Morphology of Flowering Plants: Root, Stem, Leaf, Inflorescence, Flower, Fruit, and Seed

1. The Root

The root is the descending part of the plant axis, usually found below the ground. Its primary functions are anchorage of the plant, absorption of water and minerals from the soil, and storage of reserve food materials. It also possesses certain other regulatory functions and helps in the synthesis of plant growth regulators.

1.1 Types of Root Systems

Root systems are primarily of two types: the taproot system and the fibrous root system.

Taproot System: In most dicotyledonous plants, the primary root, which grows directly from the radicle of the embryo, is called the taproot. It continues to grow for some time and produces lateral roots. These lateral roots branch into still finer branches called tertiary roots. The taproot, along with its branches, forms the taproot system. Examples include carrot, radish, and beetroot.

Fibrous Root System: In monocotyledonous plants, the primary root is short-lived and is soon replaced by a large number of roots that arise from the base of the stem. These roots originate from nodes and are collectively called the fibrous root system. Examples include wheat, rice, and maize.

Adventitious Roots: Roots that arise from parts of the plant other than the radicle are called adventitious roots. In grasses, the roots arising from the nodes of the stem are adventitious. In plants like money plant and Monstera, adventitious roots arise from nodes and internodes.

1.2 Modifications of Roots

Roots are modified to perform functions other than absorption and anchorage, such as storage, respiration, and support.

• For Storage: Taproots of some plants become fleshy and swollen to store food. Examples:
  • Conical: Carrot
  • Pencil-shaped: Raphanus sativus (Radish)
  • Napiform: Turnip
  • Tuberous: Sweet potato (though technically a modified adventitious root)
• For Respiration: In some halophytes (plants growing in saline soils), the roots grow vertically upward out of the soil, forming pneumatophores. These are cone-shaped structures with small pores called pneumatodes, which help in gaseous exchange. Example: Rhizophora.
• For Support: Adventitious roots can be modified to provide support.
  • Prop Roots: These are thick, pillar-like roots that grow downwards from the horizontal branches of a tree. Example: Banyan tree.
  • Stilt Roots: These arise from the lower nodes of the stem and grow obliquely downwards into the soil. Example: Maize, Sugarcane.
  • Climbing Roots: These roots arise from the nodes and help the plant to climb. Example: Money plant (Pothos), Betel (Piper betle).
  • Storage Roots: Modified adventitious roots that store food. Example: Sweet potato, Dahlia.
  • Haustoria: These are parasitic roots that penetrate the host tissue and absorb nutrients. Example: Cuscuta (Dodder).

2. The Stem

The stem is the ascending part of the plant axis, which develops from the plumule of the embryo. It bears leaves, flowers, and fruits. The stem is typically green when young and later becomes woody and dark brown. The main functions of the stem are to bear leaves, flowers, and fruits, and to conduct water, minerals, and photosynthates.

2.1 Modifications of Stems

Stems are modified to perform functions like storage, support, vegetative propagation, and protection.

• For Storage: Stems are modified to store food and water.
  • Underground Stems: Rhizomes (Ginger, Turmeric), Corms (Colocasia, Gladiolus), Tubers (Potato), Bulbs (Onion, Garlic). These often bear scale leaves, axillary buds, and adventitious roots.
  • Above-ground Stems: Stems modified for storage can also be above ground.
• For Support: Stems modified for climbing.
  • Tendrils: Leafy tendrils are modifications of axillary buds. Example: Grapevine, Cucumber.
  • Stem Spines: Sharp, pointed structures derived from axillary buds. Example: Bougainvillea, Citrus.
• For Photosynthesis: In some plants, the stem is modified to carry out photosynthesis.
  • Cladodes: Flattened, green stems that perform photosynthesis. Example: Asparagus. A single internode modified is a cladode.
  • Phylloclades: Green, flattened or cylindrical stems with distinct nodes and internodes, performing photosynthesis. Example: Opuntia (pads are modified stems), Cacti. These are usually found in xerophytes.
• For Vegetative Propagation: Stems modified to help in asexual reproduction.
  • Rhizomes: Ginger, Turmeric.
  • Runners: Horizontal stems that grow along the surface of the soil and produce new plants at the nodes. Example: Strawberry, Grass.
  • Suckers: Horizontal stems that grow beneath the soil surface and then emerge upwards, forming a new shoot. Example: Mint, Chrysanthemum.
  • Stolons: Similar to runners but generally arise from the base of the stem and grow horizontally, either above or below the ground. Example: Ferns, some grasses.
• For Protection: Stems modified into sharp, pointed structures.
  • Thorns: Modified axillary buds that are sharp and woody. Example: Rose, Duranta.
  • Spines: Modified leaves or stipules, but sometimes stem spines also occur. Example: Berberis (modified leaves).
• For Special Functions:
  • Phylloclade: In Opuntia, the stem is flattened and green, modified into a phylloclade for photosynthesis. The leaves are reduced to spines.
  • Cladode: In Asparagus, a branch is modified into a flattened, leaf-like structure called a cladode.

