Morphology is the study of the external form, size, shape, structure and relative arrangement of different organs of a plant. The flowering plants, or angiosperms, are the most evolved plants, and understanding their morphology is essential for the correct identification, classification and study of any plant species. The angiosperm plant body is typically differentiated into an underground root system and an aerial shoot system. The shoot system consists of stems, leaves, flowers and fruits. The study of these external features forms the basis of plant taxonomy and helps agronomists and horticulturists in crop improvement.
The morphology of flowering plants is described in relation to the vegetative organs (root, stem and leaf) and the reproductive organs (flower, fruit and seed). Flowers are the reproductive organs that give rise to fruits and seeds. The diversity seen in roots, stems, leaves and flowers across different families is enormous, but certain standard patterns can be recognised and studied. This chapter provides a detailed account of the structure, types and modifications of each organ, along with a comparative study of three economically important families, which is also called descriptive botany.
The root system is the underground, positively geotropic part of the plant that grows downwards. In dicotyledonous plants, the radicle elongates to form the primary root, which gives rise to lateral roots called secondary and tertiary roots. This type of root is called a tap root, e.g., gram, mustard. In monocotyledonous plants, the primary root is short-lived and is replaced by a large number of roots arising from the base of the stem, forming a fibrous root system, e.g., wheat, rice, maize. In some plants like grass and banyan, roots arise from parts of the plant other than the radicle; these are called adventitious roots. Fibrous roots and adventitious roots are also adventitious in origin. The root apex is covered by a thimble-like structure called the root cap, which protects the root tip. Roots absorb water and minerals from the soil, anchor the plant firmly, and sometimes store reserve food material.
The root tip is divided into distinct regions: the root cap at the tip, the region of meristematic activity (cells divide actively), the region of elongation (cells elongate and enlarge), and the region of maturation where root hairs develop and differentiation of tissues takes place.
Roots are modified to perform special functions: - Storage roots: Tap roots of carrot, radish, turnip and beet become swollen to store food. - Prop roots: In the banyan tree, hanging roots grow downwards and act as pillars supporting heavy branches. - Stilt roots: In maize and sugarcane, roots arise from the lower nodes of the stem and provide extra support. - Respiratory roots (pneumatophores): In mangroves like Rhizophora, roots grow vertically upwards to get oxygen for respiration in swampy areas. - Parasitic roots (haustoria): In Cuscuta, the parasite, roots penetrate the host plant to absorb water and nutrients.
The stem is the aerial, positively phototropic, ascending axis of the plant that develops from the plumule. It bears leaves, branches, buds, flowers and fruits. The stem is differentiated into nodes (where leaves arise) and internodes (regions between two nodes). The stem conducts water, minerals and photosynthates, provides support to leaves, flowers and fruits, and in some cases stores food and performs photosynthesis.
Stems are modified to perform functions other than support and conduction: - Underground stems: These store food and help in perennation. Examples: potato (tuber), ginger (rhizome), onion (bulb), garlic (bulb), Colocasia (corm), and zaminkand. These structures have nodes, internodes, buds and sometimes scale leaves, proving they are modified stems. - Tendrils: In plants like grapes and gourds, stems are modified into wiry, coiled structures for climbing. - Thorns: In Bougainvillea and Citrus, axillary buds are modified into pointed, woody thorns for protection. - Suckers: In mint and Chrysanthemum, underground stems grow horizontally and develop new plants. - Stem tendrils, phylloclades (flattened photosynthetic stems, e.g., Opuntia) and runners (strawberry) are other modifications.
The leaf is the lateral, flattened, generally green structure that develops from a node of the stem. Leaves are the most important photosynthetic organs. A typical leaf consists of three main parts: the leaf base, the petiole and the lamina (leaf blade). The leaf base attaches the leaf to the stem. In many legumes, the leaf base becomes swollen and is called the pulvinus. The petiole connects the leaf base to the lamina. Leaves without a petiole are called sessile. The lamina is the green, expanded part bearing veins.
Venation is the arrangement of veins and veinlets in the leaf lamina. - Reticulate venation: Veins form a network. Found in dicot leaves, e.g., mango, pea. - Parallel venation: Veins run parallel to each other. Found in monocot leaves, e.g., maize, grass.
Phyllotaxy is the pattern of arrangement of leaves on the stem or branch. - Alternate: One leaf per node, e.g., sunflower, China rose. - Opposite: Two leaves per node, e.g., Calotropis, guava. - Whorled: More than two leaves per node, e.g., Alstonia.
Leaves are modified for: - Tendrils: In peas, the terminal leaflets are modified into tendrils for climbing. - Spines: In cacti, leaves are modified into spines for protection and to reduce water loss. - Storage: Leaves of onion and garlic store food. - Insectivorous leaves: In pitcher plant, bladderwort and Venus flytrap, leaves are modified to catch and digest insects.
The inflorescence is the arrangement of flowers on the floral axis. There are two main types. In racemose inflorescence, the main axis continues to grow and flowers are borne laterally in acropetal succession (older flowers towards the base, younger at the top), e.g., radish, mustard. In cymose inflorescence, the main axis terminates in a flower and growth is basipetal (older flowers at the top, younger towards the base), e.g., jasmine, cotton.
