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Sexual Reproduction in Plants
Basic Plant Structure
• Plants have
three vegetative
organs:
– roots,
– stems,
– and leaves.
Asexual Reproduction
– All plant growth occurs by cell division (mitosis) and
cell elongation.
– Cell division occurs primarily in cells known as
meristems.
– Meristems are regions of embryonic tissue capable of
growing into new plant parts. Meristems are found in
both roots and shoots.
Primary Growth of Stems
1.Primary meristems make the shoot or root grow longer. This kind of
growth is called primary growth.
1.Secondary meristems make the stem or root grow larger in diameter.
This kind of growth is called secondary growth. Not all kinds of plants are
capable of secondary growth. Secondary growth gives rise to wood, and
plants that are not capable of secondary growth do not develop wood.
Plant Reproduction
A. Asexual Reproduction
1. Because plant growth is indeterminate, each meristem can
potentially develop into a complete plant. This means that it is very
easy to clone plants, and many plants can grow from cuttings or
broken plant parts. This is asexual reproduction (also called
vegetative reproduction).
B. Sexual Reproduction
1. Flowers are special reproductive structures found in the Flowering
Plants (=Angiosperms)
2. A flower is a specialized shoot, adapted for sexual reproduction.
3. A fruit develops from a flower following fertilization.
Major Plant Groups
• We are going to examine
several groups that show
these trends:
– 1. bryophytes: nonvascular plants
including liverworts
and mosses.
– 2. seedless vascular
plants such as ferns
and horsetails
– 3. gymnosperms,
which have seeds and a
vascular system, such
as the conifers
– 4. angiosperms, the
flowering plants that
dominate the world
today.
Bryophytes
• The bryophytes include
the mosses, liverworts,
and hornworts
• Bryophytes have no
internal vascular system.
• Bryophytes spend most of
their lives as haploids: the
body of the moss plant is
haploid.
Bryophyte Life Cycle
• The haploid gametophyte
plant bodies are either
male or female. Each
produces a different kind
of gamete (eggs or sperm)
• .
• The sperm are motile: they
swim through drops of
water (rain or dew) to
reach the eggs. The eggs
are encased within the
female gametophyte’s
body.
Bryophyte Life Cycle
•After fertilization, the
diploid sporophyte grows
as a stalk out of the
female gametophyte’s
body.
•After the diploid
sporophyte matures, the
cells in it undergo
meiosis, forming haploid
spores.
•The haploid spores
disperse in the wind, and
go on to form new
gametophyte plants.
Seedless Vascular Plants
• The seedless vascular plants
include ferns and horsetails.
• A vascular system to distribute
nutrients throughout the plant
allows them to grow tall.
Some ferns grow up to 80 feet
tall, and some extinct
horsetails were also tree-sized.
Fern Life Cycle
• Specialized structures on
the underside of the leaves
develop, and inside them
meiosis occurs.
• The haploid meiotic
products are released as
spores, which are dispersed
to new locations and
germinate into
gametophytes.
Fern Life Cycle
•The haploid
gametophytes are quite
small, a few millimeters
in diameter. They
contain structures that
produce sperm and
eggs.
•The sperm swim to the
eggs and fertilize them
•The fertilized eggs are
diploid, and they grow
into the new fern.
Seeds and Pollen
• A major development in plant
evolution was the development of
pollen grains and seeds.
• Pollen grains are the male
gametophyte packaged in a hard coat
that allows it to reach the female
without having to swim through water.
This is a large advantage on dry land.
• Seeds are diploid sporophyte embryos,
packaged to survive a period of
dormancy and bad environmental
conditions. Seeds develop from the
fertilized egg.
Pollination
• Pollination is the transfer of pollen from a
stamen to a pistil. Pollination starts the
production of seeds.
• To be pollinated, pollen must be moved from a
stamen to the stigma. When pollen from a
plant's stamen is transferred to that same plant's
stigma, it is called self-pollination.
Gymnosperms
• Gymnosperm means “naked
seed”: their seeds develop on
the outside of the plant,
instead of inside an ovary as
in the flowering plants.
• The most important
gymnosperms today are the
conifers: pines, redwoods,
cedars, etc. All are woody
plants with needles or scales
as leaves.
Gymnosperms
• In conifers (pine trees)
the tree produces both
male and female
cones.
• The male cones give
off sperm in the form
of pollen.
• The pollen grain
travels on the wind
and can land on a
female cone.
Gymnosperms
• The female cone has
water under the scales.
• The sperm gets stuck
in the water and when
the water dries up the
sperm is carried
directly to the ovaries
of the female cone.
Angiosperms
• Angiosperms are flowering plants. Most
of the plants we see are angiosperms.
• Unlike the other plant groups,
angiosperms are often fertilized with the
aid of animals.
• Some angiosperms have wind-dispersed
pollen. Flowers on these plants are
usually small and inconspicuous.
• Other angiosperms are self pollinators.
Their own sperm can be used for
fertilization.
Parts of a Flower
Fertilizing a flower
• For a seed to form the pollen must land on a
stigma and then absorb nutrients.
• As it absorbs nutrients it grows a tube down
the stigma until it reaches the ovary.
• Once it punches through to the ovary two
sperm cells (created by mitosis) make their
way to the ovary down the tube.
Fertilizing a Flower
• One sperm will fertilize the mature egg cell
while the other sperm will fertilize the polar
body.
• The polar body grows and turns into tissue
called endosperm which will be absorbed
by the mature zygote as food.