Plant Stem – Structure and Functions Notes

The Stem

The main part of the shoot is the stem that bears bud leaves, branches, flowers, fruits, etc.

The Stem Definition: The ascending, positively phototropic, and negatively geotropic axis of the plant, developing from the plumule of the embryonic axis and usually present above the soil, is known as the stem.

Characteristics of Stem:

1. The stem bears branches, leaves, buds, flowers, and fruits.
2. The stem is differentiated into nodes and internodes.
3. The apex of the stem usually contains a bud.
4. Stems may or may not bear multicellular hairs.
5. Leaves and branches grow exogenously and acropetally (upward from the origin) on the stem.
6. Young stems are green in color. Some stems gradually become dark brown with maturation.
7. On the outer surface of the stem, there is a cuticle layer.
8. The stem may be unbranched (for example many palms) or branched (for example mango).

Read and Learn More: WBCHSE Notes for Class 11 Biology

Different Parts Of A Typical Stem

A typical stem, along with all its parts, is known as a shoot. Let us learn about the different parts of a typical stem.

Node: A stem has some swollen parts at regular intervals along its length, from which leaves, buds, etc., develop. These are known as nodes. These are prominent in some plants (for example bamboo) whereas these are not visible in others (for example china-rose).

Node Function: Serves as the origin of leaves, axillary buds, branches, fruits, and flowers that develop from this region.

Internode: The region between two successive nodes is known as the internode. it does not bear any branches, leaves, flowers, or fruits.

Internode Function: It keeps the stem erect.

Leaf: These are flat, generally green, thin structures, with limited lateral outgrowth. They develop from nodes of stems or branches.

Leaf Function: It helps to produce food by photosynthesis. Leaves also help in gaseous exchange between the plants and the outer environment.

Axil: The angle formed by the leaf with the stem is known as axil.

Axil Function: Axil gives rise to leaves, branches, fruits, and flowers.

Bud: It is an immature condensed shoot. The internodes of this condensed shoot are very short and thus appear condensed. The immature leaves completely enclose the growing region at the apex of the shoot.

The buds that are found at the apical region of the mature shoot are known as apical buds. On the other hand, the buds that are found at the axils are known as axillary buds.

Bud Function:

1. Apical buds help to increase the length of the plant.
2. At certain stages, the axillary bud produces leaves and branches, then it is known as a vegetative bud.
3. During reproduction floral buds develop from the axils. These buds are also known as reproductive buds.

Branches: The stem-like structures that develop from the axil are known as branches.

Branches Function: The branches bear leaves and flowers.

Flower: The reproductive part of the plant that originates from the floral bud is known as a flower.

Flower Function: Flowers help in reproduction.

Types Of Stem

The nature, shapes, and texture of stems vary among plants. They are categorized on the basis of these features.

Classification of Stems According To Nature

Depending on their nature, the stems are classified as herbaceous and woody.

Herbaceous: The stems which are soft with less number of branching are known as herbaceous stems The plants with this type of stems are known as herbaceous plants or herbs.

Depending on the lifespan, the herbs can be divided into four groups—

1. Annuals,
2. Biennials,
3. Perennials And
4. Ephemerals.

Annuals: Herbs completing their life cycle within a year are known as annuals. example mustard (Brassica nigra), and rice (Oryza sativa).

Biennials: Herbs completing their life cycles within two successive seasons are known as biennials. They grow vegetatively in the first season and initiate reproductive growth in the following season to develop flowers and fruits. example carrot (Daucus carota), and radish (Raphanus sativus).

Perennials: Herbs living for more than two seasons are known as perennials. example turmeric (Curcuma longa) and ginger (Zingiber officinale).

Ephemerals: Herbaceous plants completing their life cycles within a few days in the favorable season are called ephemerals. example Balanites aegyptica, and Arabidopsis thaliana.

Woody stems: These stems are hard and strong.

Plants are of three types

1. Shrubs,
2. Undershrubs And
3. Trees.

Shrubs: These plants are of medium size with strong and woody stems. example china-rose (Hibiscus rosa-sinensis), rose (Rosa centifolia), custard apple (Annona squamosa), etc.

Undershrubs: These plants are intermediate between herbs and shrubs. example brinjal (Solanum melongena) and chilli (Capsicum frutescens).

Trees: These plants are taller with strong, hard, and woody stems. Mostly, the main erect axis remains unbranched for some distance and is termed a trunk.

The rest of the portion above generally becomes profusely branched to form a canopy. example mango (Mangifera indica), tamarind (Tamarindus indicus), and banyan (Ficus benghalensis).

Trees are again classified into the following types—

1. Deciduous: These are trees that shed all their leaves in a particular season of the year. example Quercus alba (oak).
2. Evergreen: These are the trees that shed their lea gradually and not all at the same time. They remain green throughout the year. example Mangifera indica.

