The body of a typical flowering plant can be divided into the underground root system and aerial shoot system. The shoot system is heterogeneous.  The shoot system (stem) is an aerial and erect part of the plant body that grows upwards. It is usually above the soil and develops from the plumule of the embryo of a germinating seed. It consists of a stem, branches, leaves, flowers, fruits, and seeds. In this article, we shall very important of a plant, the leaf.

The leaf is lateral, generally flattened structure borne on the stem. A leaf may be defined as “A dorsoventrally compressed, lateral appendage of the stem, produced at the nodes and is specialized to perform photosynthesis. It develops at the node and bears a bud in its axil. The axillary bud later develops into a branch. Leaves originate from shoot apical meristems and are arranged in an acropetal (outward) order. They are the most important vegetative organs for photosynthesis.

Characteristics of Leaf:

• The leaf is a thin, expanded, green structure. The green colour is due to the presence of chlorophyll pigment.
• An auxiliary bud is present at the axil of each leaf.
• It is borne on the stem at the node, hence it is exogenous.
• It does not contain apical bud required for continuous growth hence it has limited growth.
• The lamina possesses prominent vascular strands called veins.
• Leaf bears abundant stomata for the exchange of gases.

The Structure of the Leaf:

A typical leaf consists of three main parts: leaf base, petiole, and lamina. The leaf is attached to the stem by the leaf base (hypo-podium) and may bear two lateral small leaf-like structures called stipules. The leaves with stipules are called stipulate leaves and the leaves without stipules are called ex-stipulate leaves. The main functions of stipules are to protect the bud and carry out photosynthesis. In monocotyledons, the leaf base expands into a sheath covering the stem partially or wholly. In some leguminous plants, the leaf base may become swollen, which is called the pulvinus. It protects the young axillary bud.

The petiole (mesopodium) is a cylindrical or sub-cylindrical smooth or grooved stalk of the leaf which lifts the lamina above the level of the stem.  Leaves that possess petiole are called petiolate leaves and those without petioles are called non-petiolate or sessile leaves. In papaya the petiole is hollow. The petiole help to hold the blade to light and also in conduction. Long thin flexible petioles allow leaf blades to flutter in wind, thereby cooling the leaf and bringing fresh air to the leaf surface.

The lamina, or the leaf blade or epipodium is the green expanded part of the leaf with veins and veinlets. There is, usually, a middle prominent vein, which is known as the midrib. Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals and food materials. The shape, margin, apex, surface, and extent of incision of lamina varies in different leaves. The main functions of the lamina are photosynthesis and transpiration.

In dicots, the leaves are dorsiventral because its dorsal and ventral surfaces are structurally different. In monocots the dorsal and ventral surfaces of leaves are structurally identical, hence they are called isobilateral.

In some plants the shape and form of leaves are such that it is difficult to distinguish between the two surfaces, such leaves are called centric or cylindrical leaves. e.g. onion, garlic, etc.

Venation:

The arrangement of veins and the veinlets in the lamina of a leaf is termed as venation. Veins and veinlets are skeletal as well as conducting prominences visible on the surface of the lamina, especially the under surface in dorsiventral leaves.

Functions of Venation:

• Venation provides skeletal support to the lamina so that it can remain stretched for its optimum functioning,
• Veins and veinlets reduce the effect of wilting.
• They are important for the conduction of water and nutrients.

Types of Venation:

Reticulate Venation:

When the veinlets form a network, the venation is termed as reticulate. e.g. leaves of dicots,  Peepal (Ficus religiosa) (पीपल), Shoe-Flower (Hibiscus rosasinensis) (जास्वंदी), etc.(exceptions: Calophyllum, Corymbium, Eryngium).  It is further divided into two types.

Pinnate or Unicostate Reticulate Venation: 

The lamina has a single principal vein or midrib which extends from its base to apex. It gives rise to lateral veins along its entire length. Veins bear veinlets. The veinlets form reticulations. e.g. Mano, Peepal.

