Flowering Plant Reproduction and Development

• Reproduction and development are key to the success of flowering plants

Asexual and sexual reproduction

• Animal survival relies on herbivores/omnivores consuming plant reproductive structures.
• Includes flowers, seeds, and fruit
• Flowers are sexual reproductive structures of angiosperms that produce gametes, promote fertilization, and create fruit and seeds.
• Seeds consist of an embryo and nutrients protected by a seed coat.
• Fruit surrounds and disperses seeds.
• Angiosperms make up over 85% of land plants.
• Nearly every domesticated plant is an angiosperm

Asexual Reproduction

• Asexual reproduction results in offspring that are genetically identical to the parent plant.
• Rhizomes, horizontal underground stems, can produce a new plant.
• Asexual reproduction is efficient for spreading species.
• Asexually propagated plants can be susceptible to epidemics caused by disease or pests.

Heterosporous Alternation

• Microsporangium contains a microsporocyte (2n) that goes through meiosis to create a microspore (n)
• The microspore becomes a male gametophyte (n) that produces sperm (n) in antheridia
• Megasporangium contains a megasporocyte (2n) that goes through meiosis to create a megaspore (n)
• The megaspore becomes a female gametophyte (n) that produces eggs (n) in archegonia
• Eggs are fertilized and creates a zygote (2n) which becomes a sporophyte (2n)

Flower Structure

• A flower has highly modified leaves and four components from outside in.
• Sepals are thick, leaflike structures to protect the flower bud as it grows.
• Petals are brightly colored and scented to attract pollinators.
• Corolla: an entire group of petals.
• Stamen: male reproductive structure
• Anthers create pollen
• Filament holds anthers
• Carpel: female reproductive structure
• Stigma receives pollen
• Style is a slender stalk
• Ovary contains ovules

Flower Variations

• Complete flowers have all four parts.
• Incomplete flowers are missing a part.
• Perfect flowers have both stamens and carpels.
• Imperfect flowers have only stamens.
• Monoecious plants have both male and female reproductive structures.
• Dioecious plants have either stamens or carpels.

Female Gametophytes

• An ovary contains one or more ovules with integuments.
• There is one opening called the micropyle.
• Ovules contain a megasporangium, inside of which is a diploid megasporocyte.
• A megasporocyte divides by meiosis to produce four haploid megaspores.
• Three of the four megaspores degenerate.
• The surviving megaspore divides by mitosis three times to produce eight haploid nuclei.
• A mature female gametophyte has eight nuclei arranged inside an embryo sac.
• An egg resides directly behind the micropyle opening.
• Two polar nuclei are located in the large cell in the center of the ovule.

Male Gametophytes

• The anther has four microsporangia.
• Microsporocytes divide by meiosis to produce haploid microspores.
• Each microspore develops into a pollen grain (male gametophyte).
• A generative cell produces two sperm.
• A tube cell directs growth of the pollen tube.

Pollination

• Pollination: transfer of pollen grains from an anther to a stigma.
• Fertilization: egg unites with sperm, making a zygote.
• Water is unnecessary for sexual reproduction, allowing plants to colonize drier environments.
• Wind-pollinated plants use a lot of energy to make pollen.
• Animal-pollinated plants use less pollen but use energy to "pay" pollinators.

Pollination types

• Self-pollination: pollen from anther of an individual is transferred to the stigma of the same individual.
• Cross-pollination: pollen from one individual is carried to another individual.

Abiotic Pollination

• Abiotic Pollination uses wind for ~10% of angiosperms
• Flowers aren't showy
• Uses light weight pollen
• Has a stigma with a large surface area

Biotic Pollination

• Biotic pollination uses animals for ~90% of angiosperms
• Animals are attracted by flowers
• Sugary nectar is made for some pollinators
• Extra pollen is used as food for other pollinators
• Uses scent

Animal Pollinators

• Honeybees, 80% of all pollination, come to flowers for pollen.
• Flowers use short wavelengths; best with blue, purple, and violets
• Butterflies use perfume-y nectar, and are attracted to light scent.
• Moths are out at night, and use large white/cream flowers with heavy scent, for flowers and nectar.
• Flies are attracted to a carrion flower, use look and smell.
• pseudocopulation attracts wasps looking for a mate in orchids.
• Hummingbirds see long wavelengths, red and orange, but have a poor sense of smell.
• Bats prefer large flowers or flower clusters that are identifiable by echolocation and musky smell.
• Hummingbirds and long tubular flowers, hawkmoth species and Darwin's orchid, and pronuba moth and yucca, create pollinators.

Fertilization

• Pollen grain reaches the stigma.
• The tube cell grows a pollen tube down the style, down to the micropyle of the ovule.
• A generative cell divides to form two sperm.

Double Fertilization

• First sperm unites with egg, making a zygote (2n).
• Second sperm unites with two polar nuclei, forming the endosperm (3n).
• Endosperm cells are loaded with starch, oils, proteins, and nutrients.

Seed Components

• Embryo sporophyte: from 2n zygote.
• Endosperm: (food supply) from 3n cell resulting from fusion of 2nd sperm and 2 polar nuclei.
• Seed coat: from integuments (ovule).

Embryo Sporophyte

• Parts of the embryo sporophyte
• Cotyledons: seed leaves
• Hypocotyl: seed stem
• Radicle: embryonic root
• epicotyl: embryonic leaves

Fruit

• Fruit: ripened ovary.
• An ovule matures to form the seed, ovary ripens to form fruit.
• Functions protect the seed(s), delay germination, disperses seed.
• Seed dispersal can be via wind, water, or animals.

Seed Dormancy

• Seed dormancy allows survival through harsh conditions.
• Scarification, disruption of the seed coat so water can enter, may be necessary.
• This is done through fire, passage through animal's digestive tract, and abrasion by wind- or water-driven soil particles
• Seeds go through environmental conditions
• Requires correct temperature, water, and oxygen availability

Germination energy

• Energy for germination can come from endosperm.
• In other seeds, cotyledons break down and absorb endosperm before the seed becomes quiescent.
• The seed has large cotyledons but no endosperm.
• Germination needs a rapid influx of water.
• The seed swells and breaks open, allowing the embryo to emerge
• Geotropism: response to gravity; tips of roots and stems control roots
• Roots emerge that have positive geotropism.
• Shoots emerge that have negative geotropism.

Types of Germination

• Epigean: cotyledons brought above ground
• Hypogean: cotyledons stay below ground