Aerial stems may be strong or weak. Strong stems stand erect on the ground as weak stems need support to remain upright.

Strong (Erect) stem: These stems are more or less cylindrical and may be unbranched or branched. They are strong enough to keep the plant erect.

They are of the following types—

Excurrent: The main erect axis becomes thick at the base and gradually tapers towards the tip with racemose (indefinite) branching. The tree thus takes the shape of a pyramid. example Polyalthia longifolia, Pinus sp., etc.

Deliquescent: The stem starts branching profusely after some distance above the ground. Here, the tree canopy becomes dome-shaped. example Mangifera indica Ficus benghalensis.

Caudex: This type of stem is erect and unbranched with prominent leaf scars (impressions left by the leaf bases). It has a crown of leaves at the apex. example coconut palm (Cocos nucifera) and palmyra palm (Borassus flabellifer).

Culm or haulm: These stems are erect and hollow at the internodes but solid at the nodes. They appear to be jointed. example, bamboo (Bambusa tulda) and sugarcane (Saccharum officinarum).

Scape: This is the erect unbranched shoot, produced by the underground or underwater stem of some monocotyledonous plants. example Allium cepa.

Weak stem:

These stems are soft and delicate with less woody parts. They are not strong enough to remain erect.

The weak stems are of different types—

Trailer: This type of stem trails over the surface of the ground without producing any adventitious roots.

It is of two types—

1. Procumbent: This stem grows along the soil surface and is mostly spread in one direction. exampleIpomoea reptans, Basella rubra.
2. Decumbent: This stem grows along the ground for some distance and then rises upward at the apices. example Tridax procumbens, and Lindenbergia indica.

Creeper: This stem grows along the soil surface and develops adventitious roots from each node. It also produces small branches which are distributed in all directions. example Centello asiatka, and Phyla nodiflora.

Climber: These weak and flexible stems climb up on supports by producing certain special structures.

These items are classified as follows—

1. Stem climbers or twiners: These are climbers that climb up around the support by twining. The growing apex of the stem coils around the support, either in the clockwise (for example Dioscorea alata) or anti-clockwise (for example, Clitoria ternatea) direction.
2. Tendril climbers: When the climbers climb up with the help of a very sensitive thread-like, leafless structure called tendrils, they are known as tendril climbers. The tendrils gradually curl around the support for gripping.
   The tendrils can be modified stems (grapevine), leaves (Lathyrus aphasia), stipule {Smilax zeylanica), leaflet (Pisum sativum), leaf apex (Gloriosa superba), petiole (Clematis guarana), and inflorescence axis (Quisqualis malabaricum).
3. Root climbers: These climbers climb up with the help of adventitious roots. These roots gradually become profusely branched to anchor the support. example Pothos aureus, and Piper betel.
4. Scramblers or ramblers: This type of climber climbs up on supports with the help of thorns, spines, or prickles. example Calamus rotang, Bougainvillea spectabilis, and Capparis sepiaria.
5. Hook climbers: These climbers climb up with the help of hooks, formed by the modifications of floral stalks, i.e., pedicels (Artabotrys uncinatus), terminal leaflets (Bignonia unguiscati) and inflorescence axis (Artabotrys uncinatus).
6. Adhesive climbers: These climbers possess adhesive discs or pads on the climbing roots. These discs help them to climb up even on flat and smooth surfaces. example Parthenocissus sp.
7. Lianes: These are woody climbers who twine and climb the big trees in deep forests. example Bauhinia sp., Marsdenia volubilis.

Classification of Stems According to Texture

According to texture, stems are of the following types—

Glabrous: The outer surface of this type of stem is smooth and is devoid of any emergence (for example stem of Hibiscus rosa-sinensis).

Glaucous: These stems have a smooth and shiny outer surface (for example stems of Zea mays).

Hispid or hairy: The outer surface of this stem is covered with hairs (for example stems of Helianthus annuus).

Prickly or spiny: The outer surface of this stem is covered with prickles or spines (for example stem of a rose).

Branching Pattern Of Stem

Branches develop from the axillary buds or as bifurcation of the growing tip of the main stem. The arrangement of branches on the main stem is known as a branching pattern.

Types Of Branching

Two types of branching patterns are found in angiosperms—dichotomous and lateral.

Dichotomous branching: It occurs due to the bifurcation of the growing apex of the main axis. The branches may or may not develop equally. The main axis remains at the base of the two diverging branches and is referred to as the foot or podium. The forked region again develops branches

Monopodial dichotomy: In this type, the apex of the main stem bifurcates and gives rise to two daughter branches of equal vigor. This type of branching is found in pteridophytes. This type of branching is found in plants like Selaginella monospora, Lycopodium clavatum, Psilotum nudum, etc.

