Angiosperm Notes

Angiosperms

  • Flowering plants that produce fruits from sporophyte tissue for seed dispersal.

Angiosperm Origins

  • Complex due to a poor fossil record.
  • Molecular clocks suggest angiosperms first evolved ~275 MYA.
  • Earliest fossil from ~174 MYA.
  • Characterized by having an ovary, sepals, and style.

Flowers

  • House the gametophyte generation.

  • Composed of modified stems and leaves.

  • Organized into whorls, which is a circular organization of flower parts:

    1. Sepals: Green, modified leaves.
    2. Petals: Colorful, modified leaves to attract pollinators.
    3. Androecium: Male gametophyte or stamen, composed of anthers and filament.
    4. Gynoecium: Female gametophyte or carpel, can be composed of multiple carpels.

  • California Poppy: State flower.

Generalized Flower Anatomy

  • Flowers house gametophyte generation; composed of modified stems and leaves.
  • Key structures:
    • Carpel
    • Anther
    • Filament
    • Stamen
    • Sepal
    • Petal
    • Pedicel
    • Stigma
    • Style
    • Ovary

Carpel Structures

  • Has four main structures:

    • Style: Stalk connecting stigma to the ovary.

    • Stigma: Tip of the carpel, often sticky or feathery to collect pollen grains.

    • Ovary: Swollen base, contains ovules and develops into fruit.

    • Nectaries: Nectar-secreting glands near base of ovary and act as food source to attract pollinators.

Ovule Location and Structure

  • Ovule is located within the ovary.
  • Same ovule structures as seen in gymnosperms with a few differences.
  • Integument has two layers.
  • Ovule attached to the ovary walls via a stalk.

Female Gametophyte Development

  • Embryo sac formed from integuments, nucellus, and macrospore mother cell.

    1. Mother cell undergoes meiosis to produce haploid daughter cells. One surviving haploid daughter cell undergoes mitosis.

    2. Two daughter cells undergo mitosis, resulting in two groups of 4 haploid nuclei.

    3. One polar nucleus from each group form the central cell.

    4. Group of nuclei near the micropyle form the egg and two synergid cells.

    5. Second group forms antipodal cells.

Male Gametophyte Development

  • Pollen grains develop within anthers, which contain many microspore mother cells.
  • Anther has four chambers with many microspore mother cells that produces pollen grains.

Pollination

  • Transfer of pollen grains to the stigma.
  • Can be via self-fertilization, wind, water, or a pollinator.

Double Fertilization

  • Angiosperms exhibit double fertilization, where two non-flagellated sperm are produced by the generative cell in the pollen grain.

    1. Pollen tube grows down style, through micropyle, and destroys a synergid cell.

    2. One sperm fertilizes the egg and forms the zygote, which develops into the embryo of the new sporophyte.

    3. The second sperm unites with two polar nuclei within the central cell and forms a triploid primary endosperm.

Seed Development

  • Seeds consist of:
    • Seed coat (2n) from original sporophyte.
    • Primary endosperm (3n) from fertilization.
    • Embryo (2n) from fertilization.

Seed Embryo

  • Embryo consists of cotyledons, shoot apical meristem, and root apical meristem.
  • Apical meristem = Region of high growth rates along the axis of the plant.

Fruit Development

  • Fruits are derived from developed ovaries.
  • Three distinct layers in the ovary wall, called the pericarp, with seeds located within:
    • Exocarp: Outer layer.
    • Mesocarp: Middle layer.
    • Endocarp: Inner layer.

Angiosperm Relationships

  • Three major groups:
    1. Monocots
    2. Magnoliids
    3. Eudicots

Monocots

  • Comprise 25% of angiosperm species; includes grasses, corn, palms.
  • Parallel veined leaves, single cotyledon, scattered vascular system.

Magnoliids

  • Third largest group of angiosperms; includes magnolia trees and avocados.

Eudicots

  • Majority of angiosperms; includes oak trees, potatoes, strawberries, etc.
  • Net-like veined leaves, two cotyledons, scattered vascular system.

Angiosperm Comparisons

  • Monocots:
    • Seed: One cotyledon
    • Root: Fibrous roots
    • Vascular: Scattered
    • Leaf: Parallel veins
    • Flower: Multiples of 3
  • Magnoliids:
    • Two cotyledons
    • Tap roots
    • Scattered
    • Net-like veins
    • Multiples of 3
  • Eudicots:
    • Seed: Two cotyledons
    • Root: Tap roots
    • Vascular: Ringed
    • Leaf: Net-like veins
    • Flower: 4 or 5
  • Seed Parts:
    • Epicotyl
    • Hypocotyl
    • Cotyledons
    • Seed coat