Chapter 38:Plant Reproduction and Development

Flowers

Sexual organs of Angiosperms.

Develop from compressed shoots with four whorls of modified leaves.

Flower Whorls1. Sepals2. Petals3. Stamens4. Carpels

Sepals Whorl of sterile leaf-like structures. May be brightly colored. Function

Protect other flower parts. Attract pollinators.

Petals

Whorl of sterile flower parts.

Often brightly colored. Function

Attract pollinators.

Stamens

Male reproductive flower structure.

Function Produce pollen (sperm).

Stamen Structure

Anther - pollen producing sac.

Filament - stalk.

Carpel Female reproductive flower structure. Also called pistil. Function

Produce embryo sac (eggs). Carpel Structure

Stigma – receives the pollen. Style – stalk. Ovary – contains the ovules.

Ovule A rudimentary seed before fertilization.

Flowers Are highly variable in form, shape, and

color. Not all flowers contain the same

combination of whorls.

Flower Variations Complete Flowers - have all four whorls. Incomplete Flowers – less than all four

whorls. Perfect Flowers - have stamens and carpels. Imperfect Flowers - have stamens or carpels.

Monoecious Plants Staminate and pistillate

flowers are on the same plant.

Ex: corn Tassel – staminate flowers Ears – carpellate flowers

Dioecious Plants Have staminate or pistillate flowers, but not

both. Ex: Holly, Ginkgo Comment - "Seedless" plants may be a

staminate plant. Ex: male Ginkgo

Pollen Development Pollen is the male gametophyte. Starts with a 2N cell called a microsporocyte.

Microsporocyte Undergoes meiosis and produces 4

Microspores (1N). Each Microspore undergoes mitosis and

produces a pollen grain, which is the male gametophyte.

Mature Pollen Grain Special cell wall. Tube Nucleus (cell). Generative Nucleus (cell)

which will divide and produce two sperm nuclei.

Embryo Sac Development The Embryo Sac is the female gametophyte. Starts with a megasporocyte (2N). Megasporocyte

Undergoes meiosis and produces 4 Megaspores (1N).

Three of the megaspores abort, leaving only one to develop into the embryo sac.

Megaspore Undergoes three rounds of mitosis to produce a

“sac” with 8 nuclei. The nuclei may wall off or may remain as part of a

large cell. Embryo Sac

3 Antipodal cells. 2 Polar nuclei (1 cell) 2 Synergid cells 1 Egg cell

Pollination: The transfer of pollen from a stamen to the stigma. Pollen Vectors

Bees Flies Butterflies Moths Birds Beetles Bats Wind

Comment The flower is usually highly

adapted to the pollen vector. Ex:

Colors Rewards Scents Shape

Fertilization

The union of egg and sperm to produce a zygote (2N).

Angiosperms Have double fertilization.

1. Egg + sperm zygote

2. Polar nuclei + sperm Endosperm

Endosperm Is Triploid (3N) tissue that will be used as a

nutrition source for the embryo.

After Fertilization Zygote Embryo Ovule Seed Ovary Fruit

Embryo Development Root/shoot polarity set with 1st cell division. Mature embryo has all three primary tissues,

apical meristems etc. Endosperm

Monocots - large tissue. Dicots - transfer the energy over to the embryo's

two cotyledons.

Seed Coat Formed from the integuments of the ovule. Hard layer to protect the seed. Fruit

A mature ovary. Sometimes includes other plant parts. Ex: apple

Fruit Functions Protect the seeds. Aid in seed dispersal.

Fruit Types Fleshy: soft ovary

walls. Dry: hard ovary walls.

Fruit Types Simple Fruit: from a single ovary.

Ex: Peach, Cherry Aggregate Fruit: from a flower with multiple

carpels. Ex: Raspberry

Multiple Fruit: develops from several flowers into one structure. Ex: Pineapple

Seeds Contain a miniature plant. Main dispersal mechanism for plants. Embryo is often “dormant” when the seed is

mature.

Seed Dormancy When a seed is not actively growing. Used to increase the chances that the plant

will develop when conditions are favorable. Usually controlled by plant hormones. May require changes in temperature,

moisture, etc. before growth will continue.

Imbibition The absorption of water by a seed. First step in seedling growth. Causes “swelling” which ruptures the seed

coat. Starts metabolism to resume growth.

Germination The continuation of growth of the plant within

a seed. Root usually emerges first. Shoot tip must break through the soil surface.

Role of Light Light is the usual clue that the shoot has broken

above ground. Light causes many growth changes in the shoot

Ex. Leaves to expand Stem elongation rate decreases

Asexual Reproduction Offspring produced by mitosis. Also called “cloning”.

Normal Method Fragmentation of the plant

body so that new plants are formed.

Ex: Cuttings Grafting Offshoots

High-Tech Methods Tissue Culture. Clumps of cells grow

into embryoids which can be used to regenerate whole plants.

Often used in genetic engineering of plants.

Protoplast Fusion. A "naked" plant cell (no cell wall).

Used to create new hybrids

Plants Frequently use both sexual and asexual

reproduction depending on the environment. Asexual - stable Sexual - unstable

Plant Development Growth: increase in size Development: changes in body form and

structure. Plant Problems

Determining the direction of cell growth because of the cell wall.

Control Mechanism Cytoskeleton: determines the direction of cell

expansion. Cellulose Microfibrils: are arranged in

parallel strips as guided by microtubules in the cell membrane.

Cell Expansion Increase in cell size is usually caused by

turgor pressure. Direction of cell increase is at right angles to

the cellulose mircofibrils.

Cell Differentiation Depends on the control of gene expression. (review previous chapters on this topic)

Pattern Formation The development of specific

structures in specific locations.

Important in plants since cells don’t usually migrate.

Juvenile Mature

Positional Information Gradients of chemicals that provide clues of

position of cells to each other. Ex: why some cells develop into shoots and

others into roots.

Organ-Identity Genes Genes for normal organ development. Used in positional information to determine

which organ the cells should develop into.

Example The combination of three genes that give

rise to the flower parts. A sepals A + B petals B + C stamens C carpels

Mutations Cause other floral parts

to form in the flowers. (review Chapter 21).

Summary Know the general structures of flowers. Know the general life cycle of flowers. Review the items on plant development.