The flower is the reproductive unit in angiosperms. A typical flower has four whorls arranged on the swollen end of the stalk, called the receptacle or thalamus. The whorls are the calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels). A flower with all four whorls is complete; one lacking any whorl is incomplete. Flowers with both stamens and carpels are bisexual, while those with only one are unisexual.
Placentation is the arrangement of ovules within the ovary. - Marginal: Ovules on the margin of a single placenta (ovary one-chambered), e.g., pea. - Axile: Ovules on the central axis of a multi-chambered ovary, e.g., China rose, tomato. - Parietal: Ovules on the inner wall of the ovary or on peripheral outgrowths, e.g., mustard. - Basal: Ovules at the base of the ovary, e.g., sunflower, marigold. - Free central: Ovules on the central axis without septa, e.g., Dianthus, primrose.
A fruit is the mature, ripened ovary developed after fertilisation. If a fruit develops without fertilisation, it is called a parthenocarpic fruit (e.g., banana). The fruit consists of the fruit wall called the pericarp and the seeds. The pericarp has three layers: the outer epicarp, the middle mesocarp and the inner endocarp. In fleshy fruits, the mesocarp is juicy. Fruits may be true (developed only from the ovary) or false (developed from parts other than the ovary, e.g., apple, strawberry, cashew).
A seed is the fertilised ovule. A seed consists of a seed coat (testa and tegmen), an embryo (plumule, radicle and one or two cotyledons), and sometimes endosperm. Seeds are classified as albuminous (with endosperm) and exalbuminous (without endosperm). In dicot seeds like bean and gram, the embryo is enclosed between two fleshy cotyledons. In monocot seeds like maize, the endosperm is present and the embryo has one cotyledon called the scutellum. In some seeds like those of cereals, the outer covering is the fruit wall fused with the seed coat, forming a husk.
Vegetative features: Tap root system with root nodules containing nitrogen-fixing bacteria (Rhizobium); stem erect or climber; leaves pinnately compound or simple; leaf base pulvinate; venation reticulate. Floral features: Inflorescence racemose; flower bisexual, zygomorphic; calyx five gamosepalous; corolla five polypetalous with vexillary aestivation (largest petal, the standard, overlaps the wings and keel); androecium ten diadelphous (nine fused, one free); gynoecium monocarpellary with marginal placentation. Fruit: legume (pod), e.g., gram, pea, bean, soybean.
Vegetative features: Herb, shrub or small tree; leaves simple, alternate with reticulate venation; underground stem may be tuberous (potato). Floral features: Inflorescence cymose; flower bisexual, actinomorphic; calyx five gamosepalous, persistent; corolla five gamopetalous; androecium five epipetalous stamens; gynoecium bicarpellary, syncarpous with axile placentation. Fruit: berry or capsule, e.g., potato, tomato, brinjal, chillies.
Vegetative features: Perennial herbs with underground bulbs or rhizomes; leaves mostly basal, with parallel venation. Floral features: Inflorescence solitary or cymose; flower bisexual, actinomorphic; perianth present (tepals, six in two whorls, gamophyllous); androecium six stamens; gynoecium tricarpellary, syncarpous with axile placentation. Fruit: capsule or berry, e.g., onion, garlic, tulip, aloe, asparagus.
| Feature | Root | Stem | Leaf |
|---|---|---|---|
| Origin | Radicle | Plumule | Node of stem |
| Geotropism | Positive | Negative | - |
| Nodes/Internodes | Absent | Present | - |
| Function | Absorption, anchoring, storage | Support, conduction | Photosynthesis |
| Modification example | Carrot (storage), Banyan (prop) | Potato (tuber), Ginger (rhizome) | Pea (tendril), Cactus (spine) |
| Feature | Fabaceae | Solanaceae | Liliaceae |
|---|---|---|---|
| Root | Tap with nodules | Tap | Fibrous |
| Venation | Reticulate | Reticulate | Parallel |
| Flower symmetry | Zygomorphic | Actinomorphic | Actinomorphic |
| Androecium | Diadelphous | Epipetalous | Six stamens |
| Placentation | Marginal | Axile | Axile |
| Fruit | Legume | Berry/Capsule | Capsule/Berry |
| Example | Pea, Gram | Tomato, Potato | Onion, Tulip |
| Term | Meaning |
|---|---|
| Aestivation | Arrangement of sepals/petals in bud |
| Placentation | Arrangement of ovules in ovary |
| Phyllotaxy | Arrangement of leaves on stem |
| Actinomorphic | Radially symmetrical flower |
| Zygomorphic | Bilaterally symmetrical flower |
The morphology of flowering plants provides the descriptive foundation for understanding how the plant body is organised. The root, stem and leaf systems, along with their countless modifications, enable plants to survive in diverse habitats, from deserts to swamps. The study of inflorescence and flower structure reveals the complexity of the reproductive machinery, while fruits and seeds complete the reproductive cycle. The comparative description of the families Fabaceae, Solanaceae and Liliaceae demonstrates how morphological features are used for identification and classification. Together, these details build a clear picture of the angiosperm body plan that is essential for taxonomy, plant breeding and agricultural science, and they prepare the student for the internal structure covered in the next chapter.