Sympodial dichotomy: In this type, the main axis bifurcates into two daughter branches of unequal vigor. One of them grows more rapidly than the other.

It is further divided into two types—

1. Helicoid and scorpioid.
2. Helicoid: This branching type shows growth on any one side of the main axis.
3. Scorpioid: This branching type shows growth on both sides of the main axis.

Lateral branching: In this type, the branches develop from the sides of the main axis due to the activity of lateral axillary buds.

It may be of two kinds—

1. Racemose or monopodial or indefinite and
2. Cymose or sympodial or definite.

Racemose or indefinite: In this type, the main axis grows indefinitely and the branches develop from the axillary buds arranged in an acropetal (from the base towards the apex) manner. The terminal bud remains active throughout the lifespan of the plant.

Here the main axis forms the single foot and supports the lateral branches. This type of branching is also known as monopodial branching. The plant with this type of branching looks like a pyramid i.e., excurrent. example Polyalthia longifolia, Lawsonia alba, Pinus longifolia, etc.

Cymose or definite: In this type, the growth of the terminal bud ceases soon and the active lower lateral buds develop into branches. This type of branching is also known as sympodial branching. Due to this type of branching, the tree becomes dome-shaped i.e., deliquescent (for example mango, banyan, etc.).

It can be of three types—

1. Uniparous,
2. Biparous And
3. Multiparous.

Uniparous: In this type, only one branch develops from the axil.

It is again divided into two types—

• Helicoid, and
• Scorpioid type.
  1. Helicoid: When the successive branches develop only on one side, then it is termed as helicoid type. example Saraca indica.
  2. Scorpioid: Successive branches develop alternately from both sides (left and right). example Vitis quadrangularis.
  3. Biparous: Two branches develop from the axils of the main axis. example Carissa Carandas, and Mirabilis Jalapa.
  4. Multiparous: Many branches develop from the axils of the main axis. example, Croton bonplandianum.

Modified Stem

In certain cases stems are modified differently for different purposes such as for storing food, for protection, for reproduction, etc.

On the basis of position, modified stems are of three types—

1. Aerial,
2. Subaerial and
3. Underground or subterranean.

Modified subaerial stems

Subaerial stems develop from the axillary buds and grow along the ground. These stems give rise to new plants. Following are the different modified subaerial stems—

Runner: It is a slender, prostrate, creeping aerial stem. After running a short distance over the earth it develops roots and leaves, to form a new plant. This produces another runner from its leaf axil, which behaves similarly. example Oxalis corniculata, Centella asiatica, Ipomoea aquatica, etc.

Stolon: It is an elongated, arched runner, that does not grow horizontally above the ground. It initially grows upward like normal branches, then arches down towards the soil. At the point of contact with the soil, it produces adventitious roots. example, Mentha piperita, Fragaria vesca, etc.

Offset: These are horizontal branches with shorter I and thicker internodes. The terminal ends of the S branches develop a cluster of roots towards the lower portion and a rosette of aerial leaves. Examples,Pistia stratiotes, Eichhornia crassipes, etc.

Sucker: They originate from the underground basal part of the aerial shoot. After growing for a certain distance, the sucker develops adventitious roots, while the apex emerges upward and produces a leafy aerial shoot. example Mentha viridis, Chrysanthemum coronarium, etc.

Modified underground or subterranean stems

These stems grow fully or partially under the ground. These stems produce leaves and flowers during favorable conditions. They store food to prevent unfavorable seasons and also to help in vegetative propagation. These stems consist of apical bud, axillary bud, nodes, internodes, scale leaves, or other modified leaves.

Modified underground stems are of the following types—

Rhizome: The rhizome is an elongated, thick, fleshy, and irregularly shaped underground stem. It is differentiated into nodes and internodes. The stem bears scale leaves at the nodes and axillary buds.

Some of the axillary buds produce aerial shoots. Many adventitious roots develop from the lower portion of the rhizome. The apical bud grows to elongate and develops new aerial shoots.

Examples; are Zingiber officinale and Curcuma longa, Musa paradisiaca, etc.

Stem-tuber or tuber: These are the fleshy, round, oval, or oblong-shaped underground branches developed from the axils of leaves. The apices of these branches swell up due to stored food and are modified into tubers. Each tuber has numerous depressions known as eyes.

These stems have many nodes and internodes with rudimentary buds in the axils of scale leaves in the eyes. Example. Solarium tuberosum, Cyperus rotundus, etc.

Corm or solid bulb: The corm is more or less round, solid, stout, and fleshy underground stem or rootstock. The entire body of the corm is covered with scale leaves. Buds are developed at the nodes in the axils of the scale leaves. Some buds develop into new corm and the older portions gradually die.

