The other modifications of the stem, i.e. subaerial (Stolon, Runner and Sucker) and underground (Tuber, Bulb, Rhizome and Corm) can also be used for vegetative propagation.

(2) Subaerial modifications

Subaerial modifications do not store food, they conduct food to the daughter plant. These modifications are in the form of: Stolon, Runner and Sucker.

(i) Stolon: In this modification, the stem grows upward above the ground and then bends down. When it comes in contact with soil, it produces bud which later develop into daughter plants, e.g. strawberry, jasmine, colocasia.

(ii) Runner: It is a horizontal stem which grows parallel to the ground, almost touching it. Runner has long internodes. New roots and shoots develop on the nodes. When long internodes break, each shoot with roots forms a new plant. Example: Doob grass, strawberry, oxalis etc.

(iii) Sucker: This stem arises as a lateral branch from the underground stem at its node. It grows obliquely upwards and gives rise to a new plant.

Example: Chrysanthemum, mint, pineapple.

2. Underground modifications

The stems of some plants remain underground. They become thick and fleshy due to storage of food. The stem bears nodes, scale leaves and buds. These stems help in perennation (escaping unfavorable conditions). These underground modifications are in the form of: Tuber, Bulb, Rhizome and Corm.

(i) Tubers: It is a large underground stem containing stored food, with eyes containing a bud. Each bud can grow into a new plant in the next season. Dormant buds called ‘eye’ can be seen on tuber which can produce a shoot of the new plant on sowing, e.g. Potato.

(ii) Bulb: In a bulb, the stem is condensed into a disc. It bears thin, fibrous roots at its base. This discoid stem bears an unusually long terminal bud, which is surrounded by colourless fleshy scale leaves which store food. The bulb is protected and covered by brown, dry and scaly leaves called tunic, e.g. Onion, lily and garlic.

(iii) Rhizome: A rhizome is an irregularly thickened horizontal underground stem. It is branched and shows distinct nodes and short internodes. At every node is present a papery scale leaf and buds. The buds at nodes produce adventitious roots and aerial shoots. During unfavorable conditions the aerial part dies and underground rhizome perennates. e.g. Ginger, turmeric, bamboo, banana and ferns.

(iv) Corm: It is a vertically placed, unbranched, underground condensed stem. The stem is swollen due to the stored food and bears circular nodes and internodes. The scale leaves are borne on the nodes and protect lateral buds. The buds grow into aerial shoots after the death of the original aerial shoot, e.g. Alocassia, Gladiolus, Colocassia and Saffron.

3.0Related Questions 

1. What do you mean by offset ?

Explanation

It is a short and thick branch that develops from the axil of leaf of the main plant. Like a runner it grows almost parallel to the surface. At its apex, it bears a bunch of leaves above and a tuft of adventitious roots below. On separation from the main plant, it forms a new plant. Example: Eichhornia (water hyacinth), Pistia.

2. Why are potatoes, onions and ginger not roots?

Explanation

These have nodes and internodes and scaly leaves like stem, so they are stems.

Functions of the Stem

(i) Provides support: The stem holds and supports the branches, leaves, flowers and fruits.

(ii) Conduction: It conducts the water and minerals absorbed by the roots to the leaves. It also transports the food prepared by the leaves to the different plant parts such as the root and fruits.

(iii) Food manufacture: green stems manufacture food by photosynthesis.

(iv) Stem holds the leaves in such a way that they can be exposed to maximum sunlight for photosynthesis.

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