3. The Leaf

A leaf is a flattened, green outgrowth from the stem, arising from a node and bearing a bud in its axil. The axillary bud may develop into a branch or a flower. Leaves are the principal 'food factories' of the plant, responsible for photosynthesis. A typical leaf consists of three parts: leaf base, petiole, and leaf blade (lamina).

3.1 Parts of a Leaf

Leaf Base: The part of the leaf by which it is attached to the stem. In dicots, it often bears small leaf-like appendages called stipules. In monocots, the leaf base is often modified into a sheath that partially or wholly encloses the stem.

Petiole: A stalk that attaches the leaf blade to the stem. It helps to hold the leaf blade in a way that it gets maximum sunlight. Long, flexible petioles allow the leaf blade to flutter in the wind, which cools the leaf surface and exposes it to fresh air.

Leaf Blade (Lamina): The broad, green, expanded part of the leaf, which is the main surface for photosynthesis. It has a prominent midrib, from which lateral veins and further veinlets arise. The venation pattern is crucial for identification.

3.2 Venation

The arrangement of veins and veinlets in the leaf blade is called venation. Two types of venation are observed:

• Reticulate Venation: Veins form a network or reticulum. Typically found in dicotyledonous leaves. Example: Mango, Rose.
• Parallel Venation: Veins run parallel to each other within the lamina. Typically found in monocotyledonous leaves. Example: Grass, Banana, Wheat.

3.3 Types of Leaves

Leaves are classified based on the arrangement of their blades and venation.

• Simple Leaf: A leaf which has an undivided lamina, though it may be incised to any extent, but never down to the midrib or primary petiole. Example: Mango, Guava.
• Compound Leaf: In a compound leaf, the lamina is incised to such an extent that the midrib or primary petiole is completely cut into several leaflets.
  • Pinnately Compound Leaf: Leaflets arise from the tip of the rachis (an extension of the petiole). Example: Rose, Neem.
  • Palmately Compound Leaf: Leaflets arise from a common point at the tip of the petiole. Example: Silk cotton (Bombax).

3.4 Phyllotaxy

The arrangement of leaves on the stem is called phyllotaxy. It is mainly of three types:

• Alternate: A single leaf arises at each node, with the new leaves alternating on the stem. Example: China rose, Mustard, Sunflower.
• Opposite: A pair of leaves arises at each node. Example: Guava, Calotropis.
• Whorled: More than two leaves arise at each node, forming a whorl. Example: Alstonia.

3.5 Modifications of Leaves

Leaves are modified to perform functions other than photosynthesis, such as protection, storage, support, and trapping insects.

• For Protection: Leaves are modified into spines for protection against grazing animals and to reduce water loss. Example: Cacti, Berberis.
• For Support: The whole leaf or parts of it are modified into tendrils for climbing. Example: Peas (leaflets modified into tendrils), Smilax (entire leaf modified into tendrils).
• For Storage: Leaves store food. Example: Onion (fleshy leaves store food).
• For Trapping Insects: In insectivorous plants, leaves are modified to trap insects.
  • Pitcher Plant (Nepenthes): The leaf lamina is modified into a pitcher, and the leaf tip forms a lid.
  • Venus Flytrap (Dionaea muscipula): The leaf blade is modified into a trap with sensitive marginal hairs.
  • Sundew (Drosera): Leaves bear glandular tentacles that secrete sticky mucilage to trap insects.
• Phyllode: In some Australian Acacia species, the petiole is modified into a flattened, green structure that performs photosynthesis.
• Bracts: Modified leaves that are often brightly coloured and subtend flowers or inflorescences.

4. Inflorescence

Inflorescence is the arrangement of flowers on the floral axis. It is the part of the plant that bears the flowers. The main axis of the inflorescence is called the peduncle, and the stalk of an individual flower is called the pedicel.

