Morphology of Flowering PlantsClass 11 Biology Notes

Morphology of Flowering Plants

Morphology is the study of the external structure of organisms. In flowering plants, also known as angiosperms, this includes their roots, stems, leaves, flowers, and fruits. Despite the vast diversity in their appearance, all angiosperms share this basic body plan. Understanding these parts and their variations is crucial for classifying plants and understanding how they adapt to their environments.

A typical flowering plant is divided into two main systems:

• Root System: The part of the plant that is usually underground.
• Shoot System: The part of the plant that is above the ground, which includes the stem, leaves, flowers, and fruits.

The Root

The root system develops from the radicle (the embryonic root) of a germinating seed. Its primary functions are to anchor the plant, absorb water and minerals from the soil, store food, and synthesize plant growth regulators.

There are three main types of root systems:

1. Tap Root System: The primary root, which grows directly from the radicle, becomes the main tap root. It grows deep into the soil and gives rise to smaller lateral roots (secondary, tertiary, etc.). This is characteristic of most dicotyledonous plants, like the mustard plant.

2. Fibrous Root System: In these plants, the primary root is short-lived and is replaced by a large number of thin, branching roots that originate from the base of the stem. This system is common in monocotyledonous plants, such as wheat.

3. Adventitious Roots: These are roots that grow from any part of the plant other than the radicle. For example, they can arise from the stem or leaves. Grass, Monstera, and the hanging roots of a banyan tree are examples of plants with adventitious roots.

Regions of the Root

The tip of a root, or the root apex, has several distinct regions responsible for its growth and function.

• Root Cap: A thimble-like structure that covers and protects the tender apex of the root as it pushes through the soil.
• Region of Meristematic Activity: Located just behind the root cap, this region contains small, thin-walled cells with dense protoplasm that divide rapidly to produce new cells for growth.
• Region of Elongation: The cells just behind the meristematic region undergo rapid elongation and enlargement. This is where the root grows in length.
• Region of Maturation: In this zone, the elongated cells differentiate and mature into specialized tissues. Some of the epidermal cells in this region form very fine, delicate, thread-like structures called root hairs. These root hairs are the primary sites for absorbing water and minerals from the soil.

The Stem

The stem is the main ascending axis of the shoot system, developing from the plumule of the embryo. It supports branches, leaves, flowers, and fruits, positioning them to receive sunlight.

Key Features of a Stem:

• Nodes: The points on the stem where leaves are attached.
• Internodes: The portion of the stem between two nodes.
• Buds: Stems bear buds, which can be terminal (at the tip of the stem) or axillary (in the axil of leaves). These buds can develop into branches or flowers.
• Appearance: Stems are typically green when young and may become woody and dark brown as they age.

Functions of the Stem:

• Supports and spreads out branches, leaves, flowers, and fruits.
• Conducts water, minerals, and food (photosynthates) throughout the plant.
• In some plants, stems are modified for food storage, support, protection, and vegetative propagation.

The Leaf

The leaf is a flattened, lateral structure that grows on the stem at a node. It is the primary site of photosynthesis. Leaves develop from the shoot apical meristem and are arranged in an acropetal order (youngest leaves at the apex, oldest at the base). An axillary bud is found in the axil of each leaf, which can later grow into a branch.

A typical leaf has three main parts:

1. Leaf Base: The part of the leaf that attaches to the stem. It may have two small leaf-like structures called stipules. In monocots, the leaf base often expands into a sheath that partially or wholly covers the stem. In some leguminous plants, the leaf base is swollen and called a pulvinus.
2. Petiole: The stalk that connects the leaf base to the leaf blade (lamina). It holds the blade up to the light. Flexible petioles allow the leaf to flutter in the wind, which helps in cooling and bringing fresh air to the surface.
3. Lamina (Leaf Blade): The green, expanded part of the leaf. It contains veins and veinlets, which provide rigidity and act as channels for transporting water, minerals, and food. The prominent middle vein is called the midrib.

Venation

Venation is the arrangement of veins and veinlets in the lamina of a leaf.

• Reticulate Venation: The veinlets form a web-like network. This is typical of dicotyledonous plants.
• Parallel Venation: The veins run parallel to each other. This is a characteristic feature of most monocotyledonous plants.

Types of Leaves

Leaves can be classified as simple or compound based on the structure of the lamina.

• Simple Leaf: The lamina is entire (a single, undivided blade) or, if incised, the incisions do not reach the midrib.
• Compound Leaf: The incisions in the lamina reach the midrib, breaking it into a number of smaller "leaflets." A bud is present in the axil of the petiole for both simple and compound leaves, but not in the axil of individual leaflets.

There are two main types of compound leaves:

1. Pinnately Compound Leaf: The leaflets are present on a common axis called the rachis, which is an extension of the midrib. Example: neem.
2. Palmately Compound Leaf: The leaflets are attached at a common point, at the tip of the petiole. Example: silk cotton.

Phyllotaxy

Phyllotaxy is the pattern of arrangement of leaves on a stem or branch.

