Stem: Meaning, Types and Functions

 

Shoot system is the part of the plant which grows above the soil except the modified aerials roots. Shoot system can also be defined as that part of the plant which develops from the plumule of the embryo, normally appears above ground (at least at some stage in development) and bears the leaves, buds, flowers and possibly other appendages.

There seems to be no other way to distinguish morphology between a root system and a shoot system except by reference to appendages such as leaves and flowers, which are never borne by roots.

The shoot system consists of two main organs:-

1.  Stem

2.  The leaves. 

During the reproductive stage, flowers and fruits may be seen on different branches of the stem.

In this article, you should be able to explain the types of stems, explain the various types and purposes of stem modification, list the functions of stems and explain the internal anatomy of a stem.

 

Meaning of Stem

Stems stores food, water, and nutrients. Cells of a stem, meristems, produce new living tissues. Underground stem, Aerial stem, and subaerial stem are three different types of Stem. A stem has many important functions it performs other than letting you climb a tree. 

Let us take an in-depth look at the stem of plants.

A plant stem is one of the two main structural axes of a vascular plant. It is the part of the plant that lies above the ground. Few stems are also found underground and are considered to be stem modifications.

The stem is the ascending portion of the axis of the plant, developing directly from the plumule, and bears leaves, branches and flowers. When young, it is normally green in colour.

A stem consists of nodes and internodes alternating throughout its length. It is more or less cylindrical in appearance and ends in a terminal bud. Nodes are the slightly enlarged portion of the stem from which leaves and buds arise.

The buds occur on the axis of leaves and sometimes develop into branches and flowers. An internode is the portion between two successive nodes. The terminal bud consists of actively dividing cells (meristematic cells).

All the young tissues of the shoot system develop from these meristematic cells. Newly formed leaves can be seen covering the terminal buds. Stems are erect, decumbent, procumbent, prostrate or creeping. Stem can be smooth, hairy or rough they can bear roots or not.

 

Also Read: Photosynthesis and Its Importance

Types of Stem

There is a variety of stem structures adapted to perform diverse functions. They may be aerial or underground. Aerial stems may be erect, rigid and strong, holding themselves in an upright position; while there are some too weak to support themselves in such a position. They either trail the ground or climb neighbouring plants and other objects.

Some stems remain permanently underground and from there periodically give off aerial shoots under favourable conditions; such stems are for food storage and perennation.

There are different types of stems in flowering plants. With regard to their external characteristics and internal structures, stems can be grouped into the following.

1.  Woody Stems: These are the stems of trees and shrubs. In this group the xylem tissue is highly developed into wood which occupies a very large area of the stem, hence the stem cannot easily be bent.

2.  Herbaceous Stems: In this group the stems are thin. Wood is absent or present in very small amount; hence the stems can easily be bent. Woody and herbaceous stems belong to the dicot plants.

3. Monocotyledonous Stems (Monocot Stems): These are type of stems found in grasses, maize, lily and palms. Their vascular bundles are scattered and there is no secondary growth.

4.  Other Types of Stems: Other types found within the dicot and the monocot plants are

5. Leaning Stems (Twiners): These are tender stems with no means of support. They therefore grow by twisting round other plants for support. An example is the aerial stem of yam.

6. Creeping Stems: These are stems growing on the soil surface. They spread in all directions, producing roots at their nodes and leaves which cover the ground. A good example is seen in sweet potato.

7. Modified Stems: The normal or unmodified stems grow upright above the ground. Some stems are however modified for different purposes and functions.

These are:

(a) Tendrils: These are thin leafless and spirally curled branches with which certain plants get attached to neighbouring objects. Tendrils are therefore modified for climbing. They occur in passion flower, Curcubita and melon.

(b) Thorns and Spines: These are outgrowths of the stem. Thorns are from auxiliary buds hence they are borne on the axis of leaves. Thorns are seen in orange trees. Spines appear on the stem body. Thorns and spines are stems modified for protection against animals.

(c) Underground Stems: These are the stem tubers, such as rhizomes, corms and bulbs modified for food storage and vegetative reproduction.

i. Rhizomes: Rhizomes are prostrate, thickened stems, creeping horizontally under the surface of the soil. It is provided with distinct nodes and long or short internodes; it bears some scaly leaves at the nodes; it possess a bud in the axil of the scaly leaf; and it ends in a terminal bud. The rhizome may be unbranched or sometimes the auxiliary buds grow out into short, stout branches. Examples of rhizome are seen in ginger and canna.

ii. Tuber: This is the swollen end of a special underground branch (tubers means a swelling). The underground branch arises from the axil of a lower leaf, grows horizontally outwards and ultimately swells up at the apex. It has on its surface a number of "eye" or buds which grow up into new plants. A tuber is often very much swollen owing to a heavy deposit of food material, becoming at most spherical e.g. potato, irish potato and yam.

iii. Bulb: This is an underground modified shoot, consisting of a shortened convex or slightly conical stem, a terminal bud and numerous scale leaves. The inner scales are commonly fleshy, the outer ones thy. The fleshy scale store food (sugar in onion and mostly starch in others), while the dry scales gives protection. Examples of bulbs are Onion, Garlic and Leek.

iv. Corm: This is a condensed form of rhizome consisting of a stout, solid, fleshy, underground stem growing in the vertical direction. It is more or less rounded in shape or often somewhat flattened from top to bottom. It contains a heavy deposit of food material and often grows to a considerable size. It bears buds and adventitious roots normally developed from the sides (Example Cocoyam, Gladiolus and Amorphallus).

