Angiosperms: The Flowering Plants

Overview of Angiosperms

Angiosperms, or flowering plants, represent the largest and most diverse group of plants on Earth, comprising about 90% of all plant species. They are classified in the phylum Anthophyta and are characterized by the presence of flowers and fruit, which play key roles in their reproductive strategies.

• Species Diversity: Approximately 250,000 species.

• Key Features: Flowers, fruits, double fertilization, and highly reduced gametophytes.

Flower Structure and Function

Specialized Floral Organs

Flowers are composed of four types of specialized leaves, each with distinct functions in reproduction and protection.

• Sepals: Protect the developing flower bud.

• Petals: Attract pollinators with color and scent.

• Carpels (Female Structures):

  • Stigma: Receives pollen.

  • Style: Pollen tube grows through this tissue.

  • Ovary: Contains ovules, which house the female gametophyte and egg cell.

• Stamens (Male Structures):

  • Anther: Produces pollen grains, each containing a male gametophyte and sperm cells.

  • Filament: Supports the anther.

Perfect flowers contain both stamens and carpels, allowing for both male and female reproductive functions.

Pollination and Fertilization

Pollination Mechanisms

Pollination is the transfer of pollen from anther to stigma. It can occur via self-pollination or cross-pollination, often facilitated by animals such as bees, which are crucial for crop yields in many orchards.

• Self-pollination: Pollen from the same plant fertilizes the ovule. Some plants are self-incompatible to promote genetic diversity.

• Cross-pollination: Pollen from a different plant fertilizes the ovule.

Prevention of Polyspermy

To ensure only one sperm fertilizes each egg, angiosperms use chemical signaling to attract a single pollen tube to each ovule. Once a pollen tube enters, further entry is blocked, preventing polyspermy (fertilization by multiple sperm).

• Polytubey prevention: Ovules release attractants; only the fastest pollen tube succeeds. Extracellular signaling blocks additional tubes.

• Example: In Arabidopsis, pollen tubes can grow up to a centimeter per hour.

Angiosperm Life Cycle

Alternation of Generations

Angiosperms exhibit a life cycle with alternating multicellular haploid (gametophyte) and diploid (sporophyte) generations. The sporophyte is the dominant, visible plant.

1. Sporophyte (2n): Produces flowers containing reproductive organs.

2. Female Pathway: Carpel (megasporophyll) contains ovary → ovule (megasporangium, 2n) undergoes meiosis → four megaspores (n), one survives → female gametophyte (embryo sac, n) with egg cell.

3. Male Pathway: Stamen (microsporophyll) contains microsporangium → microsporocytes (2n) undergo meiosis → microspores (n) → pollen grains (male gametophyte, n) with two sperm and a tube cell.

4. Pollination: Pollen lands on stigma, pollen tube grows to ovule, releases two sperm cells.

5. Double Fertilization:

   • One sperm fertilizes egg → zygote (2n).

   • Other sperm fuses with two nuclei in central cell → endosperm (3n, food supply).

6. Seed Development: Zygote becomes embryo, endosperm nourishes embryo, seed coat forms.

7. Germination: Seed grows into new sporophyte.

Key Terms

• Mitosis: Cell division producing two identical diploid (2n) cells; for growth and repair.

• Meiosis: Cell division producing four non-identical haploid (n) gametes; for sexual reproduction.

• Somatic cells: All body cells except gametes.

• Germ cells: Cells that give rise to gametes (egg and sperm).

Fruit and Seed Development

Formation and Function of Fruit

After fertilization, the ovary matures into a fruit, which protects seeds and aids in their dispersal. Fruits can be fleshy or dry, and their structure is adapted to various dispersal mechanisms.

• Fleshy fruits: Grapes, tomatoes, oranges, cherries; pericarp softens during ripening.

• Dry fruits: Beans, nuts, grains; hard ovary wall may adhere to seed coat.

• Seed dispersal: By animals (ingestion or attachment), wind, or water.

• Example: Cereal grains (corn, rice, wheat) are technically fruits, not just seeds.

Not all ovules are always fertilized; some fruits may develop with missing seeds.

Evolution and Phylogeny of Angiosperms

Origin and Diversification

Angiosperms appeared in the fossil record about 140 million years ago and rapidly diversified to dominate terrestrial ecosystems by 100 million years ago. Their evolutionary origins are still under investigation, with evidence suggesting divergence from gymnosperms about 300 million years ago.

• Key evolutionary trends: Shift from gametophyte to sporophyte dominance, reduction in gametophyte size, specialization of leaves and reproductive structures.

• Major extinction events: Angiosperms became dominant around the time of the dinosaur extinction (~66 million years ago).

Angiosperm Diversity: Monocots vs. Eudicots

Major Groups

Angiosperms are divided into two major groups based on embryonic and structural features: monocots and eudicots.

• Monocots: One cotyledon (seed leaf), parallel leaf veins, scattered vascular tissue, fibrous roots, flower parts in multiples of three. Examples: corn, wheat, orchids.

• Eudicots: Two cotyledons, net-like leaf veins, vascular tissue in a ring, taproot system, flower parts in multiples of four or five. Examples: legumes, fruits, ornamental flowers.

Fun Fact: Water Lilies

• Stomata on upper leaf surface; leaves float, roots anchored.

• Among the most ancient flowering plants.

• Have diploid endosperm, unlike most angiosperms (triploid) and gymnosperms (haploid).

Economic and Ecological Importance of Angiosperms

Plants as Food and Resources

Angiosperms are vital for human nutrition, providing the majority of calories consumed worldwide, as well as resources for livestock, construction, and medicine.

• Six major crops: Corn, rice, wheat, potato, cassava, sweet potatoes (80% of human caloric intake).

• Other uses: Tea, coffee, chocolate, herbs, spices, wood, paper.

• Domestication: Humans have selectively bred crops for thousands of years (e.g., modern corn from wild ancestors).

• Livestock feed: Most livestock are fed angiosperm-derived grains; 5-7 kg of grain needed for 1 kg of beef.

Comparative Plant Diversity

Major Plant Groups and Their Features

Non-vascular vs. Vascular Plants

Additional info: The reduction in gametophyte size and the evolution of seeds and flowers are considered major factors in the evolutionary success of angiosperms.