Morphology of Flowering Plants

Learning Outcomes:

1. Understand the structural diversity among flowering plants.
2. Comprehend the different systems of roots, stems, leaves, and flowers.
3. Learn the modifications of plant organs and their adaptations.
4. Identify major plant families and describe their floral structures.

The morphology of flowering plants reveals the vast diversity of structures in higher plants. Angiosperms, despite their variety in form and external morphology, exhibit common characteristics like roots, stems, leaves, flowers, and fruits. A thorough understanding of these structures is crucial for studying plant classification and identifying adaptations to various environments.

The Root

Roots serve essential functions, including absorbing water and minerals, providing support, and storing nutrients. Roots are classified based on their origin and modifications.

Types of Roots

1. Tap Root System: Found in dicotyledonous plants, such as mustard, this system originates from the radicle and grows vertically down, developing lateral roots.
2. Fibrous Root System: Monocotyledonous plants like wheat exhibit a fibrous system, where the primary root is short-lived and replaced by adventitious roots originating from the stem base.
3. Adventitious Roots: Found in plants like grass and banyan trees, these roots arise from parts of the plant other than the radicle.

Regions of the Root

1. Root Cap: The root’s apex is covered by this thimble-like structure, protecting it as the root grows through the soil.
2. Region of Meristematic Activity: Cells here are small, thin-walled, with dense protoplasm, and divide continuously.
3. Region of Elongation: Cells elongate here, contributing to root growth.
4. Region of Maturation: Cells differentiate into mature structures. Root hairs emerge from this region, aiding in water and mineral absorption.

Modifications of Root

Roots adapt to perform additional functions:

1. Storage Roots: Examples include carrot and sweet potato, where roots store food.
2. Prop Roots: These are aerial roots that provide support, as seen in the banyan tree.
3. Stilt Roots: Found in maize and sugarcane, these roots grow from the lower stem nodes to support the plant.
4. Pneumatophores: Found in swampy plants like Rhizophora, these roots grow upward to facilitate gaseous exchange.

The Stem

The stem is a crucial structure that supports leaves, flowers, and fruits. It develops from the plumule and is characterized by nodes and internodes.

Features of the Stem

1. Nodes: Points where leaves attach to the stem.
2. Internodes: The regions between two nodes.
3. Buds: Found at the stem’s apex or axillary positions, developing into flowers or branches.

Modifications of Stem

Stems are modified to adapt to various functions:

1. Storage Stems: Underground stems like potato, ginger, and turmeric store food and act as organs of perennation.
2. Stem Tendrils: These slender, spirally coiled structures help plants like grapevines and pumpkins climb.
3. Thorns: Found in Citrus and Bougainvillea, they protect the plant from animals.
4. Cladodes: Modified stems, such as in Opuntia, which perform photosynthesis in arid regions.
5. Runners and Stolons: Horizontal stems that spread and develop new plants, as seen in strawberry.

Stem Modifications	Examples	Function
Storage Stems	Potato, Ginger	Food storage and perennation
Tendrils	Grapevines, Watermelon	Climbing
Thorns	Bougainvillea, Citrus	Protection
Cladodes	Opuntia, Euphorbia	Photosynthesis

The Leaf

Leaves are essential for photosynthesis and arise from the stem at the nodes.

Structure of the Leaf

1. Leaf Base: Connects the leaf to the stem. In monocots, it forms a sheath, while in leguminous plants, it may become swollen.
2. Petiole: The stalk that attaches the blade to the stem.
3. Lamina: The flat, green part responsible for photosynthesis, containing veins and veinlets.

Venation

1. Reticulate Venation: Seen in dicots, where veins form a network, as in neem.
2. Parallel Venation: Found in monocots, where veins run parallel to each other, as in grass.

Types of Leaves

1. Simple Leaf: The lamina is entire or incised, but the incisions do not reach the midrib.
2. Compound Leaf: The lamina is divided into leaflets.

• Pinnately Compound: Leaflets arranged along a rachis, as in neem.
• Palmately Compound: Leaflets radiate from a single point, as in silk cotton.

Phyllotaxy

The arrangement of leaves on the stem is classified into:

1. Alternate: One leaf per node, alternating along the stem, as in mustard.
2. Opposite: A pair of leaves arise at each node, opposite to each other, as in guava.
3. Whorled: More than two leaves arise at a node, forming a whorl, as in Alstonia.

Modifications of Leaves

1. Tendrils: Found in peas, modified for climbing.
2. Spines: In cacti, modified leaves provide protection.
3. Fleshy Leaves: Onion and garlic store food in their modified leaves.
4. Insectivorous Plants: Leaves of plants like pitcher plants and Venus flytraps are modified to trap insects.

