Flower Reproduction Guide

Lesson Overview

Reproduction in Flowering Plants

Lesson Overview
24.1 Reproduction in
Flowering Plants

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THINK ABOUT IT
What makes a flower beautiful?
To a plant, the whole point of a flower is to bring gametes together for
reproduction and to protect the resulting zygote and embryo.

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The Structure of Flowers
What are flowers?

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What are flowers?
Flowers are reproductive organs that are composed of four different kinds
of specialized leaves: sepals, petals, stamens, and carpels.

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Flowers are reproductive organs that are composed of four different kinds
of specialized leaves: sepals, petals, stamens, and carpels.
This diagram shows the parts of a typical angiosperm flower.

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Sepals and Petals
The outermost circle of floral parts contains the sepals.
Sepals enclose the bud before it opens, and they protect the flower
while it is developing.

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Sepals and Petals
Petals, which are often brightly colored, are found just inside the sepals.
The colors, number, and shapes of such petals attract insects and other
pollinators to the flower.

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Stamens
The stamens are the male parts of the flower—each stamen consists of
a stalk called a filament with an anther at its tip.
Anthers are the structures in which pollen grains—the male
gametophytes—are produced.

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Carpels
The innermost floral parts are the carpels, which produce and shelter
the female gametophytes and, later, seeds.
Each carpel has a broad base forming an ovary, which contains one or
more ovules where female gametophytes are produced.

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Carpels
The diameter of the carpel narrows into a stalk called the style. At the
top of the style is a sticky or feathery portion known as the stigma,
which is specialized to capture pollen.

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Carpels
Botanists sometimes call a single carpel or several fused carpels a pistil.

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Variety in Flowers
Flowers vary greatly in shape, color, and size.

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The Angiosperm Life Cycle
How does fertilization in angiosperms differ from that of other plants?

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How does fertilization in angiosperms differ from that of other plants?
The process of fertilization in angiosperms is distinct from that found in
other plants. Two fertilization events take place—one produces the zygote
and the other a tissue, called endosperm, within the seed.

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Angiosperms have a life cycle that shows an alternation of generations
between a diploid sporophyte phase and a haploid gametophyte stage.
Male and female gametophytes live within the tissues of the sporophyte.

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Development of Male Gametophytes
The male gametophytes—the pollen grains—develop inside anthers.
First, meiosis produces four haploid spore cells.

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Development of Male Gametophytes
Each spore undergoes one mitotic division to produce the two haploid
nuclei of a single pollen grain.
The two nuclei are surrounded by a thick wall that protects the male
gametophyte.

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Development of Female Gametophytes
Female gametophytes develop inside each carpel of a flower.
The ovules—the future seeds—are enveloped in a protective ovary—the
future fruit.

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A single diploid cell goes through meiosis to produce four haploid cells,
three of which disintegrate.

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The remaining cell undergoes mitosis, producing eight nuclei. These
eight nuclei and the surrounding membrane are called the embryo sac.
The embryo sac, contained within the ovule, makes up the female
gametophyte of a flowering plant.

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Cell walls form around six of the eight nuclei.
One of the eight nuclei, near the base of the gametophyte, is the
nucleus of the egg—the female gamete.
If fertilization takes place, this egg cell will fuse with the male gamete to
become the zygote that grows into a new sporophyte plant.

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Pollination
Pollination is the transfer of pollen to the female portions of the flower.
Some angiosperms are wind pollinated, but most are pollinated by
animals.
Because wind pollination is less efficient than animal pollination, windpollinated plants, such as oak trees, rely on favorable weather and
sheer numbers of pollen grains to get pollen from one plant to another.

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Pollination
Animal-pollinated plants have a variety of adaptations, such as bright
colors and sweet nectar, to attract and reward animals.
Animals have evolved body shapes that enable them to reach nectar
deep within certain flowers.

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Pollination
Insect pollination is beneficial to insects and other animals because it
provides a dependable source of food—pollen and nectar.
Plants benefit because the insects take the pollen directly from flower
to flower.
Insect pollination is more efficient than wind pollination, giving insectpollinated plants a greater chance of reproductive success.

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Fertilization
If a pollen grain lands on the
stigma of a flower of the same
species, it begins to grow a
pollen tube.
Of the pollen grain’s two cells,
one cell—the “generative” cell—
divides and forms two sperm
cells. The other cell becomes the
pollen tube.

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Fertilization
The pollen tube contains a tube
nucleus and the two sperm cells.
The pollen tube grows into the
style, where it eventually reaches
the ovary and enters an ovule.

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Fertilization
Inside the embryo sac, two distinct fertilizations take place—a process
called double fertilization.
First, one of the sperm nuclei fuses with the egg nucleus to produce a
diploid zygote, which will grow into the new plant embryo.

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Fertilization
Second, the other sperm nucleus fuses with two polar nuclei in the
embryo sac to form a triploid (3N) cell.
This cell will grow into a food-rich tissue known as endosperm, which
nourishes the seedling as it grows.

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Fertilization
The endosperm and embryo of a corn seed are shown.

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Fertilization
By using endosperm to store food, the flowering plant spends very little
in the way of food resources on producing seeds from ovules until
double fertilization has actually taken place.
The resources saved can be used to make many more seeds.

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Vegetative Reproduction
What is vegetative reproduction?

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What is vegetative reproduction?
Vegetative reproduction is the formation of new individuals by mitosis. It
does not require gametes, flowers, or fertilization.

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Many flowering plants can reproduce asexually. This process, known as
vegetative reproduction, enables a single plant to produce offspring
genetically identical to itself by mitosis. It does not require gametes,
flowers, or fertilization.
This process takes place naturally in many plants.

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Types of Vegetative Reproduction
New plants may grow from roots, leaves, stems, or plantlets.
A potato is an underground stem that can grow whole new plants from
buds, called “eyes.”

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Strawberry plants send out long, trailing stems called stolons. Nodes
that rest on the ground produce roots and upright stems and leaves.

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New plants may grow from roots, leaves, stems, or plantlets.
Many cactus species can reproduce by dropping sections of their stems.
The small individuals growing at the base of the larger adults are clones

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Because vegetative reproduction does not involve pollination or
seed formation, a single plant can reproduce quickly.

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Asexual reproduction allows a single plant to produce genetically
identical offspring, enabling well-adapted individuals to rapidly fill a
favorable environment.
One drawback of asexual reproduction is that it does not produce new
combinations of genetic traits, which may be valuable if conditions in
the physical environment change.

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Plant Propagation
To propagate plants with desirable characteristics, horticulturists use
cuttings or grafting (shown) to make many identical copies of a plant or
to produce offspring from seedless plants.

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Plant Propagation
One of the simplest ways to reproduce plants vegetatively is by cuttings.
A grower cuts from the plant a length of stem that includes a number of
buds containing meristem tissue.
That stem is then partially buried in soil or in a special mixture of
nutrients that encourages root formation.

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Plant Propagation
Grafting is a method of
propagation used to
reproduce seedless plants
and varieties of woody plants
that cannot be propagated
from cuttings.
To graft, a piece of stem or a
lateral bud is cut from the
parent plant and attached to
another plant, as shown.

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Plant Propagation
Grafting works only when the two plants are closely related, such as
when a bud from a lemon tree is grafted onto an orange tree.
Grafting usually works best when plants are dormant, which allows the
wounds created by the cut to heal before new growth starts.

Flower Reproduction Guide

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