Palmate or Multicostate Reticulate Vena­tion: 

A number of prominent or principal veins arise from the tip of the petiole and reach either the apex or margins of the lamina. They give rise to lateral veins connected by reticulations of veinlets. It may be convergent (as in Ziziphus (बेर), Smilax) or divergent ( as in grapevine, lufia).

Parallel Venation:

When the veins run parallel to each other within a lamina, the venation is termed as parallel venation. There is a single principal vein or midrib that runs from base to the apex of the lamina. The lateral veins run parallel to one another without forming anastomoses e.g. leaves of monocots, maize, grass, banana (Musa paradisiaca), canna. etc. It is further divided into two types.

Pinnate or Unicostate parallel Venation: 

There is a single principal vein or midrib that runs from base to the apex of the lamina. The lateral veins run parallel to one another without forming anastomoses e.g. banana, canna

Palmate or Multiicostate convergent parallel Venation: 

Several parallel principal veins arise from the base of the lamina and converge towards the apex, e.g., Bamboo, Grass, etc.

Palmate or Multiicostate Divergent parallel Venation: 

Several parallel principal veins arise from the base of the lamina and proceed towards the margins, e.g., Fan Palm (Livistonia).

Furcate Venation:

The veins branch dichotomously. The finer branches do not form a reticulum. (e.g., ferm Adiantum, Circeaster)

Types of Leaves:

A leaf is said to be simple, when its lamina is entire or when incised, the incisions do not touch the midrib.

When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called a compound.

A bud is present in the axil of petiole in both simple and compound leaves, but not in the axil of leaflets of the compound leaf.

Types of Compound Leaves:

Pinnately Compound Leaves:

In a pinnately compound leaf, a number of leaflets are present on a common axis, the rachis, which represents the midrib of the leaf as in neem. They are further classified as

Unipinnate Compound Leaves: If the leaflets arise on primary rachis itself then it is known as unipinnate compound leaf. If the number of leaflets is odd it is known as imparipinnate compound leaf and if it is an even then it is known as a paripinnate compound leaf. Example: Azadirachta indica (Neem)

Bipinnately Compound Leaves: In this type, the primary rachis gets branched once and the leaflet arises on the secondary rachis. Example: Mimosa pudica (Touch me not)

Tripinnately Compound Leaves: In this type, the rachis gets branched twice and the leaflet arises on the tertiary rachis. Example: Moringa oleifera (drumstick)

Decompound Compound Leaves:

In this type, the rachis gets branched thrice or more than thrice and the leaflets arise on the ultimate branches. Example: Coriandrum sativum (coriander)

Palmately Compound Leaves:

In palmately compound leaves, the leaflets are attached at a common point, i.e., at the tip of the petiole, as in silk cotton.  On the basis of the number of leaflets they are further classified as

• Unifoliate Palmately Compound Leaves: With a single leaflet. Examples: Citrus limon (lemon), Citrus maxima (papanas).
• Bifoliate Palmately Compound Leaves: With two leaflets. Example: Bauhinia Yunnanensis (butterfly tree / kanher).
• Trifoliate Palmately Compound Leaves: With three leaflets. Example: Clover (Ran methi).
• Quadrifoliate Palmately Compound Leaves: With four leaflets. Example: Oxalis, MarsileaMultifoliate Palmately Compound Leaves: With more than four leaflets. Examples: Baobab, Umbrella plant.

Phyllotaxy:

Phyllotaxy is the pattern of arrangement of leaves on the stem or branch. This is usually of three types alternate, opposite and whorled.

• In an alternate type of phyllotaxy, a single leaf arises at each node in an alternate manner, as in China rose, mustard and sunflower plants.
• In the opposite type, a pair of leaves arise at each node and lie opposite to each other as in Calotropis and guava plants.
• If more than two leaves arise at a node and form a whorl, it is called whorled, as in Alstonia.

