Lecture Notes: Seed Plants - Angiosperms, Part I

Angiosperms: Introduction

Almost all plants used for food, clothing, medicine, etc., are flowering plants (angiosperms), representing over 250,000 out of 300,000 land plant species.

Evolution of Flowers

Flowering plants produce diverse secondary metabolites for various functions:

• Attracting mutualistic partners

• Aiding growth and development

• Deterring herbivores and microbes

Flower Structure

A flower is considered a modified strobilus.

• Key Structures:

  • Stamen: Includes the anther and filament; condensed microsporophylls bearing pollen.

  • Carpel (Pistil): Modified megasporophyll containing the ovary, style, and stigma. The ovary surrounds the ovules, a key adaptation for angiosperms.

  • Petals: Sterile modified leaves that attract pollinators.

  • Sepals: Sterile modified leaves; enclose the flower before opening.

  • Receptacle: Point of attachment for flower parts.

  • Bracts: Modified leaves beneath the flower.

Homologous Structures

Understanding homology between flower parts is crucial.

Arrangement of Flower Parts

Flower parts are arranged in a specific order: sepals, petals, stamens, and carpels.

Modified Leaves

Some modified leaves, like bracts, are adapted for reproduction but are not flower parts. Phyllotaxy helps identify modified flower parts.

Flower Adaptations and Co-evolution

Flowers are often specialized for animal pollination, representing a significant case of co-evolution. This is more efficient than wind pollination.

Pollination Specificity and Speciation

Specific pollination strategies can lead to genetic isolation and speciation.

Flower Diversity

• Symmetry:

  • Radial (actinomorphic): Affects how pollinators touch the flower.

  • Bilateral (zygomorphic): Reduces gene flow between diverging populations.

• Completeness:

  • Perfect Flowers: Contain both male (stamens) and female (carpels) parts.

  • Imperfect flowers: Male (staminate) flowers lack carpels, female (pistillate) flowers lack stamens. Plants with imperfect flowers can be monoecious or dioecious.

• Fusion: Floral organs can fuse, forming a hypanthium at the base of the flower

• Tepals: When there is little difference between sepals and petals, they are called tepals.

• Inflorescence: A macro-structure consisting of many small flowers born on smaller stalks called pedicels. Expansion or contraction of different parts leads to different morphologies, such as Raceme, Spike, Panicle, Umbel, and Head.

Angiosperm Diversity

Angiosperms are traditionally grouped into monocots and dicots based on the number of cotyledons.

• "Dicots" are not monophyletic; the term Eudicots is now preferred.

Monocots vs. Eudicots

Important Angiosperm Families

• Eudicot Families:

  • Rose family

  • Bean (legume) family

  • Potato family

• Monocot Families:

  • Orchid family

  • Grass family

Secondary Growth

Monocots lost bifacial vascular cambium, resulting in no secondary growth.

Angiosperm Life Cycle

Key derived traits compared to gymnosperms should be noted.

• Mature flower on sporophyte plant (2n).

• Microsporocytes (2n) undergo meiosis to produce microspores (n).

• Megasporangium (2n) contains a surviving megaspore (n) after meiosis.

• Male gametophyte (in pollen grain) includes a generative cell and a tube cell.

• Female gametophyte (embryo sac) contains antipodal cells, polar nuclei in the central cell, synergids, and an egg nucleus (n).

• Double fertilization: Results in a zygote (2n) and a triploid endosperm (3n).

Seed Development

Includes the ovary and ovule.

Angiosperm Innovations

• Double Fertilization:

  • Advantage: Triploid endosperm.

  • Cotyledons: Embryonic leaves inside seeds.

Angiosperm Dominance

Angiosperms dominate land plants due to their reproductive success and rapid speciation.
Angiosperms are the source of nearly all food and plant products and produce diverse secondary metabolites with medicinal and other uses.