REPRODUCTION IN FLOWERING PLANTS
REPRODUCTION IN FLOWERING PLANTS
Objectives for Today
Understand the following key concepts:
Functions of each part of a flower.
Location of egg and pollen grain formation within the flower.
Difference between pollination and fertilization.
Sexual Reproduction in Plants
Life Cycle Stages: Plants exhibit a life cycle consisting of two distinct stages:
Diploid Stage: Known as the sporophyte.
Haploid Stage: Known as the gametophyte.
Overview of the Plant Life Cycle
Angiosperms: Display an alternation between a conspicuous sporophyte and a reduced gametophyte.
Alternation of Generations:
A type of life cycle where subsequent generations alternate between diploid and haploid organisms.
Diploid Sporophyte:
Produces spores through meiosis.
Spores grow into the haploid gametophyte which produces gametes.
Fertilization: During this event, gametes fuse to form a diploid zygote.
Sporophyte Generation
For flowering plants, the sporophyte represents the dominant, flowering-producing generation:
Produces spores that develop into male gametophytes (pollen grains).
Produces female gametophytes (embryo sacs).
Gametophyte Generation
The gametophyte is significantly reduced and exists solely inside the reproductive structures of the flower:
Male Gametophyte: Pollen grain.
Female Gametophyte: Embryo sac located within the ovule.
Flowers
Definition: The reproductive structures of angiosperms.
Triggered by environmental signals, such as day length (photoperiodism).
Types of Spores Produced:
Microspore: Male gametophyte that undergoes mitosis to become a pollen grain.
Megaspore: Female gametophyte, an embryo sac within an ovule located within an ovary.
Outcome:
Ovule matures into a seed.
Ovary develops into a fruit.
Pollen transports sperm to the flower egg in the embryo sac.
Functions of Flowers
Key Functions:
Attraction of pollinators.
Production of spores.
Protection for gametophytes.
Dispersal of pollen.
Flower Structure of Angiosperms
General Patterns:
Flower parts typically occur in threes (or multiples) in monocots.
Flower parts typically occur in fours or fives (or multiples) in eudicots.
Flower Parts
Sepals: Enclose the bud before it opens, protecting the developing flower.
Petals: Often brightly colored, located inside sepals to attract insects and other pollinators.
Stamen: The male parts of the flower, composed of the anther and filament.
Anther: An oval sac where meiosis occurs to produce pollen grains (male sex cells).
Filament: A long, thin stalk supporting the anther.
Carpels (Pistils): Innermost floral parts that produce female gametophytes.
Ovary: Broad base of the carpel containing ovules.
Style: Narrow stalk of the carpel.
Stigma: Sticky portion on top of the style where pollen grains land.
Classification of Flowers
Complete Flower: Contains all four essential parts: petal, sepal, stamen, and pistil.
Incomplete Flower: Lacks one or more of the four floral parts.
Perfect Flower: Contains both stamen and pistil; may lack sepal or petal (bisexual).
Imperfect Flower: Contains either stamen or pistil; may be staminate (male) or pistillate (female - unisexual).
Dioecious and Monoecious Plants
Dioecious Flowers: Flowers are borne on separate plants (e.g., papaya - Carica papaya).
Monoecious Flowers: Both male and female flowers are present on the same plant (e.g., corn - Zea mays).
Size and Shape of Flower Parts
Regular Flower: The corolla comprises similarly-shaped petals that are equally spaced from the center.
Irregular Flower: At least one member of at least one whorl differs in form or size from others (e.g., bean flower).
Formation of Pollen and Eggs Within the Flower
Pollen:
Forms within pollen sacs located in the anther.
Contains two cells:
One cell generates two sperm cells.
The other forms a pollen tube that transports sperm cells to the ovule.
Pollen Formation Process:
Involves meiosis of microspores in pollen sacs located in the anther.
Each microspore mother cell undergoes meiosis.
Results in four microspores; these undergo mitosis to form pollen grains.
Eggs:
Formed within the ovule:
Contains 1 egg and 2 polar nuclei along with additional nuclei participating in fertilization.
Events Inside the Ovule:
Forming cells on the inner wall of the ovary initiate the development of ovules.
Protective layers (integuments) surround each cell mass as it grows.
Meiosis produces four haploid spores (megaspores); all but one disintegrates.
The remaining megaspore undergoes mitosis three times without cytoplasmic division resulting in an eight-nuclei cell that develops into a seven-celled female gametophyte (embryo sac).
Contains six cells, including the egg; each with one nucleus.
The central cell contains two polar nuclei.
The egg cell fuses with one sperm during fertilization to produce the zygote.
The central cell fuses with another sperm to form triploid endosperm (3n), which nourishes the developing embryo.
Pollination vs. Fertilization
Pollination:
Defined as the transfer of pollen grains to a receptive stigma.
Various agents facilitate pollen transfer.
Germination occurs when a pollen grain lands on the stigma.
Fertilization:
Takes place after pollination.
Involves the fusion of gametes.
Double Fertilization in Plants
Process:
Pollen is deposited on the stigma.
A pollen tube grows down through ovary tissue, carrying two sperm nuclei.
Upon reaching an ovule, the pollen tube penetrates the embryo sac to deposit sperm.
One sperm fertilizes the egg, while the other fuses with both nuclei of the endosperm mother cell.
Endosperm Formation:
Occurs in angiosperms only.
Creates a triploid (3n) cell which becomes the nutritive tissue of the seed.
Flower Characteristics and Adaptations
Fragrance and Coloration of Flowers:
Some epidermal cells of petals generate fragrant oils.
Petals receive their shimmer and color from pigments such as carotenoids and anthocyanins, along with light-refracting crystals.
Fragrance and coloration attract pollinators.
Pollinating Agents
Insects:
Different insects prefer specific flower traits:
Butterflies are attracted to red or orange flowers that are fragrant and have distinctive patterns.
Bees are drawn to yellow, blue, or purple flowers, which are scented and can absorb ultraviolet light.
Flies and beetles are attracted to flowers that emit scents resembling rotten meat or decaying materials.
Birds:
Flowers typically appear yellow, orange, or red with little to no scent, as resources are not diverted to fragrance creation.
Bats:
Flowers have dusky white petals and are strongly scented.
Wind:
Wind-pollinated flowers produce large pollen quantities, might have smaller or no petals, and usually lack scent and nectar.
Inflorescence and Pollination
Inflorescence: Refers to the arrangement of flowers on the floral axis in specific formations to maximize reproduction success.
Helps in attracting pollinators and can lead to increased seed production or efficient seed dispersal.
Provides stable platforms for insects or birds to land on.
Might offer protection to floral parts from adverse environmental factors or from pollinating mammals.
Coevolution
Definition: Reciprocal adaptation in two interdependent species (flowering plants and animal pollinators) influences one another's evolution.
Mechanism:
A heritable change in one species affects the selection pressures on the other, prompting evolutionary changes in both species.
Examples:
Plants may evolve bright colors to attract specific pollinators, while pollinators such as bees can detect ultraviolet light, leading to the evolution of UV patterns on flowers.
Specific Case: The long floral tube of the Madagascar orchid (Angraecum sesquipedale) has coevolved with the 28 cm-long proboscis of its pollinator, the hawkmoth (Xanthopan morganii praedicta), named in homage to Darwin's prediction of its existence.
Relationships Among Ovules, Ovaries, Seeds, and Fruits
Definitions:
Ovules: Potential to develop into seeds.
Ovaries: Potential to develop into fruits.
Fruits: Mature, ripened ovaries containing seeds.
Seeds: Enclosed within fruits, representing the embryo and its surrounding nutritive tissue.