Biology 200 Chapter 23: Angiosperms

Introduction to Angiosperms

• Definition: Angiosperms are the flowering plants.

• Seed Protection: Seeds are enclosed within a carpel. A carpel is defined as a leaf that has folded over and fused at the margins.

• Pistil Structure: A pistil can be composed of either a single carpel or two or more united carpels.

• Developmental Outcomes:

  • The seed develops from the ovule which is located within the carpel.

  • The ovary matures into a fruit.

• Source Citation: Kingsley Stern.

General Characteristics and Classification

• Phylum Magnoliophyta: All angiosperms are grouped within this single phylum.

• Classes of Magnoliophyta: The phylum is historically divided into two large classes:

  • Magnoliopsida (Dicots): Modern DNA and cladistic evidence suggest that two distinct groups of dicots should be recognized.

  • Liliopsida (Monocots): Characterized by single cotyledons.

• Floral Morphology: The flower itself is a modified stem bearing modified leaves.

• Primitive Floral Characteristics: These traits are found in the most primitive flowers:

  • Presence of a long receptacle.

  • Numerous flower parts that are separate and spirally arranged.

  • Lack of differentiation between sepals and petals.

  • Flattened and numerous stamens and carpels.

Variation and Specialized Lifestyles

• Size Variation: There is great variation in size among Phylum Magnoliophyta members.

• Parasitic Angiosperms: Approximately $4,000$ species are parasitic. These plants intercept food and water from the host by invading the xylem and phloem.

• Saprophytic Angiosperms: Certain plants obtain nutrients from decaying matter, including specific orchids and the snowplant.

• Images/Credits: Phil Schermeister/Getty Images.

Sporophyte and Gametophyte Generations

• Heterospory: Angiosperms are heterosporous, producing two types of spores.

• Dominance: The sporophyte generation is dominant.

• Female Gametophyte: Wholly enclosed within the sporophyte tissue and significantly reduced in size, consisting of only a few cells.

• Male Gametophyte: At maturity, it consists of a germinated pollen grain containing $3$ nuclei.

Female Gametophyte Development

• Megasporocyte: A diploid ($2n$) megasporocyte differentiates within the ovule.

• Meiosis: The megasporocyte undergoes meiosis to produce $4$ haploid ($n$) megaspores.

• Degeneration: $3$ of the megaspores degenerate.

• Nuclear Division: The remaining cell enlarges, and its nucleus undergoes division to produce $8$ nuclei (initially without cell walls).

• Integuments: The two outer layers of the ovule differentiate into integuments, which later develop into the seed coat.

• Micropyle: A small opening called the micropyle is located at one end of the ovule.

Structure of the Mature Megagametophyte

• Nuclei Organization: The $8$ nuclei form two groups of $4$ at each end of the cell.

• Central Cell: One nucleus from each group migrates to the middle of the cell, forming the central cell (containing $2$ polar nuclei).

• Cell Wall Formation: Cell walls eventually form around the remaining $6$ nuclei.

• Micropyle End: Contains the egg cell and $2$ synergids.

• Opposite End: Contains $3$ antipodal cells, which have no apparent function.

• Final Assembly: The female gametophyte (also known as the megagametophyte or embryo sac) is a large sac containing $8$ nuclei and $7$ cells.

• Source Citation: Kingsley Stern.

Male Gametophyte Development

• Location: Development occurs within the anthers.

• Microsporocytes: Four patches (pollen sacs) of microsporocyte cells differentiate in the anther.

• Meiosis: Each microsporocyte undergoes meiosis to produce a quartet of haploid ($n$) microspores.

• Transformation into Pollen:

  • Microspores divide once via mitosis to form a small generative cell inside a larger tube cell.

  • The tube cell nucleus is also called the vegetative nucleus.

  • The members of the microspore quartet separate.

  • The wall thickens into two layers. The outer layer is the exine, which is finely sculptured and contains chemicals that react with the stigma.

  • The generative nucleus later divides to produce $2$ sperm cells.

• Source Citation: Kingsley Stern.

Pollination and Fertilization

• Pollination: The transfer of pollen grains from the anther to the stigma.

  • Self-pollination: Occurs when pollen grains germinate on the stigma of the same flower.

  • Vectors: Pollination occurs via insects, wind, water, animals, or gravity.

• Fertilization: Defined as the union of sperm and egg.

• Pollen Tube Growth:

  • Further development may only occur if the pollen is from a different plant of the same species or a different variety.

  • The pollen tube grows between the cells of the stigma and style to reach the micropyle.

  • The vegetative nucleus stays at the tip of the tube, while the generative cell follows and divides into $2$ sperm.

  • Upon entering the female gametophyte, the pollen tube destroys a synergid to discharge the sperm.

• Mature Male Gametophyte Definition: A germinated pollen grain with its vegetative nucleus and $2$ sperm cells within the tube cell.

Double Fertilization and Seed Development

• Mechanism of Double Fertilization:

  • One sperm unites with the egg to form a diploid ($2n$) zygote, which develops into the embryo.

  • The other sperm unites with the $2$ central cell nuclei to produce a triploid ($3n$) endosperm nucleus.

• Endosperm:

  • Serves as nutritive tissue for the embryo.

  • In many monocots (corn, grasses), it becomes an extensive part of the seed.

  • Wheat, rice, and corn are major human nutrition sources due to the nutritional quality of the endosperm.

  • In most dicots, the endosperm is absorbed into the cotyledons.

• Maturation: The ovule becomes the seed; the ovary matures into the fruit; the integuments harden into the seed coat.

