Plant Reproduction Flashcards

Plant Reproduction

Let's delve into the fascinating world of plant reproduction. We often overlook the intricate structures and processes happening right before our eyes.

Upcoming topics

After the mid-session break, we'll explore:
Transport systems:
Xylem and Phloem
Plant growth and hormones.

Lecture outline:

Plant life cycle (high-level overview)
Flower structures and terminology
Angiosperm double fertilization
Seeds, germination, and dormancy

Plant Life Cycle: A Familiar Process

Plants, like us, have a life cycle:

Gametes (pollen and embryos) unite during fertilization.
The embryo develops into a seed – a protected embryo.
The seed germinates, growing into a seedling.
The seedling matures into a plant, alternating between:
Vegetative stage (non-reproductive)
Reproductive stage (flowering)

Some plants, such as wheat, flower once, triggered by environmental cues. Others, like canola, flower repeatedly within a limited window. Perennials, like apple trees, flower for decades.

Alternation of Generations

Plants exhibit a unique life cycle called alternation of generations. Sexual reproduction involves:

Meiosis: Diploid cell \rightarrow Haploid cell
Fertilization: Two haploid cells \rightarrow Diploid cell

Humans undergo mitosis only after fertilization to develop from a zygote to an adult. Plants, however, undergo mitosis immediately after meiosis, resulting in multicellular haploid phases.

Imagine we divide the process in two with a blue line.
Above the line: Diploid (2 sets of chromosomes)
Below the line: Haploid (1 set of chromosomes)

Terminology:

Sporophyte: Diploid phase
Gametophyte: Haploid phase

The "-phyte" suffix means "plant". The Gametophyte makes gametes and the sporophyte makes spores.

Ferns as an Example

Ferns illustrate this concept well. The brown spores on the underside of fern leaves grow from the adult fern through meiosis. These spores germinate into haploid gametophytes, which are small, heart-shaped structures often found in rainforests. These gametophytes produce sperm and eggs, which then fertilize to produce a new diploid fern (sporophyte).

Flowering Plants

In flowering plants, this process occurs within the flower. The sporophyte (the main plant) produces spores via meiosis in the flower's pollen or ovaries (sporogenesis). These spores divide by mitosis to form gametophytes like pollen grains (male) or egg sacs (female).

Inside the gametophytes, gametes are formed by gametogenesis, leading to fertilization and the development of a zygote, which grows into an adult plant via mitosis.
The key difference is the mitosis at the haploid stage.

Mosses

In mosses, the gametophyte is the dominant, visible form, while the sporophyte appears as small "moss heads". In flowering plants and conifers, these gametophyte structures are protected inside the flowers and cones.

Flower Structure and Morphology

A typical flower consists of:

Petals: Attract pollinators with bright colors.
Sepals: Protect the flower bud.
Stamen (male parts):
Anther: Holds pollen.
Filament: Supports the anther.
Carpels (female parts):
Ovary: Contains ovules (egg cells).
Style: Connects the ovary to the stigma.
Stigma: Sticky surface for pollen reception.

All these parts are attached to the stalk at the receptacle.

Flower Completeness and Sexuality

Complete flowers: Possess all the above parts.
Incomplete flowers: Lack one or more parts.
Perfect (hermaphroditic) flowers: Have both male and female parts.
Imperfect flowers: Have only male or female parts.

Examples:

Complete flower: canola.
Incomplete flower (but still perfect): eucalyptus (lacks petals).
Imperfect flower: zucchini (separate male and female flowers on the same plant).
Monoecious: Having separate male and female flowers on the same plant (one house).
Dioecious: Having male and female flowers on separate plants (two houses), e.g., cannabis, mulberry, walnuts, and almonds.

Grasses

Grasses, including wheat and other grain crops, feature inflorescences with spikelets containing florets (equivalent to flowers). These florets are protected by glumes, palea, and lemma. Anthers hang on filaments, and stigmas sit atop the ovary.

Pollination and Fertilization

Pollen, containing sperm cells, can be used to identify plants due to its unique structure. Pollen grains land on the stigma, triggering germination and the growth of pollen tubes down the style. These tubes deliver sperm cells to the ovules in the ovary.

Double Fertilization

Angiosperms undergo double fertilization:

One sperm cell fertilizes the egg cell \rightarrow zygote (new generation).
The other sperm cell fertilizes the central cell \rightarrow endosperm (food source for the seed).

This endosperm serves as nourishment for the developing embryo within the seed.

Seeds and Germination

Seeds, as Thor Hanson beautifully stated, "nourish, unite, endure, defend, travel," enabling land plants to thrive.

Seed Development and Structure

Seeds develop from ovules after fertilization and provide protection, food reserves (endosperm), dispersal mechanisms (via fruits), and dormancy.

A seed consists of:

Plant embryo
Protective outer shell (seed coat or testa)
Cotyledons (embryonic leaves)
Radicle (embryonic root)
Hypocotyl (region connecting radical and cotyledons)

Dicots have two cotyledons, while monocots have one. Monocots also feature additional protective layers like the coleoptile.

Fruits

Fruits are enlarged ovaries that aid in seed dispersal via wind (e.g., dandelion), water (e.g., coconut), or animals (e.g., burrs).

Germination Process

Germination is the irreversible process where a seed sprouts into a plant, triggered by:

Suitable environmental conditions (water, oxygen, temperature, light)
Imbibition of water, leading to metabolic activity and radical (root) growth
Emergence of cotyledons

Seedling Development

Key terms:

Epigeal germination: Cotyledons emerge above ground.
Hypogeal germination: Cotyledons remain below ground.

The position of the cotyledons affects the plant's ability to recover from herbivore damage. Epigeal germination: plant is more susceptible to grazing/herbivore. Hypogeal germination: plant has more energy reserves and is more stable.
Terminology recap: epigeal - above the ground, and hypogeal - below the ground (geo=earth).
Cotyledons nourish the developing plant, and those above ground can perform early photosynthesis.

Seed Dormancy

Dormancy is a mechanism to prevent germination in unfavorable conditions, allowing for:

Dispersal
Prevention of simultaneous germination
Time for conditions to become suitable

Dormancy can last days, weeks, months, or even years, influencing the persistence of weed seed banks. Dormant seeds are difficult to kill!

Examples of hard to kill weeds: ryegrass (40yr seed dormancy), and docks.

Conclusion

The success of seed plants lies in their protected gametophytes. Flowers are reproductive structures that attract pollinators and give rise to fruits. Fruits, enlarged ovaries containing seeds, facilitate dispersal. Seeds protect and nourish the growing embryo, requiring specific conditions for germination. Be comfortable with the terms "epigeal" and "hypogeal". Dormancy is beneficial for plants but problematic for weed management.