4.1 Types of Inflorescence

Inflorescences are broadly classified into two main types: Racemose and Cymose.

• Racemose Inflorescence: The main axis continues to grow indefinitely, and the flowers are borne in an acropetal succession (older flowers at the base, younger flowers towards the apex). The flowers are stalked (pedicellate). Examples: Mustard, Radish, Solanum.
  • Raceme: Unbranched main axis with pedicellate flowers.
  • Spike: Similar to raceme, but the flowers are sessile (stalkless). Example: Achyranthes.
  • Spadix: A spike with a fleshy peduncle and a large, often colourful bract called a spathe. Example: Colocasia, Maize.
  • Catkin: A pendulous spike or raceme, usually bearing unisexual flowers. Example: Oak, Willow.
  • Compound Raceme (Panicle): The main axis is branched, and each branch bears a raceme of flowers. Example: Mango, Grasses.
  • Corymb: The main axis is short, and all the pedicellate flowers are borne at the same level due to the unequal length of pedicels (lower pedicels are longer). Example: Candytuft.
• Cymose Inflorescence: The main axis terminates in a flower, which is the oldest. Therefore, the growth of the main axis is limited. The flowers are borne in a basipetal succession (younger flowers towards the base, older flowers towards the apex). The central flower is borne on a pedicel, while the lateral flowers are sessile. Examples: Solanum, Jasmine, Dianthus.
  • Uniparous Cyme: The main axis produces only one lateral branch.
  • Biparous Cyme: The main axis produces two lateral branches.
  • Multiparous Cyme: The main axis produces more than two lateral branches.

4.2 Special Types of Inflorescence

Some inflorescences have unique structures:

• Hypanthodium: A fleshy, cup-shaped receptacle that encloses many flowers, both male and female. It is characteristic of the family Euphorbiaceae. Example: Ficus (e.g., Banyan, Fig).
• Cyathium: Found in Euphorbia species. It resembles a single flower but is actually an inflorescence. It consists of a cup-shaped involucre formed by fused bracts, enclosing a single female flower (pistillate) and several male flowers (staminate). Each male flower consists of a single stamen, and the female flower consists of a stalked ovary.

5. The Flower

The flower is the reproductive unit of angiosperms. It is essentially a modified shoot, where the apical meristem differentiates to produce floral organs instead of leaves. A typical flower is borne on a stalk called the pedicel. If the pedicel is absent, the flower is called sessile. The swollen tip of the pedicel where the floral organs are attached is called the receptacle.

5.1 Parts of a Typical Flower

A typical flower consists of four whorls of floral organs, arranged successively on the receptacle.

• Calyx (Sepals): The outermost whorl, consisting of sepals. Sepals are usually green, leaf-like structures that protect the flower in the bud stage. If all sepals are fused, it is called gamosepalous. If they are free, it is called polysepalous.
• Corolla (Petals): The second whorl, consisting of petals. Petals are often brightly coloured to attract pollinators. If all petals are fused, it is called gamopetalous. If they are free, it is called polypetalous.
• Androecium (Stamens): The third whorl, consisting of stamens, which are the male reproductive organs. Each stamen consists of a stalk (filament) and an anther. The anther produces pollen grains. If stamens are fused, various types of fusion occur:
  • Adelphous: Stamens fused to form bundles. E.g., Monadelphous (one bundle, e.g., China rose), Diadelphous (two bundles, e.g., Pea), Polyadelphous (many bundles, e.g., Lemon).
  • Syngenesious: Anthers fused, filaments free. E.g., Asteraceae family (Sunflower).
  • Gynandrous: Stamens fused with the pistil.
• Gynoecium (Pistil/Carpels): The innermost whorl, consisting of pistils or carpels, which are the female reproductive organs. Each pistil typically consists of three parts: stigma (the receptive tip for pollen), style (a stalk connecting the stigma to the ovary), and ovary (the swollen basal part containing ovules).
  • Pistil: The female reproductive organ.
  • Carpel: A unit of the gynoecium. A flower may have one or more carpels.
  • Monocarpous: Gynoecium consists of a single carpel.
  • Polycarpous: Gynoecium consists of many carpels.
  • Syncarpous: Carpels are fused.
  • Apocarpous: Carpels are free.

5.2 Types of Flowers

Flowers can be classified based on symmetry and the presence of floral organs.