• Alternate: A single leaf arises at each node in an alternate manner. Examples: china rose, mustard, sunflower.
• Opposite: A pair of leaves arises at each node, opposite to each other. Examples: Calotropis, guava.
• Whorled: More than two leaves arise at a node and form a whorl or circle. Example: Alstonia.

The Inflorescence

An inflorescence is the arrangement of flowers on the floral axis. A flower is essentially a modified shoot where the shoot apical meristem transforms into a floral meristem. The axis gets condensed, and the internodes do not elongate.

There are two major types of inflorescences:

1. Racemose: The main axis continues to grow indefinitely. Flowers are borne laterally in an acropetal succession, meaning the older flowers are at the base and the younger ones are at the top.
2. Cymose: The main axis terminates in a flower, so its growth is limited. Flowers are borne in a basipetal order, where the older flower is at the top and younger ones develop below it.

The Flower

The flower is the reproductive unit in angiosperms, designed for sexual reproduction. A typical flower consists of four whorls of floral appendages arranged on a swollen end of the stalk (pedicel) called the thalamus or receptacle.

The Four Whorls:

• Calyx: The outermost whorl, composed of sepals.
• Corolla: The whorl inside the calyx, composed of petals.
• Androecium: The male reproductive part, composed of stamens.
• Gynoecium: The female reproductive part, composed of carpels.

Calyx and corolla are accessory organs, while the androecium and gynoecium are the reproductive organs. In some flowers like the lily, the calyx and corolla are not distinct and are collectively called the perianth.

Flower Characteristics:

• Bisexual Flower: Has both androecium and gynoecium.
• Unisexual Flower: Has either only stamens (male) or only carpels (female).
• Symmetry:
  • Actinomorphic (Radial Symmetry): Can be divided into two equal radial halves in any plane passing through the center. Examples: mustard, datura, chilli.
  • Zygomorphic (Bilateral Symmetry): Can be divided into two similar halves in only one particular vertical plane. Examples: pea, gulmohur, bean.
  • Asymmetric (Irregular): Cannot be divided into two similar halves by any vertical plane. Example: canna.
• Position of Floral Parts on Thalamus:
  • Hypogynous: The gynoecium is at the highest position, and other parts are below it. The ovary is said to be superior. Examples: mustard, china rose, brinjal.
  • Perigynous: The gynoecium is in the center, and other parts are located on the rim of the thalamus at almost the same level. The ovary is half-inferior. Examples: plum, rose, peach.
  • Epigynous: The margin of the thalamus grows upward, enclosing the ovary completely and fusing with it. Other floral parts arise above the ovary. The ovary is inferior. Examples: guava, cucumber, ray florets of sunflower.

Parts of a Flower

Calyx

The calyx is the outermost whorl, made of sepals. Sepals are typically green and leaf-like, and their main function is to protect the flower in the bud stage.

• Gamosepalous: Sepals are united.
• Polysepalous: Sepals are free.

Corolla

The corolla is composed of petals. Petals are usually brightly colored to attract insects for pollination.

• Gamopetalous: Petals are united.
• Polypetalous: Petals are free. The shape of the corolla can be tubular, bell-shaped, funnel-shaped, or wheel-shaped.

Aestivation is the mode of arrangement of sepals or petals in a floral bud with respect to other members of the same whorl.

• Valvate: Sepals or petals just touch each other at the margin without overlapping. Example: Calotropis.
• Twisted: One margin of an appendage overlaps the margin of the next one. Examples: china rose, lady's finger, cotton.
• Imbricate: Margins overlap one another but not in any particular direction. Examples: Cassia, gulmohur.
• Vexillary (Papilionaceous): Found in pea and bean flowers. There are five petals: the largest (standard) overlaps two lateral petals (wings), which in turn overlap the two smallest anterior petals (keel).

Androecium

The androecium is composed of stamens, the male reproductive organs. Each stamen consists of a stalk called the filament and a tip called the anther. The anther is usually bilobed, and each lobe contains two pollen-sacs where pollen grains are produced. A sterile stamen is called a staminode.

• Adhesion of Stamens:
  • Epipetalous: Stamens are attached to the petals. Example: brinjal.
  • Epiphyllous: Stamens are attached to the perianth. Example: lily.
• Cohesion of Stamens:
  • Polyandrous: Stamens are free.
  • Monoadelphous: Stamens are united into one bundle. Example: china rose.
  • Diadelphous: Stamens are united into two bundles. Example: pea.
  • Polyadelphous: Stamens are united into more than two bundles. Example: citrus.

Gynoecium

The gynoecium is the female reproductive part, made of one or more carpels. A carpel has three parts:

• Ovary: The enlarged basal part containing ovules.
• Style: The elongated tube connecting the ovary to the stigma.
• Stigma: The receptive tip of the style where pollen grains land.

Ovules are attached to a cushion-like tissue called the placenta inside the ovary.

• Apocarpous: Carpels are free. Examples: lotus, rose.
• Syncarpous: Carpels are fused. Examples: mustard, tomato.

Placentation is the arrangement of ovules within the ovary.