(d) Others: Runners, offsets, suckers and stolons are various types of stems also modified for vegetative reproduction.

i. Runner: This is a slender, prostrate branch with long or short intemodes, creeping on the ground and rooting at the node. The runner arises as an auxiliary bud and creeps some distance away from the mother plant, then strikes roots and grows into a new plant. The runner may break off from the mother plant and grow up as independent daughter plants. Examples are seen in wood-sorrel (oxalis).

ii. Offset: Like runners, this originates in the axil of a leaf as a short, more or less thickened, horizontal branch. The offset often breaks away from the mother plant and then the daughter plant grows into a new plant. Common examples are water lettuce (pistia) and water hyacinth.

iii. Sucker: Sucker is a lateral branch developing from the underground part of the stem at its node. But it grows obliquely upwards and directly gives rise to a leafy shoot or a new plant. Examples of suckers are seen in chrysanthemum, banana and pineapple.

iv. StoIon: Like the runner this is also a slender lateral branch originating from the base of the stem. Many stolons are provided with long or short internodes which may grow out as a mother plant and spread out in different direction.

 

Also Read: Root System: Definition, Types and Functions

The Internal Anatomy of the Stem

A transverse section (T.S) of a dicot stem under the low power or electronic microscope shows three major regions- the epidermis (on the outside), the cortex (the middle) and the endodermis (on the inside). The cortex is further sub-divided into collenchyma and parenchyma. A ring of vascular bundles is arranged inside the endodermis or starch sheath.

The vascular bundle consists of phloem (outside), a layer of cambium (middle) and xylem tissue (inside). In monocots however, the T.S. of the stem shows that vascular bundles are scattered without cambium, hence, no increase in thickness in monocots. Also there is no central pith and the cortex is thin.

 

Functions of Stem

I. They hold the leaves in the best position for light which is used in photosynthesis

2. They hold the flowers and fruits in the best position for pollination and dispersal respectively

3. They conduct water and mineral salts from the roots to the leaves and manufactured food from the leaves to the roots, growing regions and storage organs of the plant.

4. Some stems are modified for special functions such as climbing, protection, food storage, water storage and vegetable reproduction 5. Young parts of stems contain chlorophyll; hence they take part in photosynthesis.

 

Structure of a Stem

The stem divides into nodes and internodes. The nodes give rise to the leaves and hold the buds which grow into branches. The internodes separate two nodes.

Internally, it contains three basic types of tissues: Dermal tissue, Ground tissue, and Vascular tissue all of which are made of simple cells.

1. Epidermis: The epidermis is a single layer of cells that make up the external tissue of the stem called dermal tissue. This tissue covers the stem and protects the underlying tissue. Woody plants have an extra layer of protection on top of the epidermis known as bark. In some cases, the bears’ multi-cellular hairs and a few stomata.

2. Ground tissue divides into two- the central portion is known as the pith and the cortex which lies between the vascular tissue and the epidermis.

The cortex can be further divided into three layers:

1. Hypodermis: It is the outermost layer of the cortex. It is formed of 4 to 5 cell thick layer of collenchymatous cells. These cells are living and contain chloroplasts.

2. General cortex: Lies below the hypodermis. It consists of thin-walled parenchymatous cells with intercellular spaces. Some of the cells have chloroplasts and are known as chlorenchyma.

3. Endodermis: The innermost layer of the cortex. It is made up of a single row of compact barrel-shaped cells without intercellular spaces. The cells of endodermis store starch grains and so they are known as the starch sheath. Casparian strips are distinctly visible in endodermal cells.

4. The vascular tissue of the stem consists of the complex tissues xylem and phloem which carry water and nutrients up and down the length of the stem and are arranged in distinct strands called vascular bundles. Cambium is a strip of thin-walled cells that lie between the xylem and phloem in dicot plants. Cambium is made up of merismatic cells and is responsible for secondary growth. It is absent in monocots.

 

Read On: Flowering Plants: Structure and Characteristics

Growth in a Stem

Growth in stems occurs in two ways:

1. Primary growth occurs at the apical tips of the stem by virtue of the rapidly dividing merismatic tissue in these regions of the stem.

2. Secondary growth is actually the increase in the thickness of the stem by virtue of the lateral meristems. These are absent in the herbaceous plants as they lack cambium which is responsible for this type of growth.