Angiosperm Reproduction Overview

Angiosperm Reproduction

• Angiosperms can reproduce asexually or sexually.
• Asexual reproduction is advantageous in stable environments as it involves cloning parts of a parent plant.
• Vegetative reproduction creates new individuals by cloning parts of the parent.
• Asexual reproduction requires less energy than sexual reproduction.

Forms of Vegetative Reproduction

• Runners: Slender stems growing along the soil surface (e.g., strawberries).
• Rhizomes: Underground horizontal stems creating a network and giving rise to new shoots (e.g., irises, potatoes).
• Suckers (or sprouts): Produced by roots, leading to new plants (e.g., cherry, apple, raspberry).
• Adventitious plantlets: Arise from meristematic tissue in leaf notches (e.g., Kalanchoë daigremontiana).

Sexual Reproduction in Angiosperms

• Involves an alternation of generations.
• The diploid sporophyte generation produces a haploid gametophyte generation.
• Male gametophytes are pollen grains developing from microspores.
• Female gametophytes are embryo sacs developing from megaspores.
• These gametophytes are found in specialized structures of the flower.
• Reproductive organs are produced seasonally.

Flower Structure

• Most flowers contain male and female parts.
• Stamens: Male parts, with anthers at the tips.
• Carpels: Female parts, consisting of an ovary (lower bulging part), style (slender stalk), and stigma (sticky tip).
• Some flowers contain only male OR female parts and are called imperfect flowers.
• Plants with both male and female imperfect flowers on the same plant are called monoecious.

Pollen Formation

• Pollen sacs inside anthers contain microspore mother cells.
• Microspore mother cells undergo meiosis creating four haploid microspores.
• Microspores then undergo mitosis to produce pollen grains, which include generative cells and a tube cell nucleus.
• The tube cell nucleus develops into the pollen tube.
• The generative cell divides into two sperm cells.

Egg Formation

• Ovules within the ovary contain diploid megaspore mother cells.
• Meiosis in megaspore mother cells produces four haploid megaspores.
• One megaspore undergoes multiple mitotic divisions creating eight haploid nuclei.
• These nuclei are arranged within an embryo sac.

Pollination

• Pollen transfer from the anther to the stigma.
• Self-pollination occurs when pollen from a flower's anther pollinates the same flower's stigma, leading to self-fertilization.

Animal Pollination

• Many angiosperms use animals (pollinators) to carry pollen from one flower to another.
• Food (e.g., nectar) rewards the pollinators.
• Flower color and form are shaped by evolution to attract specific pollinators.
• For effective animal pollination, the insect or animal must visit plants of the same species.

Wind Pollination

• In some angiosperms and all gymnosperms, pollen is dispersed by wind and passively reaches the stigmas.
• Plants must grow relatively close together in areas with ample wind.
• Wind-pollinated angiosperm flowers are typically small, green, and odorless.

Fertilization

• Pollen grains adhere to the stigma and grow a pollen tube.
• The pollen tube travels down the style reaching the ovule.
• Two sperm cells are released from the pollen tube.
• One sperm fertilizes the egg, while the other fuses with the polar nuclei to form endosperm.
• This process is called double fertilization.

Seed Development

• After fertilization, cell division forms the embryo.
• Embryo develops and enters a dormant stage due to drying.
• Embryo development usually pauses shortly after the apical meristems and seed leaves, called cotyledons, have developed.

Seed Coat and Germination

• The outer covering of the ovule forms the seed coat.
• Seed coat is impermeable and encloses the dormant embryo and food source.
• Germination, the resumption of metabolic activity leading to growth, only occurs when water and oxygen reach the embryo.
• Germination ensures seed growth when conditions are favorable for plant survival.

Fruit Formation and Dispersion

• The flower ovary develops into fruit during seed formation.
• Fruits contain seeds and form in multiple ways with varied forms.
• Fleshy fruits are generally dispersed by birds and other vertebrates.
• Some fruits disperse through the wind, attaching to mammals or bird feathers.
• Some fruits disperse via water.

Germination

• A seed absorbs water when conditions are suitable for germination.
• Aerobic respiration starts after the seed coat ruptures.
• Roots emerge first, followed by shoot growth in eudicots (cotyledons emerge with the stem) and monocots (coleoptile emerges).

Plant Hormones

• Pattern of growth and differentiation in the embryo is repeated indefinitely after germination.
• Entire plants can be regenerated from differentiated plant tissues.
• A plant's development depends on meristematic tissue activity with interactions mediated through plant hormones.
• Differentiation in plants is largely reversible.
• Scientists have shown differentiated plant cells can express hidden genetic information given right environmental signals.
• Plant hormones control the expression of some plant genes.
• All plant hormones are produced in various tissues that have multiple functions.
• F.C. Steward successfully regenerated plants from isolated phloem tissue.

Auxin

• Phototropism: Plant growth toward light.
• Charles and Francis Darwin's experiments suggested a light-sensitive substance in plants that caused bending.
• This substance was later identified as auxin.
• Frits Went researched how auxin governs plant growth.
• Auxin promotes growth on the side of the seedling exposed to it.
• Auxin causes the shaded side to elongate more, resulting in plant bending toward light.
• Synthetic auxins are used in weed control by causing plant death and reducing ATP production.
• 2,4-D kills broadleaf eudicots with this mechanism.

Photoperiodism and Dormancy

• Photoperiodism: Organisms measure seasonal changes in day and night length.
• Plants' flowering responses are categorized in three areas related to day length:
  • Long-day plants: Flower when days become longer (summer).
  • Short-day plants: Flower when days become shorter (fall).
  • Day-neutral plants: Flower regardless of day length.
• Dormancy: Plants stop growing under unfavorable conditions.
• Dormancy in temperate zones is linked to winter where low temperatures and water freezing limit plant growth.

Tropisms

• Tropisms: Directional and irreversible growth responses to external stimuli.
• Types of tropisms include:
  • Phototropism: Growth toward light.
  • Gravitropism: Growth related to gravity (stems upward; roots downward).
  • Thigmotropism: Response to touch.

Inquiry and Analysis

• Irwin and Strauss: Studied wild radish flower color, its response to selection by pollinators.
• Compared the frequencies of different flower colors in two populations of wild radishes.
• One group's flower colors reflected bee preference for color.
• Another group was created by hand pollinating with no selection, based on color.