Angiosperms Lecture Notes

Angiosperms

General Overview

  • Phylum Classification: All angiosperms classified in the phylum Magnoliophyta (also known as Phylum Anthophyta).

  • Definition: Angiosperms are seeded plants characterized by their reproductive structures known as flowers and fruits.

  • Diversity: They represent the most widespread and diverse group of plants.

Characteristics of Angiosperms

  • Term: The term angiosperms translates to “covered seed.”

  • Vascular Tissue: Contains xylem and phloem for nutrient and water transport.

  • Life Cycle:

    • Dominant sporophyte generation, with a reduced, microscopic gametophyte generation.

  • Reproductive Strategy:

    • Heterosporous: Capable of producing two different types of spores: microspores (male) and megaspores (female).

  • Evolutionary Advances: Key evolutionary features include the development of flowers and fruits, contributing to their success among plant groups.

  • Dominance: They account for approximately 95% of all plant species.

  • Seed Formation: All angiosperms possess seeds housed within fruits, specifically within ovaries.

Types of Angiosperms

Classification by Cotyledons

  • Angiosperms are divided into two primary groups based on the number of cotyledons present in the embryo:

Monocots

  • Definition: Seeds typically contain only one embryonic leaf, or cotyledon.

  • Examples: Includes plants such as grasses, orchids, irises, onions, lilies, and coconut.

  • Seed Composition: Endosperm is present in the seed.

  • Vascular Structure: Root vascular bundles are scattered; leaf veins are typically parallel; flower parts often occur in threes or multiples of three.

Dicots

  • Definition: Seeds contain two embryonic leaves or cotyledons.

  • Examples: Includes tomato plants, oaks, roses, mustards, cacti, blueberries, and sunflowers.

  • Species Diversity: Dicots are more diverse, encompassing more species as compared to monocots.

  • Seed Composition: Endosperm is absent in the mature seed, having been absorbed by the two cotyledons.

  • Vascular Structure: Root vascular bundles are organized in a ring; leaf veins form a net; flower parts typically occur in fours or fives and their multiples.

Anatomy of Angiosperm Flowers

  • Key Flower Structures:

    • Carpel: Female parts of the flower, including the ovary, stigma, and style.

    • Ovary: Contains ovules.

    • Stigma: Receives pollen.

    • Style: The structure connecting the stigma and the ovary.

    • Stamen: Male parts of the flower, composed of anther and filament.

    • Anther: Produces pollen.

    • Filament: Supports the anther.

    • Calyx: Composed of sepals; protects the flower bud.

    • Corolla: Composed of petals, attracting pollinators.

  • Pistil: Collective term for the carpels in a flower.

Flowering Processes

Microsporogenesis

  1. The anther houses microsporangium, which contains microspore mother cells (2n).

  2. These divide via meiosis to form microspores (n).

  3. The microspores develop into pollen grains (microgametophytes).

Megasporogenesis

  1. The ovary contains ovules, each with a megaspore mother cell (2n).

  2. The megaspore mother cell undergoes meiosis to produce megaspores (n).

  3. One megaspore divides via mitosis to produce the embryo sac (or megagametophyte), which has seven cells and eight nuclei.

Double Fertilization

  • Definition: A unique fertilization process in angiosperms.

  • Mechanism:

    1. The pollen tube discharges two sperm cells into the female gametophyte within an ovule.

    2. First Division: One sperm fertilizes the egg cell, resulting in a zygote (2n).

    3. Second Division: The other sperm fuses with polar nuclei to form the endosperm (3n), which nourishes the developing embryo.

Life Cycle of Angiosperms

  • Key Components:

    • Microsporangium and microsporocytes (2n) leading to meiosis to create microspores (n).

    • Pollination resulting in fertilization: formation of zygotes (2n) and developing endosperms (3n).

  • Pathways: Includes significant stages of fertilization and seed development.

Pollination Mechanisms

  • Main Pollinators: Key pollinators include birds, bats, and insects.

    • Approximately 75% of angiosperms rely on insects for pollination.

Fruit Development and Types

  • Fruit Definition: A fruit is a mature ovary that may also include other flower parts, protecting seeds and aiding their dispersal.

  • Types of Mature Fruits:

    • Dry Dehiscent: Split open when dry (e.g., peanuts).

    • Dry Indehiscent: Do not split open when dry (e.g., walnuts).

    • Fleshy Fruits: Soft and edible (e.g., apples).

  • Types of Fruits Based on Structure:

    • Simple Fruit: Develops from a single ovary.

    • Aggregate Fruit: Formed from multiple ovaries from a single flower (e.g., raspberry).

    • Multiple Fruit: Results from the ovaries of multiple flowers (e.g., pineapple).

    • Accessory Fruit: Involves other flower parts (e.g., apple).

Seed and Seed Dispersal in Angiosperms

  • Pollen Grain: Replaces swimming sperm; essential for reproduction

  • Seed Composition: Contains a young plant embryo and nutritional tissue (endosperm), surrounded by a seed coat.

    • Monocots: Endosperm persists as the food source in the mature seed.

    • Dicots: Endosperm nourishes the embryo during development and is stored in the cotyledons.

  • Seed Dispersal Mechanisms: Seeds can be dispersed through various means, including:

    • Wind: e.g., dandelion, maple.

    • Animals: Seeds may stick to fur or be ingested.

    • Water: Some seeds can float and travel by water.

    • Bursting Mechanism: Pods that burst open to release seeds (e.g., lotus).

    • Human Activities: Seeds can also be dispersed intentionally or unintentionally by humans.