Diversity in Gametophyte Development (Lilies)

• Nuclear Variation: Female gametophytes can have between $4$ and $16$ nuclei or cells at maturity.

• Endosperm Ploidylevels: Endosperm can be $5x$, $9x$, or $15x$.

• Example: Lily ($Lilium$):

  • All $4$ haploid megaspore nuclei remain functional.

  • Three nuclei unite to form a $3x$ nucleus, while the fourth remains haploid ($1x$).

  • This results in a gametophyte with four $3x$ nuclei and four $1x$ nuclei.

  • One central cell nucleus is $3x$ and the other is $1x$.

  • Fertilization ($1x$ sperm + $3x$ nucleus + $1x$ nucleus) results in a $5x$ endosperm nucleus.

• Source Credit: James E. Bidlack.

Apomixis and Parthenocarpy

• Apomixis: Development of an embryo without the fusion of gametes, though normal structures are involved. Common in some grasses. The embryo develops from a diploid nutritive cell or other diploid cell of the ovule, resulting in a vegetatively propagated (clonal) plant.

• Parthenocarpy: Fruits that develop from ovaries containing unfertilized eggs, resulting in seedless fruits. Examples include navel oranges and bananas.

Evolutionary Trends and Specialization

• Fossil Record: Flowering plants appeared approximately $160$ million years ago during the late Jurassic; they diversified during the Cretaceous and Cenozoic to become dominant.

• General Evolutionary Trends:

  • Reduction in the number of flower parts.

  • Fusion of flower parts.

  • Compression of spiral arrangements into whorls.

  • Shift from radial symmetry to bilateral symmetry.

• Evolution of the Carpel: The first pistil evolved from a leaflike structure with ovules along the margins. The edges of the blade rolled inward and fused. Primitive flowers with separate carpels eventually fused to form compound pistils.

Floral Symmetry and Ovary Position

• Symmetry:

  • Radial (Regular): Symmetrical in all directions.

  • Bilateral (Irregular): Symmetrical in only one plane.

• Ovary Placement:

  • Superior Ovary (Hypogynous): Ovary is produced on top of the receptacle; other parts are attached at the base.

  • Inferior Ovary (Epigynous): Receptacle or other parts are fused to and grown around the ovary; calyx and corolla appear attached to the top.

  • Perigynous Flower: Flower parts are attached to a corolla tube made of fused petals, creating a floral tube not attached to the ovary.

Flower Classifications

• Completeness:

  • Complete: Has calyx, corolla, stamens, and pistil.

  • Incomplete: Missing corolla or other parts.

• Sexuality:

  • Perfect: Both stamens and pistil are present.

  • Imperfect: Either stamens or pistil is missing.

• Species Sexual Distribution:

  • Monoecious: Male and female imperfect flowers on the same individual plant.

  • Dioecious: Plants bear either only male flowers or only female flowers.

Pollination Ecology and Coevolution

• Pollinators: Approximately $20,000$ species of bees currently act as pollinators.

• Bee-Pollinated Flowers: Brightly colored (blue/yellow), with honey guides. Visible under UV light, which bees can perceive but humans cannot.

• Beetle-Pollinated Flowers: Strong yeasty, spicy, or fruity odors. White or dull in color. Sometimes provide food on petals in special storage cells instead of nectar.

• Fly-Pollinated Flowers: Dull red or brown; smell like rotten meat (e.g., $Stapelia$ species).

• Butterfly and Moth Flowers: Often have sweet fragrances. White/yellow for nocturnal moths; red, blue, yellow, or orange for butterflies. Nectaries are at the base of corolla tubes or spurs.

• Bird-Pollinated Flowers (Hummingbirds/Sunbirds): Bright red or yellow, odorless (birds have a poor sense of smell). Sturdy inflorescences with copious nectar and long floral tubes.

• Bat-Pollinated Flowers: Tropical, nocturnal, dull in color. Large enough for a bat's head or consist of ball-like inflorescences.

Orchid Pollinating Mechanisms

• Pollinia: Pollen grains produced in sacs called pollinia (singular: pollinium) with sticky pads at the base.

• Mechanism 1: A flap yields to an insect's weight; the insect exits through a center opening near the stigma and anthers, brush-cleaning sticky pollen onto its head/back.

• Mechanism 2: An insect follows a nectar-secreting furrow. Pollinia adhere to the insect after an "explosion" triggered by the insect touching a sensitive point below the anther.

• Mechanism 3: Moths/butterflies touch a sensitive area, and a clamp attaches the pollinia to the insect's tongue.

• Mechanism 4 (Showy Orchids): Pollinia attach to an insect's head, then twist outward/forward. When the insect visits another flower, the pollinia are deposited in two separate stigma patches while new ones attach.

• Mechanism 5 (Ophrys/Wasp Mimicry): A modified petal resembles a female wasp or bee. Male insects attempt to copulate and pollinia are glued to their heads.

• Mechanism 6 (Bucket Orchids): Faucet glands fill a bucket-like petal with fluid. Insects fall in and escape through a trapdoor, contacting the stigma and anthers near the exit.

Herbaria and Plant Preservation

• Definition: Herbaria are libraries of dried, pressed plants, algae, and fungi, arranged and labeled.

• Preservation Methods:

  • Fungi and Bryophytes: Dried and stored in small packets.

  • Vascular Plants: A plant press is used for drying; specimens are mounted on $100\%$ rag herbarium paper.

• Storage: Specimens are organized so retrieval is easily accomplished.

• Source Citation: Kingsley Stern.