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In this article, we shall study the modification in stem in some plants for performing the functions other than its primary function. Primary functions of the stem are

• To support and orient the leaves in a manner that they are exposed to maximum sunlight and for efficient gaseous exchange during photosynthesis and respiration.
• To conduct water and minerals from roots to leaves and manufactured food from leaves to different parts of the plant.
• To bear flowers and fruits

Some stems perform the function of storage of food, support, protection, and vegetative propagation. The secondary functions of the stem are

Underground Modification of Stem:

Since underground, they may seem like roots but they have characteristics of the stem, like the presence of nodes and internodes, scaly non-green leaves and buds. This modification serves two functions it acts as perennating structures by remaining leafless and dormant in winter but giving off aerial shoots under favourable conditions (next season) and Store food and become thick and fleshy.

Rhizome:

It is a prostrate, dorsoventrally thickened brownish stem, which grows horizontally under the surface of the soil. It shows distinct nodes and internodes. It shows distinct nodes and internodes. It bears scale leaves on nodes, It possesses terminal bud and axillary buds in the axil of each scale-leaf present at the node. Adventitious roots are present. e.g. Ginger (अद्रक), Turmeric (हलदी).

Stem Tuber:

Tubers are actually the swollen tips of ends of special underground branches swollen due to the storage of food as starch. The tubers show nodes and internodes. Nodes bear scale leaves with axillary buds, commonly called eyes. Under favourable conditions eyes sprout and produce aerial roots. Thus tubers help in vegetative propagation. They do not produce adventitious roots. e.g. Potato (आलू).

Bulb:

It is a condensed disc like an underground stem. The upper surface of the disc-like stem is conical and bears centrally placed apical bud and many concentrically arranged overlapping scale leaves. Scale leaves store food.  When the scale leaves surround the apical bud in the form of concentric rings, it is called a tunicated bulb. The lower surface of the stem produces adventitious roots. e.g. onion (प्याज).

When the scale leaves partially surround the apical bud by overlapping each other, it is called a scaly bulb. The lower surface of the stem produces adventitious roots. e.g. garlic (लहसुन).

Corm:

It is condensed disc-like underground, fleshy, spherical stem with flattened base, It grows vertically, bears many scale leaves, distinct nodes and internodes, buds and adventitious roots. e.g.  Saffron (केसर), yam (जिमीकंद), gladiolus.

Sub Aerial Modification Of Stem:

Stems are weak, therefore lie prostrate on the ground or may get partially buried in the upper layer of soil. The plants bearing such stems are called creepers. Their stems serve the function of vegetative propagation.

Runner:

The basal internodes of the bud elongate horizontally and trail along the soil carrying the bud to a distance from the mother plant where it gets fixed to the soil by means of adventitious roots and develops a new daughter plant. This branch carrying the bud is called a runner.  It grows in all the direction and a single plant soon covers a large area by its pro­geny. e.g. Grass, Oxalis, Centella asiatica, strawberry, etc.

Stolon:

When a weak lateral branch which grows upwards then arches down to meet the soil, strike roots and produce daughter plants. The difference between the runner and stolon is, runner grows horizontally, while stolen grow obliquely upward and then arches to the ground. e.g. Mint (‘Pudina’), Jasmine.

Offset:

Like runner but thicker and shorter, grow for a short distance then produce cluster (rosette) of leaves above and adventitious roots below; generally in aquatic plants. It is just like the runner, only it is shorter and thicker. e.g. water hyacinth (Eichhornia crassipes), pistia (Pistia stratiotes), water lettuce

Sucker:

An underground runner that grows horizontally for a distance under soil then emerges obliquely upwards, strikes roots and forms daughter plants. e.g. Chrysanthemum, Mentha arvensis, banana, pineapple etc.

Aerial Modification of Stem:

The whole stem or its part (axillary or terminal bud) gets modified to perform definite functions. It is a stem because they show characteristics like a) Arise in the axil of leaf b) Bear nodes and internodes c) may bear leaves, buds, flowers.

