Flower Structure and Gametophyte Development

Flower Structure and Function

• Flowers are reproductive structures of angiosperms, attaching to the stem's receptacle.
• Composed of four main organs: sepals, petals, stamens, and carpels; sepals and petals are sterile.
• Stamens have filaments topped by anthers, which produce pollen.
• Carpels consist of a style with a stigma for pollen reception; the ovary at the base contains ovules.
• A single carpel or fused carpels form a pistil.
• Complete flowers have all four organs, while incomplete flowers lack one or more.
• Clusters of flowers are called inflorescences.

Male and Female Gametophyte Development

• Pollen develops from microspores in anthers’ pollen sacs, undergoing mitosis to form generative and tube cells.
• A pollen grain consists of these two cells encased in a spore wall; pollen tubes grow down to ovary following successful pollination.
• The female gametophyte is the embryo sac within an ovule surrounded by integuments.
• A single cell in the megasporangium undergoes meiosis, producing four megaspores where only one survives and divides to form a cell with eight nuclei.

Pollination and Coevolution

• Pollination is the transfer of pollen from an anther to a stigma, facilitated by wind, water, or animals.
• Wind-pollinated species tend to produce large amounts of pollen.
• Coevolution occurs between flowering plants and specific pollinators, resulting in flower shapes aligning with pollinator anatomy; an example includes Darwin's prediction regarding a moth with specific tongue length.

Double Fertilization

• Involves two sperm from the pollen tube: one fertilizes the egg, and the other combines with polar nuclei to form triploid endosperm (nutritional support for the developing embryo).

Seed Development, Form, and Function

• Endosperm development typically occurs before embryo development; it serves as nutrient storage for monocots and some eudicots while in others, nutrients are transferred to cotyledons.
• The zygote divides into a basal cell that forms a suspensor and a terminal cell that develops into the embryo.
• Mature seeds contain the embryo enclosed in a protective seed coat and are generally 5–15% water.
• Embryonic structures include hypocotyl (below cotyledons), epicotyl (above), and plumule (young leaves and meristem).
• Monocots have one cotyledon; grasses feature a scutellum and protective sheathes (coleoptile for shoot and coleorhiza for root).

Seed Germination

• Germination requires imbibition, where seeds absorb water and swell.
• The radicle emerges first, followed by the shoot tip breaking through the soil.
• Eudicot germination involves a hook formation in the hypocotyl, while in monocots like maize, the coleoptile emerges through the soil.

Fruit Structure and Function

• Fruits are derived from the ovary, providing protection for seeds and facilitating their dispersal via wind or animals.

Plant Reproduction Overview

• Angiosperms, or flowering plants, are the most successful group of land plants due to their ability to reproduce through flowers.
• Animals are multicellular and diploid, creating haploid gametes via the process of meiosis.

Plant Life Cycle

• The lifecycle includes a multicellular sporophyte, which is diploid and responsible for producing haploid spores through meiosis.
• Spores undergo mitosis to develop into multicellular gametophytes; in angiosperms, this gametophyte stage is microscopic.

Generational Alternation

• Plants alternate between a diploid sporophyte and a haploid gametophyte.
• Sporophytes produce spores that develop into gametophytes, which generate gametes, including sperm and eggs.

Ferns vs. Angiosperms

• In ferns, the gametophyte structure is significantly larger than in flowering plants.
• Fern gametes are produced within specialized structures: antheridia (male) and archegonia (female).

Angiosperms

• Male gametophytes (pollen) are produced in anthers, while the ovary contains ovules, representing female gametophytes.
• Fusion of sperm and egg leads to the formation of a zygote, which undergoes mitosis and develops into a new sporophyte.
• The ovary matures into fruit, which protects seeds and aids in dispersal.

Key Terms

• Gametophyte: The haploid phase in the plant lifecycle that is responsible for gamete production.
• Sporophyte: The diploid phase that arises from the zygote and produces spores through meiosis.
• Anther: The part of the stamen that produces pollen.
• Carpel: The female reproductive organ of the flower, including stigma, style, and ovary.

Seed and Fruit Development

• Seeds are formed from fertilized ovules, and the ovary subsequently develops into fruit.
• Fruits facilitate seed dispersal, with some plants depending on animals for pollen spread, while others rely on wind.

Plant Structures

• Stamen: Composed of anther and filament; the anther generates pollen (sperm).
• Carpel: The essential female reproductive structure that captures pollen on the stigma and contains ovules.

Evolutionary Advantages

• The reduction in size of the gametophyte enables better protection for embryos and eggs.
• Angiosperms exhibit adaptations for diverse environments, including variations in floral color and structure that attract specific pollinators.

Pollen and Fertilization

• Germinated pollen grain is haploid, containing a single set of chromosomes.
• Upon landing on the stigma, pollen divides into two sperm nuclei traveling down the style to reach the ovule.
• Ovules, located within the ovary, house eggs that are produced through mitosis.
• Male reproductive structures include the stamen, which consists of the anther (pollen producer) and filament.
• Female structures comprise the style, stigma, and ovary containing ovules with eggs.

Pollination and Double Fertilization

• Pollination is facilitated by wind or animals that land on the stigma.
• Formation of the pollen tube allows sperm nuclei to access the embryo sac within the ovule.
• Double fertilization consists of one sperm fertilizing the egg to create a diploid zygote, while another sperm fuses with two polar nuclei to produce a triploid endosperm.

Seed Development

• The fertilized ovule develops into a seed, while the ovary evolves into fruit.
• Endosperm provides essential nutrients for the developing embryo within the seed.
• Development of the zygote may result in a dicot (two cotyledons) or a monocot (one cotyledon).

Prevention of Self-Pollination

• Genetic diversity is enhanced by mechanisms preventing self-fertilization, including:
  • Temporal separation of male and female gametophyte maturity.
  • Preference of pollinators for specific reproductive structures.
  • Biochemical self-incompatibility that prevents fertilization between gametes from the same plant.

Anatomy of Seeds and Fruits

• Seeds are composed of endosperm, an embryonic plant, and various structures.
• The ovary wall thickens to form fruit, serving to protect seeds and assist in their dispersal.
• Diverse fruit formations exist, as exemplified by peaches or apples containing multiple seeds derived from several carpels.

Coevolution and Seed Dispersal Strategies

• Coevolution signifies the adaptation of plants to attract animal pollinators using vivid colors or nectar.
• Plants employ various seed dispersal strategies:
  • Lightweight seeds promote wind transport.
  • Buoyant seeds, like coconuts, are adapted for water dispersal.
  • Edible fruits attract animals, facilitating seed deposition and fertilization via animal waste.

Key Definitions

• Double fertilization: Process in which one sperm fertilizes the egg while another combines with polar nuclei to form endosperm.
• Polar nuclei: Two nuclei within the ovule that unite with a sperm to create triploid tissue.
• Sporophyte: The diploid plant arising from the zygote that contains embryonic and seed structures.

Review and Summary

• Explore the functions of various plant structures in reproduction.
• Understand the mechanisms of fertilization and evolutionary adaptations in plant-animal interactions.
• Recognize the anatomy of seeds and the role of fruits in the plant lifecycle and dispersal.