Adventitious roots originate from the base or from the whole body. Corm bears a large apical bud that develops into large foliage in early spring and finally blooms. example Amorphophallus campanulatus, Colocasia sp., Crocus sativus, etc.

Bulb: Bulb is a modified underground stem in which the stem is extremely reduced to a small convex disc with compressed internodes. Thick and fleshy scale leaves are produced from the upper portion of the stem.

A bulb consists of apical buds and axillary buds. Axillary buds may develop into new bulbs. Adventitious roots are produced from the lower surface of the stem.

Bulb is of two types—

Tunicated or coated bulb: In this type, the fleshy leaves are arranged on the disc in a concentric manner, The outer leaves become dry and scaly to form tunica (covering sheath).

Example Allium cepa. Scaly or naked bulb: In this type, the fleshy leaves overlap each other on the margins and do not remain covered with a tunica. example Lilium candidium and Tulipa gesneriana.

Metamorphosed (highly modified) aeria stems: In some cases, the stem shows extreme structural modifications. Its nature can be determined by its origin and internal structure. Such extremely modified stems are termed metamorphosed stems.

They are of various types—

Thom or stem spine: In some plants, the axillary buds are metamorphosed into hard and sharp pointed thorns or spines. These thorns act as a defense organ. They may be modified axillary buds, as in Duranta plumieri, Hygrophyla auriculata, Aegle marmelos etc. They may also be modified terminal buds, as in Carissa carandas. As they grow in the leaf axils, they often bear foliage and lateral branches. They also have anatomical features like the stems.

Stem tendril: Some parts of a weak stem or branch sometimes metamorphose into a tendril to climb the support. It may be axillary (for example, Passiflora suberosa) or terminal (for example Vitis quadrangularis).

Phyllodade or mesophyll: This is formed from the metamorphosis of stem, into a flat leaf-like structure consisting of many nodes and internodes. It performs the functions of a leaf. In this case, the original leaves may either fall off or become very rudimentary or modified into spines.

This helps to reduce the rate of I transpiration. Sometimes it may store water or food and become succulent. These are mostly found in xerophytes. This type of metamorphosis is observed in plants like Opuntia dillenii, and Muehlenbeckia platyclados.

Cladodes: These are the metamorphosed leaf-like stems or branches consisting of only one internode Cladodes are green in colour, flattened or cylindrical in shape. Cladodes develop in the axils of very small scaly or spiny leaves. They perform the function of foliage leaves, i.e., photosynthesis. example Asparagus racemosus.

Pseudobulbs: It is a modified stem found in some orchids. Generally, the internodes swell up into a fleshy and tuberous structure. They serve as storage organs, primarily for water, which helps the plants to survive during drought. example Bulbophyllum sp.

Bulbil: In some plants, the axillary buds develop into swollen structures known as bulbil. They serve as organs of food storage. They also help in vegetative propagation by producing new plants. example Dioscorea.

Thalamus: It is the modified shoot on which floral leaves develop. It is a condensed axis with very closely placed nodes and suppressed internodes.

At the condensed nodes, the thalamus bears different parts of the flower (floral leaves)- calyx, corolla, stamen, and carpel, which are actually modified leaves. It is found in most of the typical flowers like Hibiscus rosa-cinensis.

Functions of stem

Functions of the stem can be divided into two groups.

They are as follows—

Primary functions:

The primary functions of stems are as follows—

Mechanical: Stem bears fruits, leaves, flowers, buds, and branches on it. Stem provides mechanical support to all the other parts of the plant.

Physiological: It helps in the conduction of water containing dissolved minerals to different parts of the plant body through xylem vessels. It also transports food and many other macromolecules prepared by leaves to different parts of the plant body through phloem tissue.

Special functions:

The special functions of stems are as follows—

Food and water storage: Stores water (as seen in many cacti) and food (as seen in many underground modified stems).

Photosynthesis: Young stems and some modified leaf-like stems (cladodes and phylloclades) are involved in the production of food through photosynthesis.

Protection: In some plants stems get modified into thorns which protect the plants from animals. example Aegle I marmelos and Duranta plumieri.

Support: The stems are modified to various structures such as tendrils, thorns or hooks. These structures provide support to the weak plants. example Vitis vinifera.

Vegetative reproduction: The underground and subaerial-modified stems help in vegetative repro(junction or propagation. example Runner in grass sto|ons |n Mentha sp., and tuber in potato.

Perennation: The function of perennation (survival during unfavorable environmental conditions) is found in underground modified stems such as corms, rhizomes, tubers, etc.

Floating: Specially modified stems of certain aquatic plants contain aerenchyma. This helps the plant to float on water.