Introduction to the Angiosperms

Home > Biology 103: Botany > Exam Three

Learning Objectives

1. Compare and contrast fertile and infertile parts of the flower.
2. Compare and contrast male versus female parts of the flower.
3. Describe the variation in flower morphology.
4. Describe how the male gamete (sperm) is formed.
5. Describe how the female gamete (egg) is formed.
6. Describe the process of double fertilization and the function of the products.

Solutions

1. Sepals and petals are infertile. Stamens and carpels/pistils are fertile.
2. Male parts are the stamen. The filament holds the anther with it's pollen away from the female parts. Female parts are the carpels in the pistil. The stigma catches pollen, and the style brings the pollen to the ovary.
3. Morphology:
• Complete: has all 4 whorls
• Incomplete: missing a whorl
• Perfect: male and female parts
• Imperfect: missing male or female parts
• Monoecious: male and female parts on one flower
• Diaecious: male and female parts oon separate flowers
4. Male gametes are formed in the anther with 4 pollen sacs with microsporocyte cells inside each. Each microsporocyte → meiosis → 4 haploid microspores. 1 microspore → mitosis → 1 vegetative cell and 1 generative cell. Each generative cell → mitosis → 2 generative cells. Each one of these 3-cell matrixs is 1 sperm cell.
5. Female gametes are formed in the ovule. Megasporocyte → meiosis → 4 haploid megaspores, 3 degenerate. The remaining cell enlarges. Nucleus undergoes mitosis 3 times for 8 nuclei and 7 cells (3 antipodal cells, 2 polar nuclei [1 cell], 2 synergid cells, and 1 egg cell).
6. Double fertilization occurs when the second generative cell and the 2 polar nuclei meet to become 3n endosperm tissue, which is nutritive for the embryo. In monocots, this stays liquid. In eudicots, this gets absorbed into the cotyledons.

Notes

Angiosperms

• Flowering plants
• Seeds enclosed in fruits
• Unique: flowers produce fruits that contain seeds

Plant Evolutionary Trees

• Has:
• Chlorophyll a & b
• Land-dwelling adaptations (cuticle, gametophyte, and embryo)
• Vascular systems (xylem & phloem)
• Seeds
• Flowers

Flower Organization

• Whorls
• Counted up from bottom point of attachment
• 1^st whorl: below sepals
• 2^nd whorl: below petals
• 3^rd whorl: below stamens
• 4^th whorl: below carpels

Flower Parts

• Pedicel is the stem of the plant from which flowers develop
• Receptacle is the point of attachment where whorls are attached
• Whorls:
• Sepals can be green or flower colored
• Petals are thinner tissue and colored to attract pollinators
• Stamens are male parts (filament and anther)
• Carpels are female parts (style, stigma, and ovary)
• Male Stamen
• Filament holds the anther with its pollen away from flower structure, enough for pollinators to access and away from the ovary
• Looks more attractive outside of the main flower
• Female Parts
• In pistil
• Stigma is sticky for pollen to attach to
• Base is the ovary with a single egg inside each ovule
• Style separates stigma from ovary for same reason as filament
• The number of ovaries equals the number of pistils

Structure of Flowers

• Complete flower: has all parts
• Has sexual and asexual parts
• Incomplete flower: 1 or more whorls are missing
• "Spp" that is pollinated by wind; might be missing petals (that attract insects)
• Perfect flower: has both stamens and pistil
• Imperfect flower: is missing either stamen or pistil
• Wind pollinated flower can be incomplete and perfect
• If imperfect, must be incomplete because it would be missing a whorl
• Monoecious species: male and female flowers on the same plant (ex: corn, pumpkins)
• Dioecious species: male and female flowers on separate plants (ex: holly)
• Plant breeders have overcome this through grafting

Ovaries

• Superior ovary (hypogynous flower)
• Variablity around receptacle, flower at left sepals attached, petals arrached to receptable, stamen attached
• Half-inferior/Half-superior (perigynous flower)
  • Evolutionarily, flowers develop different receptacle tissue, expanded around ovary, spp along that path
• Inferior ovary (epigynous flower)
  • All whorls attached to above ovary
  • Ovary below receptacle

Angiosperm Life Cycle

• Microgametophyte
• Produces sperm
• 3 cells
• Megagametophyte
• produces egg
• 7 cells
• Not a values judgement
• Plants form zygote through fertilization, that zygote goes through mitois to become a diploid sporophyte (multicellular).
• Cell from that sporophyte goes through meiosis, where we gain 4 haploid spores (does not directly make the gametes).
• Spores are haploid; they go through mmitosis to become multicellular haploid individuals that then produce gametes.
• Microgametophytic development: flower has microsporophyte that goes through meiosis, that will produce 4 microspores. Each of those microspores will go through mitosis to make the microgametophyte.
• Keep this in mind for angiosperm reproduction!

Microgametophyte Development

• Takes place in the anthers
• 4 pollen saccs with microsporocyte cells inside
• Each microsporocyte undergoes meiosis to produce 4 haploid microspores.
• Male microscopres:
• Divide once by mitosis to form generative cell inside the larger vegetative cell.
• Generative nucleus will later divide to produce 2 sperm.
• Generative cell in vegetative cell has its own nucleus
• In normal mitosis, you get 2 cells that are next to each other; flower microspores have a slightly altered version of mitosis
• The vegetative cell, also called "tube cell", continues on and does not divide further. The generative cell divides one more time to make 2 sperm cells.
• Mathematically: 3 cells (2 sperm, 1 vegetative)
• Pollen sacs filled with many microsporocytes go through meiosis to produce haploid sperm that are in pollen.

Megagametophyte Development

• The diploid megasporoyte differentiates in ovule.
• Undergoes meiosis and and produces 4 haploid megaspores
• 3 degenerate
• Remaining cell enlarges and nucleus divides three times to produce 8 nuclei without cell wall
• Remains in that single cell
• Example: peas
• Inside of ovary is ovules
• At this point, walls have formed with 7 cells, and 2 nuclei are in a single cell
• Each cell:
• 3 antipodal cell at the top (regulates development)
• 2 polar nuclei (together in the center)
• 2 synergid cells (back-up cells)
• 1 egg cell (which will get fertilized)
• micropyle opening and integuments (will be important)

Pollination

• Transfer of pollen from anther to stigma
• Fertilization: union of sperm and egg
• Pollen tube has to grow down the style
• Pollen grain is the vegetative cell
• Why are there 2 sperm cells? Probably for backup

Fertilization

• Pollen tube grows down style to ovule micropyle
• Enters the ovule, destroying synergids in the process
• Double fertilization
• One sperm unites with one egg
• Develops into a zygote
• Other sperm unites with 2 polar nuclei
• Developes into a triploid (3n) endosperm tissue
• Endosperm: nutritive tissue for the embyro

Endosperm

• Nutritive tissue for the embyro
• Monocots: endosperm stays as a liquid inside of the seed (ex: corn)
• Eudicots: endosperm absorbed into cotyledons (peanuts)

Seed to Fruit

• Ovule becomes seed
• Integuments harden into seed coat
• Integuments: outer 2 layers of the ovule
• Ovary matures into fruit

Monocots versus Eudicots