• Symmetry:
  • Actinomorphic: Radial symmetry. A flower can be divided into two equal halves by any radial plane passing through the centre. Example: Mustard, Petunia.
  • Zygomorphic: Bilateral symmetry. A flower can be divided into two equal halves only by a single plane. Example: Pea, Bean, Gulmohar.
  • Asymmetrical: Irregular symmetry. A flower cannot be divided into two equal halves by any plane. Example: Canna.
• Presence of Floral Organs:
  • Trimerous: Floral parts arranged in multiples of three. Common in monocots.
  • Tetramerous: Floral parts arranged in multiples of four. Common in dicots.
  • Pentamerous: Floral parts arranged in multiples of five. Common in dicots.
• Sexuality:
  • Unisexual: Flowers having either stamens or pistils, but not both. Male flowers have stamens only (staminate flowers), and female flowers have pistils only (pistillate flowers).
  • Bisexual: Flowers having both stamens and pistils. Example: Hibiscus, Mustard.
  • Monocious: Unisexual flowers are borne on the same plant. Example: Maize (male and female flowers on the same plant).
  • Dioecious: Unisexual flowers are borne on different plants. Example: Papaya, Date Palm.
• Position of Gynoecium:
  • Hypogynous: The gynoecium is situated at the highest level, while the other floral parts are below it. The ovary is superior. Example: China rose, Pea, Cucumber.
  • Perigynous: The gynoecium is situated in the centre, and other floral parts are on the rim of the ovary, almost at the same level. The ovary is half-inferior. Example: Rose, Peach, Plum.
  • Epigynous: The gynoecium is situated at the lowest level, and the other floral parts are above the ovary. The ovary is inferior. Example: Sunflower, Apple, Guava.

5.3 Placentation

Placentation is the arrangement of ovules within the ovary. The point where the placenta attaches to the inner wall of the ovary is called the placenta.

• Marginal: Ovary is unilocular, and placenta forms a ridge along the ventral suture. Ovules are borne on this ridge. Example: Pea, Bean.
• Axile: Ovary is bilocular or multilocular. The placenta develops from the base and grows upwards, meeting in the centre. Ovules are borne on the placenta at each angle. Example: China rose, Lemon, Tomato.
• Parietal: Ovary is unilocular but becomes bilocular due to the formation of a false septum. Placenta develops on the inner wall of the ovary. Ovules are borne on the placenta. Example: Mustard, Argemone.
• Free Central: Ovary is unilocular, and ovules arise from the centre of the ovary, not attached to the wall. Example: Dianthus, Primrose.
• Basal: Ovary is unilocular, and a single ovule is borne at the base of the ovary. Example: Sunflower, Marigold.

6. The Fruit

The fruit is the mature ovary, developed after fertilization. It typically consists of a pericarp (wall of the ovary) and seeds. The pericarp can be differentiated into the outer epicarp, middle mesocarp, and inner endocarp.

6.1 Types of Fruits

Fruits are classified into three main types based on their origin and structure:

• Simple Fruits: Develop from a monocarpellary ovary or a flower with a superior ovary. They can be fleshy or dry.
  • • Fleshy Simple Fruits:
      • Berry: Fleshy pericarp, usually containing many seeds. Example: Tomato, Grape, Guava, Chilli.
      • Drupe: Fleshy pericarp, with a hard, stony endocarp enclosing a single seed. Example: Mango, Peach, Plum, Coconut (fibrous mesocarp).
      • Pome: Fleshy part develops from the thalamus, not the ovary wall. Ovary is inferior. Example: Apple, Pear.
      • Hesperidium: A modified berry with a leathery rind, typically found in citrus fruits. Example: Orange, Lemon.
      • Pepo: A modified berry with a hard, thick rind, characteristic of the gourd family. Example: Watermelon, Cucumber, Pumpkin.
    • Dry Simple Fruits: The pericarp is dry and may be dehiscent or indehiscent.
      • Dehiscent Dry Fruits: Fruits that split open at maturity to release seeds.
        • Legume: Develops from a monocarpellary, unilocular ovary with marginal placentation. Splits along both sutures. Example: Pea, Bean.
        • Capsule: Develops from a syncarpous ovary. Splits in various ways. Example: Okra, Cotton.
        • Siliqua: Develops from a bicarpellary, syncarpous ovary with parietal placentation. Splits along both sutures, leaving a central partition called the replum. Example: Mustard.
      • Indehiscent Dry Fruits: Fruits that do not split open at maturity.
        • Achene: Unilocular, superior ovary with a single seed. Pericarp is free from the seed coat. Example: Clematis.
        • Caryopsis: Unilocular, superior ovary with a single seed. Pericarp is fused with the seed coat. Characteristic of grasses. Example: Wheat, Rice, Maize.
        • Samara: An achene with a wing-like outgrowth of the pericarp. Example: Maple, Ash.
        • Nut: A unilocular, superior ovary with a single seed, enclosed by a hard, stony pericarp, often surrounded by a husk. Example: Hazelnut, Acorn.
• Aggregate Fruits: Develop from a single flower having multiple ovaries (apocarpous gynoecium). Each ovary develops into a small fruitlet, and these fruitlets together form an aggregate fruit. Example: Raspberry, Strawberry (a false aggregate fruit where the fleshy part is the thalamus).
• Composite or False Fruits: Develop from an entire inflorescence or from more than one flower.
  • Sorosis: Develops from a racemose inflorescence. The peduncle becomes fleshy and fuses with the ovaries. Example: Pineapple, Mulberry.
  • Syconus: Develops from a hypanthodium inflorescence. The receptacle becomes fleshy and cup-shaped, enclosing many achenes. Example: Ficus species (Fig).
• Parthenocarpic Fruits: Fruits that develop without fertilization. These fruits are seedless. Example: Banana, Grapes, Orange.

