Morphology of Flowering Plants
1. Introduction to Morphology
Morphology refers to the study of the form and structure of organisms. In flowering plants (angiosperms), it deals with the organization of various vegetative and reproductive structures.
The angiosperm plant body consists of:
- Root system: The underground subterranean part of the plant.
- Shoot system: The aerial part that bears stems, branches, leaves, flowers, and fruits.
2. The Root
In majority of dicotyledonous plants, the direct elongation of the radicle leads to the formation of the primary root, which grows inside the soil.
Regions of the Root
- Root cap: The tender apex of the root is protected by a thimble-like structure called the root cap, which shields the root meristem as it plows through the soil.
- Region of meristematic activity: Cells of this region have small, thin walls and dense protoplasm, dividing repeatedly.
- Region of elongation: Cells proximal to meristematic zone undergo rapid elongation and enlargement, responsible for root length growth.
- Region of maturation: Elongated cells gradually differentiate and mature. Root hairs (very fine, delicate, thread-like structures) arise from this region to absorb water and minerals.
Types of Root Systems
| Root System Type | Description | Example |
|---|---|---|
| Tap Root System | Primary root persists and gives off lateral roots (secondary, tertiary). Forms the main subterranean axis. | Mustard plant |
| Fibrous Root System | In monocots, the primary root is replaced by a large number of roots originating from the base of the stem. | Wheat plant |
| Adventitious Root System | Roots arise from parts of the plant other than the radicle (e.g., stem or leaves). | Grass, Monstera, Banyan tree |
Modifications of Root
Roots are modified to perform functions other than absorption and conduction, such as support, storage of food, and respiration.
- Storage roots: Tap roots of carrot, turnips, and adventitious roots of sweet potato get swollen and store food.
- Prop roots: Hanging support structures arising from tree branches, e.g., banyan tree.
- Stilt roots: Supporting roots coming out of the lower nodes of the stem, e.g., maize and sugarcane.
- Pneumatophores: Respiration roots that grow vertically upwards out of the water or swampy soil to get oxygen, e.g., Rhizophora.
3. The Stem
The stem is the ascending part of the axis that bears branches, leaves, flowers, and fruits. It develops from the plumule of the embryo of a germinating seed.
Key features include:
- Nodes: Regions of the stem where leaves are born.
- Internodes: Portions between two consecutive nodes.
- Buds: May be terminal or axillary, developing into branches or floral structures.
Modifications of Stem
Stems are modified to perform varied functions:
- Underground stems: Modified for food storage and perennation during unfavorable conditions, e.g., potato, ginger, turmeric, zaminkand, colocasia.
- Stem tendrils: Slender, spirally coiled structures developed from axillary buds to help climbing plants, e.g., gourds (cucumber, pumpkins, watermelon) and grapevines.
- Thorns: Woody, straight, and pointed structures modified from axillary buds for protection against browsing animals, e.g., citrus, bougainvillea.
- Phylloclade: Flattened or fleshy cylindrical stems containing chlorophyll to perform photosynthesis in arid regions, e.g., Opuntia, Euphorbia.
- Runners/Stolons/Offsets/Suckers: Creeping stems for vegetative propagation, e.g., mint, jasmine, pistia, banana.
4. The Leaf
A leaf is a lateral, generally flattened structure borne on the stem. It develops at the node and bears a bud in its axil. Leaves are the primary vegetative organs for photosynthesis.
Parts of a Leaf
- Leaf base: The part by which the leaf is attached to the stem. May bear two lateral small leaves called stipules. In monocots, the leaf base expands into a sheath covering the stem partially or wholly. In some leguminous plants, the leaf base may become swollen, called pulvinus.
- Petiole: The long, thin, flexible stalk that helps hold the blade to light and flutter in wind.
- Lamina (Leaf blade): The green expanded part of the leaf with veins and veinlets. The middle prominent vein is the midrib.
Venation
The arrangement of veins and the veinlets in the lamina of leaf is termed venation.
- Reticulate venation: Veinlets form a network; characteristic of dicotyledonous leaves.
- Parallel venation: Veins run parallel to each other within a lamina; characteristic of monocotyledonous leaves.
Types of Leaves
- Simple leaf: A leaf is simple when its lamina is entire or when incised, the incisions do not touch the midrib.
- Compound leaf: When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called compound. Compound leaves can be pinnately compound (leaflets present on a common axis, the rachis) or palmately compound (leaflets are attached at a common point, i.e., at the tip of petiole).
Phyllotaxy: The pattern of arrangement of leaves on the stem or branch. Types include alternate (single leaf per node alternately, e.g., china rose), opposite (a pair of leaves at every node opposite each other, e.g., Calotropis), and whorled (more than two leaves arise at a node and form a whorl, e.g., Alstonia).
Modifications of Leaves: Leaves are modified into structures like tendrils for climbing (pea), spines for defense (cacti), fleshy leaves for food storage (onion and garlic), and pitcher/trap structures for catching insects in insectivorous plants (pitcher plant, venus flytrap).
5. Inflorescence
A flower is a modified shoot wherein the shoot apical meristem changes to floral meristem. Internodes do not elongate and the axis gets condensed. The arrangement of flowers on the floral axis is termed inflorescence.
- Racemose: The main axis continues to grow, flowers are borne laterally in an acropetal succession (older flowers at the base, younger flowers at the tip).