Stem Tendrils:

Stem or its branches get modified into green threadlike, spirally coiled leafless structures called tendrils which are meant for climbing. They twine around neighbouring objects and help weak plants to climb. These may be branched or unbranched. A scale leaf is always present at the point of branching of the tendril. e.g. Grapevine, cucumber, pumpkins, watermelon, etc.

Thorns:

These are straight, pointed, hard or woody structures sometimes they bear leaves, flowers or may be branched. Axillary buds of stems get modified into thorns (e.g. Citrus, Duranta, and Aegel). In Carrissa, terminal buds get modified into thorns. Thorns are used as organs of defence against grazing animals or climbing (e.g. Bougainvillea) and to check transpiration.

Phylloclade:

These are fleshy, green flattened or cylindrical branches of unlimited growth with nodes and internodes. The leaves are modified into spines or scales to check transpiration. This modification of stem is observed in plants growing in dry regions. The stem takes part in photosynthesis and stores water. e.g. Opuntia, Euphorbia, Casuarina, Cocoloba etc.

Cladode or Cladophylls:

It is a phylloclade with limited growth i.e. with only one or two internodes; help in photosynthesis. These are a green cylindrical or flattened leaf-like branches. In Asparagus, the cladodes are one internode long and in Ruscus, the cladodes are two internodes long. They help in photosynthesis.

Bulbil:

These are modified vegetative or floral buds with stored food and meant for vegetative propagation. In Dioscorea, bulbils are condensed axillary buds while in Agava and lily the floral buds develop into bulbil. They detach to develop into a new plant.

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The body of a typical flowering plant can be divided into the underground root system and aerial shoot system. The shoot system is heterogeneous. The shoot system (stem) is an aerial and erect part of the plant body that grows upwards. It is usually above the soil and develops from the plumule of the embryo of a germinating seed. It consists of stem, branches, leaves, flowers, fruits, and seeds.

The Stem:

The stem is negatively geotropic (moves above the soil), negatively hydrotropic (moves away from the water) and positively phototropic (moves towards the light). Branches arise from axillary buds present in the axil of leaves. Each axillary bud is a small, compact, underdeveloped shoot covered with a large number of overlapping leaf primordia. Internodes of this bud enlarge and develop into a branch. Therefore the development of branches is exogenous (exo = outside).

Characteristics of Stem:

• Stem arises as a prolongation of plumule (one end of an embryo).
• The shoot system is heterogeneous and consists of stem, branches, leaves, and flowers.
• The stem is negatively geotropic (moves above the soil), negatively hydrotropic (moves away from the water) and positively phototropic (moves towards the light). 
• The stem bears nodes and internodes. The regions of the stem where leaves are born are called nodes while internodes are the portions between two nodes.
• The stem bears vegetative buds which could be terminal (apical bud) for the plant to grow upwards or axillary (bud in the axil of the leaf) which give rise to lateral branches. The stem bears floral buds (terminal or axillary) that grow into flowers
• The young stem is green in colour and thus it is photosynthetic. The mature stem is generally green when young and later often become woody and dark brown.
• Lateral branches of the stem are exogenous in origin.

Functions of the Stem:

Primary functions of the stem are

• To support and orient the leaves in a manner that they are exposed to maximum sunlight and for efficient gaseous exchange during photosynthesis and respiration.
• To conduct water and minerals from roots to leaves and manufactured food from leaves to different parts of the plant.
• To bear flowers and fruits

Some stems perform the function of storage of food, support, protection and vegetative propagation. The secondary functions of the stem are

• Storage:  Some stems store food and water in some plants e.g. potato
• Perennation: The underground stems help tide over the unfavourable growing periods e.g. ginger, turmeric.
• Vegetative propagation: e.g. rose, and sugarcane the stem can be used for vegetative propagation.
• Photosynthesis: In xerophytes (desert plants) the leaves are reduced to thorn, the stem possessing chlorophyll takes up the function of photosynthesis. e.g.Opuntia
• Protection: In some plants, the axillary bud modifies into the thorn and protects the plants from grazing animals e.g. citrus, Duranta.
• Support, Climbing and Clinging: Tendrils or hooks are modified branches or buds. They coil around
  the support and help the plant to climb e.g. grapevine.

