The Stem

The stem is the ascending part of the axis bearing branches, leaves, flowers, and fruits. It develops from the plumule of the embryo of a germinating seed. The stem bears nodes and internodes. The stem is generally green color when young and later often become woody and dark brown.

Node: The region of the stem where leaves are born is called nodes.

Internode: Internodes are the region between two nodes.

Buds: The stem bears buds. Bud is a condensed, immature, or embryonic shoot with close nodes. An example of the largest bud is Cabbage. Based on its position on plants these are of the following types:

  1. Apical or terminal buds: present on the apex of the main stem and branches are responsible for the increased length of the plant. It is the main area of growth in most plants. (Auxin hormones produced in this area of plants).
  2. Lateral buds: these buds are found on either the main stem or branches on the left or right side. Branches or flowers rise from them. these are two types
    • Axillary buds: born in the axil of a leaf and produce branches.
    • Collateral buds: arises on lateral sides of axillary buds e.g. lilies
  3. Superposed buds: these are present above the axillary buds, e.g. Aristolochia
  4. Adventitious buds: These develop at any part of the plants other than the nodes. They mean for asexual reproduction in plants. Adventitious buds may be foliar e.g. Myriophyllum, Cauline e.g. rose, and radical e.g. Dalbergia

 

Main functions of stem
  • The main function of the stem is to support and spreading out branches bearing leaves, flowers, and fruits.
  • It conducts water, minerals, and photosynthates. Transport food produced by leaves to different parts of plants.
  • Some stems perform the function of storage of food, support, protection, and vegetative propagation.

 

Some specialized functions of stem

There are some special functions of stems discussed as follows:

  1. Phylloclade or cladophyll: In many xerophytic plants stem becomes green, flat,, or fleshy and carries out photosynthesis, called phylloclade or cladophyll. Example Opuntia.
  2. Cladode: short, green cylindrical needle-like (ex. in Asparagus) or flattened leaf-like (Ruscus) branches develop from nodes of stem or branches in the axil of a leaf, reduced to small scales and performs photosynthesis called cladode. The smallest Cladode is found in Lemna.
  3. Tendril: It is a thread-like leafless, green, spirally coiled structure and sensitive to touch. Tendrils help in the climbing of weak stems. e.g. Grapevine (Vitis vinifera)
  4. Stems can be modified for food storage e.g. potato, ginger, turmeric, zimikand.
Modification of stem

In some plants, the stems modify to perform some other functions like storage or food, support, vegetative propagation, protection, etc. are called modified stems. There are three types of stem modification is found:

  1. Underground modification
  2. Subaerial modification
  3. Aerial modification
Underground modification of stem

Underground modification

sub aerial modification of stem

Subaerial modification

aerial modification of stem

Aerial modification

Type of stem modification Characteristics features example
A.  Underground stem
Underground modification of stems adapted for penetration and storage of food. Aerial shoots are produced annually. Nodes and internodes are found. Scale leaves, buds, adventitious roots at the nodes are shown. They counter xeric and other unfavorable conditions. These are of four types given below;
I.   Rhizome These are horizontal (under the soil), non-green, bearing nodes, and internodes. Rootstock rhizome grows obliquely and straggling rhizomes grow horizontally. Rootstock rhizome: Banana (Musca paradisiaca), Alocasia

Straggling rhizome: Ginger (zingiber Officinale), turmeric

II.  Bulb modification Fleshy leaf base, look like discoid stem, internodes suppressed, terminal buds are present, encircled by leaves, and adventitious roots formed at the base.  Axillary buds are found on the axis of leaves. These  are of two types:
a.  Imbricate bulb or scaly bulb No tunics are present, scaly bulbs are loosely arranged overlap only at the margin. Lily (Lilium)
b.  Tunicate bulb Tunicate bulbs coved by dry membranous scale leaves called a tunic. Scale leaves arranged in the form of a ring. Axil or fleshy scale contains buds Onion ( Allium cepa) and Garlic ( Allium sativum)
III.   Corm The corm is a short stem that grows in the vertical direction, loaded with food reserves, nodes bear buds, and produce adventitious roots. Various shapes like spherical or flattened, nodes consisting of scaly leaves, axillary buds, and adventitious roots. Crocus sativus (saffron ) colocasia
IV.             Tuber The flattened and enlarged stem that store starch, eye-like nodes are found, in the axil of leaves 1-3 buds are found.

A big scar, apical bud marks the endpoint on germination the eye develops into stolon. In tubers, adventitious roots are generally absent.

Potato (Solanum tuberosum), Jerusalem artichoke (Helianthus tuberosus)
B.     Subaerial stem
 They are short aerial branches in which adventitious roots develop to nodes. The nodes can grow into a fully developed plant, called creepers. These stem are meant for vegetative propagation. Four types of subaerial stems:
I.                 Runner These grow horizontally or in all directions. Creeping stem with long internodes. Multidirectional above soil.

Scale leaves at nodes. long internodes, adventitious roots emerge from nodes, these grow into new plants.

Oxalis (wood-sorrel family Oxalidaceae), Cynodon (grass family), and Centella Asiatica – Indian Pennywort
II.               Stolon Stolon is a lateral stem rise from the main stem, touches the soil, forms shoot from its terminal buds. Root produced and anchors the plant to soil. there is no specific direction to grow. Jasmine (jasminum) and fragaria, colocasia
III.              Offset A thick, succulent, cluster of leaves found above water soil, adventitious roots emerge from all nodes. these are aquatic plants. Pistia and Eichhornia (water hyacinth)
IV.             Sucker It is a horizontal non-green stem below the soil. Adventitious roots emerge from the lower side of nodes. Some produce green leaves from the aerial shoot, and some bear adventitious roots before separation from the mother plant. Chrysanthemum, Mentha (pudina), and Musa.
C.    Aerial stem
Modifies for special functions like protection of plant, food storage, vegetative propagation, climbing, etc. These are of the following types:
I.       Stem Tendrils It is a modification of the stem into a threadlike leafless structure called tendrils. These are meant for climbing. These may not necessarily contain a branch. Stem tendrils may be: Axillary, Extra-axillary, Apical bud, Floral Bud Axillary: E.g., Passiflora

Extra-axillary: E.g., Luffa

Apical bud: E.g., Grapevine

Floral Bud: E.g., Antigonon

 II. Thorns These are pointed, hard structures that may or may not bear leaves, branches, and flowers. The terminal bud gets modified into thorns. Thorns are used as defense organs and help to check transpiration. Bougainvillea
III. Phylloclades These are leaves modified into scales or spines to check transpiration. This is done by controlling the growth of the leaves. These are fleshy, green, and take part in photosynthesis. This modification is seen in xerophytic plants and stores water. It consists of fleshy internodes and the leaves are modified into spines or small scaly leaves. Opuntia
 

IV. Cladodes

It is a type of phylloclade consisting of only one internode. These are non-fleshy cylindrical. These help in photosynthesis. In this, the leaves are modified into a prickle  Asparagus
V . Bulbils These are modified axillary vegetative or floral buds meant for food storage. Bulbils undergo vegetative propagation to form a new plant. In this, the bulbils are condensed auxiliary buds. Dioscorea
VI . Thalamus It is the axis of the flower and contains floral organs such as calyx, corolla, androecium, and gynoecium. exhibits clear nodes and internodes.  calyx, corolla, androecium, and gynoecium

 

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