7. The Seed

The seed is the final product of sexual reproduction in angiosperms. It is a mature ovule, containing an embryo, endosperm (food reserve), and is enclosed within a protective seed coat.

7.1 Structure of a Typical Seed

A typical seed consists of:

• Seed Coat: The protective outer covering of the seed, derived from the integuments of the ovule. The outer coat is called the testa, and the inner coat is called the tegmen. The seed coat is usually hard and protects the embryo from mechanical injury, desiccation, and pathogens.
• Embryo: The embryonic plant, consisting of an embryonic axis and one or two cotyledons.
  • Embryonic Axis: Consists of the radicle (embryonic root), plumule (embryonic shoot), and one or two cotyledons.
  • Cotyledons: Fleshy structures that store food or absorb food from the endosperm.
• Endosperm: A nutritive tissue formed during double fertilization. It stores food reserves, which are utilized by the developing embryo during germination.
  • Albuminous Seeds: Seeds that retain a persistent endosperm at maturity. Example: Maize, Wheat, Castor.
  • Non-albuminous Seeds: Seeds where the endosperm is completely consumed by the developing embryo during its development. The cotyledons store food. Example: Pea, Bean, Groundnut.

7.2 Structure of Dicotyledonous and Monocotyledonous Seeds

Dicotyledonous Seed (e.g., Bean Seed):

• Two prominent, fleshy cotyledons that store food.
• The seed coat consists of the tough testa and the delicate tegmen.
• On the testa, there are two distinct parts: a scar called the hilum, where the seed was attached to the placenta, and a small pore called the micropyle, located above the hilum.
• The embryo consists of an embryonic axis with a radicle at one end and a plumule at the other.
• The radicle is enclosed in a protective sheath called the coleorhiza.
• The plumule is enclosed in a protective sheath called the coleoptile.
• The endosperm is usually absent or consumed by the cotyledons.

Monocotyledonous Seed (e.g., Maize Grain):

• It is a single-seeded fruit (caryopsis) where the pericarp is fused with the seed coat.
• The seed is albuminous, with a large, starchy endosperm.
• A protective sheath for the plumule is called the coleoptile.
• A protective sheath for the radicle is called the coleorhiza.
• A single, shield-shaped cotyledon called the scutellum is present, which absorbs food from the endosperm and transfers it to the embryo.
• The scutellum is located in a lateral position.

7.3 Seed Dormancy

Seed dormancy is a condition where a viable seed fails to germinate under favourable conditions. This dormancy can be due to various factors, such as an impermeable seed coat, the presence of germination inhibitors, or the immaturity of the embryo.

7.4 Significance of Seeds

Seeds play a crucial role in the life cycle of plants and human life.

• Dispersal: Seeds facilitate the dispersal of plants to new areas.
• Survival: Dormant seeds can survive adverse environmental conditions, germinating when conditions become favourable.
• Food Source: Seeds are a vital source of food for humans and animals (cereals, pulses, oilseeds).
• Breeding: Seeds are essential for plant breeding programs.
• Economic Importance: Many industries rely on seeds (e.g., oil industry, food processing).