The Inflorescence

Inflorescence refers to the arrangement of flowers on the floral axis.

Types of Inflorescence

1. Racemose: The main axis grows continuously, and flowers are arranged in an acropetal sequence.
2. Cymose: The main axis terminates in a flower, and flowers are arranged in a basipetal sequence.

The Flower

A flower is the reproductive structure in angiosperms, composed of calyx, corolla, androecium, and gynoecium.

Parts of a Flower

1. Calyx: The outermost whorl made up of sepals.
2. Corolla: The whorl of petals, usually brightly colored.
3. Androecium: The male reproductive organ, composed of stamens.
4. Gynoecium: The female reproductive organ, made up of carpels.

Flower Symmetry

1. Actinomorphic: Radial symmetry, where the flower can be divided into equal halves along any plane.
2. Zygomorphic: Bilateral symmetry, where the flower can be divided into equal halves only along one plane.

Important Note: Flowers with both male and female reproductive structures are bisexual, while those with only one type of reproductive structure are unisexual.

Aestivation

Aestivation refers to the arrangement of sepals and petals in the floral bud:

1. Valvate: Sepals or petals just touch without overlapping.
2. Twisted: One margin overlaps the next, as seen in hibiscus.
3. Imbricate: Margins overlap in no specific direction.
4. Vexillary: Found in pea and bean flowers, where the largest petal overlaps the two lateral petals, which in turn overlap the smallest anterior petals.

Placentation

The arrangement of ovules within the ovary is known as placentation. Types include:

1. Marginal: Ovules attached to a ridge along the ventral suture, as in pea.
2. Axile: Ovules attached to a central axis in a multilocular ovary, as in tomato.
3. Parietal: Ovules attached to the inner wall of the ovary, as in mustard.
4. Free Central: Ovules attached to a central axis without septa, as in primrose.
5. Basal: A single ovule attached at the base of the ovary, as in sunflower.

Type of Placentation	Description	Example
Marginal	Ovules attached to a ridge	Pea
Axile	Ovules attached to central axis	Tomato
Parietal	Ovules attached to ovary wall	Mustard
Free Central	Ovules on central axis, no septa

 | Primrose             |

| Basal | Ovule attached at ovary base | Sunflower |

The Fruit

A fruit is a matured ovary, often containing seeds. In some cases, fruits develop without fertilization, known as parthenocarpic fruits.

Types of Fruits

1. Drupe: Seen in mango and coconut, where the pericarp is divided into an epicarp, mesocarp, and endocarp.

The Seed

Seeds develop from fertilized ovules and contain an embryo. They are classified based on the number of cotyledons.

Structure of Dicotyledonous Seed

1. Seed Coat: Comprising the outer testa and inner tegmen.
2. Embryo: Consists of a radicle, plumule, and two cotyledons.

Structure of Monocotyledonous Seed

1. Endosperm: Stores food and is covered by a protein layer.
2. Embryo: Composed of a single scutellum and enclosed by coleoptile and coleorhiza.

Important Note: In some monocots like maize, the seed coat is fused with the fruit wall.

Semi-Technical Description of a Typical Flowering Plant

Plants are described starting with vegetative characters, followed by floral characters. A floral diagram and floral formula summarize the structure.

Symbol	Meaning
⊕	Actinomorphic flower
%	Zygomorphic flower
Br	Bracteate flower
K	Calyx
C	Corolla
G	Gynoecium
A	Androecium

Description of Some Important Families

Fabaceae

1. Vegetative Characters: Includes trees, shrubs, and herbs. Leaves are alternate, pinnately compound, and have reticulate venation.
2. Floral Characters: Flowers are zygomorphic and bisexual with papilionaceous corolla.
3. Economic Importance: Includes food crops like gram and soybean, medicinal plants, and sources of fiber and dye.

Solanaceae

1. Vegetative Characters: Includes herbs, shrubs, and occasionally small trees. Leaves are simple or pinnately compound.
2. Floral Characters: Flowers are actinomorphic and bisexual.
3. Economic Importance: Contains important crops like potato, tomato, and chili.

Liliaceae

1. Vegetative Characters: Perennial herbs with rhizomes, bulbs, or corms.
2. Floral Characters: Flowers are actinomorphic with six tepals.
3. Economic Importance: Includes ornamental plants like tulip and medicinal plants like aloe.

MCQ: Which type of placentation is found in a pea plant?
Answer: Marginal