• Marginal: The placenta forms a ridge along the ventral suture of the ovary, and ovules are borne in two rows. Example: pea.
• Axile: The placenta is axial (central), and ovules are attached to it in a multilocular (multi-chambered) ovary. Examples: china rose, tomato, lemon.
• Parietal: Ovules develop on the inner wall of the ovary. The ovary is one-chambered but can become two-chambered due to a false septum. Examples: mustard, Argemone.
• Free Central: Ovules are borne on a central axis, and septa are absent. Examples: Dianthus, Primrose.
• Basal: The placenta develops at the base of the ovary, with a single ovule attached to it. Examples: sunflower, marigold.

The Fruit

A fruit is a mature or ripened ovary that develops after fertilization. It is a characteristic feature of flowering plants. A fruit formed without fertilization is called a parthenocarpic fruit.

A fruit typically consists of a fruit wall (pericarp) and seeds. The pericarp can be dry or fleshy. A thick, fleshy pericarp is differentiated into:

• Epicarp: The outer skin.
• Mesocarp: The middle, fleshy layer.
• Endocarp: The inner layer.

The Seed

After fertilization, ovules develop into seeds. A seed is made up of a seed coat and an embryo. The embryo consists of a radicle (embryonic root), a plumule (embryonic shoot), an embryonal axis, and one or two cotyledons (seed leaves).

Structure of a Dicotyledonous Seed

• Seed Coat: The outermost covering, with two layers: the outer testa and the inner tegmen.
• Hilum: A scar on the seed coat where the seed was attached to the fruit.
• Micropyle: A small pore above the hilum.
• Embryo: Consists of an embryonal axis and two cotyledons, which are often fleshy and store food.
• Endosperm: In some seeds like castor, a food-storing tissue called endosperm is present (endospermic seeds). In others like bean, gram, and pea, the endosperm is absent in the mature seed (non-endospermic seeds).

Structure of a Monocotyledonous Seed

• Most monocot seeds are endospermic (e.g., maize), but some are non-endospermic (e.g., orchids).
• Seed Coat: Often membranous and fused with the fruit wall.
• Endosperm: Bulky and stores food. Its outer covering is a proteinous layer called the aleurone layer.
• Embryo: Small and situated in a groove. It has one large, shield-shaped cotyledon called the scutellum. The embryonal axis has a plumule and a radicle.
• Coleoptile and Coleorhiza: The plumule is enclosed in a sheath called the coleoptile, and the radicle is enclosed in a sheath called the coleorhiza.

Semi-Technical Description of a Typical Flowering Plant

To describe a flowering plant scientifically, botanists use a standard sequence and technical terms, often summarized in a floral formula and a floral diagram.

Floral Formula Symbols:

• Br: Bracteate (flower has bracts)
• K: Calyx (number of sepals)
• C: Corolla (number of petals)
• P: Perianth (when calyx and corolla are not distinct)
• A: Androecium (number of stamens)
• G: Gynoecium (number of carpels)
• $\underline{\mathbf{G}}$: Superior ovary
• $\overline{\mathbf{G}}$: Inferior ovary
• $\boldsymbol{O}^{\boldsymbol{7}}$: Male flower
• $\underset{+}{Q}$: Female flower
• $\underset{+}{7}$: Bisexual flower
• $\oplus$: Actinomorphic (radial symmetry)
• %: Zygomorphic (bilateral symmetry)

Fusion is shown by enclosing the number in brackets, e.g., $K_{(5)}$ means 5 united sepals. Adhesion (fusion between different whorls) is shown by a line drawn above the symbols, e.g., $\widehat{C_{(5)} A_5}$ indicates stamens are attached to the corolla.

A floral diagram is a visual representation showing the number of parts of a flower, their arrangement, and their relationship with each other and the mother axis (represented by a dot).

Solanaceae (The 'Potato Family')

This is a large family of plants found in tropical, subtropical, and temperate zones.

Vegetative Characters:

• Habit: Mostly herbs and shrubs, rarely small trees.
• Stem: Herbaceous or woody, aerial, erect, branched. Underground stem in potato (Solanum tuberosum).
• Leaves: Alternate, simple (rarely pinnately compound), exstipulate (no stipules), with reticulate venation.

Floral Characters:

• Inflorescence: Solitary, axillary, or cymose.
• Flower: Bisexual, actinomorphic.
• Calyx: Five sepals, united (gamosepalous), persistent, valvate aestivation.
• Corolla: Five petals, united (gamopetalous), valvate aestivation.
• Androecium: Five stamens, epipetalous (attached to petals).
• Gynoecium: Two carpels (bicarpellary), fused (syncarpous), with a superior, bilocular (two-chambered) ovary. The placenta is swollen with many ovules in axile placentation.
• Fruit: Berry or capsule.
• Seeds: Many, endospermous.

Floral Formula: $\oplus \widehat{\chi}^{7} \mathrm{~K}_{(5)} \widehat{\mathrm{C}_{(5)}} \mathrm{A}_{5} \underline{\mathrm{G}}_{(2)}$

Economic Importance:

• Food: Tomato, brinjal, potato.
• Spice: Chilli.
• Medicine: Belladonna, ashwagandha.
• Fumigatory: Tobacco.
• Ornamentals: Petunia.