Types of Stem: 

The stem may be

• aerial (erect, rigid, strong and upright as in herbs, shrubs, and trees) or
• subaerial (weak, unable to stay upright and trail on the ground as creepers or climb up as climbers) or
• underground (buried in soil and produces aerial branches under favourable conditions only

Branching of Stem:

There are three main forms of branching.

• Excurrent:  the main trunk goes the entire height of the tree, with branches forming patterns; e.g. Evergreens, pinus.
• Decurrent: The main trunk continues up about halfway, then splits into more than one main branch; eg. fruit trees.
• Columnar or Coudex: The main trunk continues the full height of the tree, with the branches forming only at the top; eg. Palm trees, coconut tree.

Lateral Branching:

Branching is racemose or cymose according as the lateral buds are less vigorous or more vigorous than the apical bud.

Racemose Branching:

In racemose or monopodial branching the apical bud of the plant grows indefinitely giving rise to a straight stem axis or podium on which the lateral buds grow in acropetal succession, i.e. older branches are borne near the base and younger branches towards the apex.

This type of branching is the commonest among young Phanerogams. as the plant grows older, the apical bud often gets lost and some side branches become very strong so that the plant loses its monopodial character.

Instead of there being a single leaf at each node there be a whorl of leaves, the branches at each node will also be whorled. Then it is called whorled racemose.

Types of Buds:

Small lateral or terminal protuberance on the stem of a vascular plant that may develop into a flower, leaf, or shoot is called bud. Buds arise from meristem tissues. Flower buds are modified leaves. Buds are classified as follows:

Types of Buds on the Basis of Their Function:

Vegetative buds:

They contain embryonic leaves or shoots. They grow into new branches and shoots.

Floral buds:

They contain embryonic flowers. They grow into flowers. They are also referred as reproductive buds.

Mixed buds:

They contain both embryonic leaves and flowers. They may gro into vegetative parts like soot or leaf or into a flower part.

Tendrillar buds:

They grow into tendrils.

Types of Buds on the Basis of Its Position:

On the basis of the position of the buds, they are classified as terminal or apical buds and lateral buds.

Terminal or apical buds:

They develop at the apex of the main stem or branch. e,g, cabbage is a large apical bud

Lateral buds:

They develop into lateral branches or flowers. They are further classified as axillary buds, accessory buds, and sub-petiolar buds. Axillary buds are lateral buds that arise in the axil of the leaf, e.g. Sunflower, Citrus, Rose. Accessory buds are more than one bud that arises in the axil of a single leaf. e.g. Cucurbita, Luffa, Brinjal, Chilly, Bougainvillaea. Subpetiolar buds are axillary buds those are covered by a sheathing leaf base and appear to arise from the leaf base. As they remain dormant for a quite long time they are also referred as dormant buds. When they gro they are referred as sprouting buds. e.g. Dalbergia sissoo (Shisam) and Mulberry.

Bud Scales:

In some plants, buds are covered by overlapping scales, these scales are called bud scales. These bud scales are tough and may be modified leaves, leaf bases, petioles or stipules. e.g. Jackfruit, Ficus, and Magnolia.

Winter Buds:

In extremely cold regions plants bear specially protected buds called winter buds. During cold period leaves fall but the winter buds remain intact. During spring the bud opens with developing young leaves. e.g. Cauliflower, Cabbage.

Adventitious Buds:

They arise at a position other than their normal position. Epiphyllous or foliar buds arise on leaves. e.g. Begonia, Bryophyllum, etc. Cauline buds arise from the cut or pruned end of branches. e.g. Sweet potato, Coffee, Aegle, etc. Radical buds develop on the roots. e.g. Sweet potato, Dahlia, Dalbergia.

Turions Buds: 

A turion bud is a resistant plant bud that is found in certain aquatic plants and can allow the plant to survive winter in the vegetative state. e.g. Ultricularia, Potamogeton.

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