Stem | Definition, Structure, Functions & its Modifications
The ascending part of the plant axis which bears branches, leaves, flowers and fruits is called stem. It generally grows above the ground and hence is considered as the aerial part of the plant. The plumule of the embryo, present in the germinating seed gives rise to the stem.
The stem is generally green in colour at the initial young stage but later it becomes woody and dark brown. It is differentiated into nodes and internodes. The region bearing leaves, present at regular intervals on the stem and its branches are called nodes and the part of stem present between the two nodes is called internode. The stem bears buds which may be terminal or axillary. A bud is defined as the young, immature, under developed, compact shoot. The buds present on the stem are of two types namely:
(i) Terminal bud : The bud present at the tip of the stem is called terminal bud. The growth of the stem and its branches is accomplished through the terminal bud. The terminal bud is also called apical bud.
(ii) Axillary bud: The leaf makes an angle with the upper part of the stem. The angle made between the leaves and the upper part of the stem is called the axil. The bud which is present at the axil is called axillary bud.
Functions of the Stem
- Stem bears and supports leaves, flowers and fruits.
- It conducts water and minerals salts from roots to leaves and fruits.
- The food manufactured in the leaves is transported to the roots, fruits and organs of storage through the stem.
Modifications of Stem
The stem of some plants is modified to perform different functions in order to help plants to adapt to the present environmental conditions. Some of the modification occurring in plants are :
(i) Underground stem : Stem is generally the aerial part of the plant i.e., it is present above the ground but in some plants it is modified for storing food materials, where it forms underground stem which penetrates the soil and lies below it e.g., underground stem of potato, ginger etc. store food material. They also act as organs of perenation to tide over conditions unfavourable for growth.
- Rhizome : It grows parallel or horizontal to soil surface. It bears nodes, internodes, buds and scaly leaves e.g., Ginger, Banana, Turmeric, Ferns.
- Tuber: It is terminal portion of underground stem branch which is swollen on account of accumulation of food, e.g., Potato, Helianthus tuberosus (Jerusalem artichoke).
- Corm: It grows vertically beneath soil surface. It is usually unbranched. It bears nodes, internodes,buds and scale leaves, e.g., Colocasia, Gladiolus, Colchicum, Crocus, Amorphophallus (Zaminkand).
- Bulb: Stem is reduced and disc shaped. The bud is surrounded by many concentric scale leaves.Leaf bases of inner ones are fleshy and edible and of outer ones are dry, e.g., onion, lily, garlic.
It is of two types — tunicated and scaly. Tunicated bulb is covered by a sheath of membranous scales called tunic. It may be simple tunicated bulb– covered bya sheath, e.g, onion and Narcissus or compound tunicated bulb- concentric rings of bulblets surrounded by a white membranous sheath or tunic e.g. garlic. Scaly or naked bulbs do not have tunic, e.g., lily.
(ii) Stem tendrils : In some plants the axillary buds present on the stem modify to form tendrils. Tendrils are long, thin, thread-like spirally coited, sensitive structures. They are the climbing organs of the plant which coil around the nearby support. They provide support to the weak and tender stem e.g., tendrils are present in grape vine, gourds (pumpkins, watermelon, cucumber).
|Tendrils are formed as modifications of :
(iii) Thorn : The axillary buds of plants like Citrus and Bougainvillea lose their ability to grow and form hard, woody and pointed structures caled thorns. These thorns protect the plants from browsing animals. Thus, these thorns are protective in function.
(iv) Sub-aerial Weak Stem
- (a) Offsets : Aquatic plants such as Pistia and Eichhornia contain a lateral branch which bear short internodes. In these lateral branches the distance between the two nodes decreases and each node bears a rosette of leaves above and a cluster (tuft) of roots below.
- (b) Suckers: In plants like banana, pineapple, Chrysanthemum, the lateral branches originate from the basal and underground portion of the main stem. They grow below the surface of the soil to some distance and then emerges out obliquely to form the aerial shoot.
- (c) Runners : It is elongated, prostrate branch with long internodes and roots at nodes. e.g., grasses.
- (d) Stolons : In plants like mint and jasmine a slender lateral branch arises from the base of the main axis and after growing aerially for some time arch downwards to touch the ground. Example — Jasmine, Mint.
(v) Aerial stem : Plants present in arid regions modify their stem into flattened (Opuntia) or fleshy cylindrical structures (Euphorbia), called phylloclade. These structures are green in colour due to the presence of photosynthetic pigments. These are green stems have unlimited growth. These structures perform the function of photosynthesis.