- Cymose: The main axis terminates in a flower, hence is limited in growth. Flowers are borne in a basipetal order (older flowers at the top, younger flowers at the base).
6. The Flower
A flower is the reproductive unit in the angiosperms meant for sexual reproduction. A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk or pedicel, called the thalamus or receptacle.
The four whorls are:
- Calyx (non-essential/accessory whorl)
- Corolla (non-essential/accessory whorl)
- Androecium (reproductive whorl)
- Gynoecium (reproductive whorl)
Note: When calyx and corolla are not distinct, they are termed perianth (e.g., lily). A flower having both androecium and gynoecium is bisexual. A flower having either only stamens or only carpels is unisexual.
Symmetry of Flowers
- Actinomorphic (Radial symmetry): Can be divided into two equal radial halves in any radial plane passing through the centre, e.g., mustard, datura, chilli.
- Zygomorphic (Bilateral symmetry): Can be divided into two similar halves only in one particular vertical plane, e.g., pea, gulmohur, bean, cassia.
- Asymmetric (Irregular): Cannot be divided into two similar halves by any vertical plane passing through the centre, e.g., canna.
Position of Floral Parts on Thalamus
| Condition | Description | Gynoecium Position | Example |
|---|---|---|---|
| Hypogynous | Gynoecium occupies the highest position while other parts are situated below it. | Superior | Mustard, China rose, Brinjal |
| Perigynous | Gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level. | Half-inferior | Plum, Rose, Peach |
| Epigynous | The margin of thalamus grows upward enclosing the ovary completely and fusing with it, other parts arise above the ovary. | Inferior | Guava, Cucumber, Ray florets of sunflower |
Parts of a Flower In-Depth
- Calyx: The outermost whorl of the flower, members are called sepals. Generally green and leaf-like. May be gamosepalous (sepals united) or polysepalous (sepals free).
- Corolla: Composed of petals. Usually brightly colored to attract insects for pollination. May be polypetalous (petals free) or gamopetalous (petals united). Aestivation is the mode of arrangement of sepals or petals in floral bud with respect to the other members of the same whorl (Valvate, Twisted, Imbricate, Vexillary).
- Androecium: Composed of stamens. Each stamen represents the male reproductive organ consisting of a stalk (filament) and an anther. An anther is usually bilobed, with each lobe having two pollen sacs. A sterile stamen is called staminode.
- Gynoecium: Female reproductive part of the flower made up of one or more carpels. A carpel consists of three parts: stigma, style, and ovary. Apocarpous when carpels are free (e.g., lotus and rose); syncarpous when carpels are fused (e.g., tomato and mustard). After fertilization, the ovules develop into seeds and the ovary develops into the fruit.
7. The Fruit
The fruit is a characteristic feature of flowering plants. It is a mature or ripened ovary, developed after fertilization. If a fruit is formed without fertilization of the ovary, it is called a parthenocarpic fruit.
A fruit consists of:
- Pericarp: Wall of the fruit, which may be dry or fleshy. When fleshy, pericarp is often differentiated into an outer epicarp, middle mesocarp, and inner endocarp.
8. The Seed
The ovule after fertilization develops into a seed. A seed is made up of a seed coat and an embryo.
- Structure of a Dicotyledonous Seed: Outer covering is the seed coat. The seed coat has two layers, the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle. Inside the seed coat is the embryo, consisting of an embryonal axis and two cotyledons (often fleshy and full of reserve food materials). At the ends of the embryonal axis are the radicle and plumule.
- Structure of a Monocotyledonous Seed: Generally endospermic (except orchids which are non-endospermic). The outer covering of the endosperm separates the embryo by a proteinous layer called the aleurone layer. The embryo is small and situated in a groove at one end of the endosperm. It consists of one large and shield-shaped cotyledon known as scutellum and a short axis with a plumule and a radicle. The plumule and radicle are enclosed in sheaths called coleoptile and coleorhiza respectively.
9. Description of Family Solanaceae
Solanaceae is a large family, commonly called the 'potato family'. It is widely distributed in tropics, subtropics and even temperate zones.
Vegetative Characters
- Plants: Herbs, shrubs and rarely small trees.
- Stem: Herbaceous rarely woody, aerial, erect, cylindrical, branched, solid or hollow, hairy or glabrous, underground stem in potato (Solanum tuberosum).
- Leaves: Alternate, simple, rarely pinnately compound, exstipulate; venation reticulate.
Floral Characters
- Inflorescence: Solitary, axillary or cymose as in Solanum.
- Flower: Bisexual, actinomorphic.
- Calyx: Sepals five, united, persistent, valvate aestivation.
- Corolla: Petals five, united; valvate aestivation.
- Androecium: Stamens five, epipetalous.
- Gynoecium: Bicarpellary, syncarpous; ovary superior, bilocular, placenta swollen with many ovules, axile placentation.
- Fruits: Berry or capsule.
- Seeds: Many, endospermous.
Floral Formula of Solanaceae: Br (circle with plus sign) K(5) C(5) A5 G(2)
Economic Importance of Solanaceae
Many plants belonging to this family are sources of:
- Food: Potato (Solanum tuberosum), Tomato (Solanum lycopersicum), Brinjal (Solanum melongena).
- Spices: Chilli (Capsicum frutescens).
- Medicine: Belladonna, Ashwagandha (Withania somnifera).
- Fumigatory: Tobacco (Nicotiana tabacum).
- Ornamentals: Petunia.