Download 2.3 Sexual Reproduction in Plants

2.3 Sexual Reproduction in
Plants
How Plants Meet the Challenge of
Sexual Reproduction
• Plants can also reproduce by sexual
reproduction. The offspring have the
genes from two parents - their genes
are different, so the offspring will not be
identical. There is variety in the species.
• Movie
• Sexual Reproduction cross-word
How Plants Meet the Challenge of
Sexual Reproduction
• Seeds are the products of sexual
reproduction in most plants.
• A seed is a complete reproductive
package that contains an embryo, a
food supply, and a seed coat, which
protects it from drying out.
• Angiosperms – flowering plants
• Gymnosperms – seeds inside cones
Angiosperms
• Flowering plants contain the most diverse
and widespread members of the plant
kingdom:
monocots
dicots
• The greatest contribution to the success of
the angiosperms was the evolution of the
flower, which greatly improved its
reproductive efficiency.
Angiosperms
• Sexual Reproduction Angiosperms
• The angiosperms took it one step further. They not only
have seeds, but they also have flowers.
• What kind of an advantage is that? Well what if
you didn't need to rely on the wind to spread your pollen around
anymore? What if someone could do it for you? Maybe an
insect? Sounds like a new advantage. That's right, those
specialized flowers are able to attract organisms to help
pollinate and distribute seeds. Another cool advantage it the
fruit/seed packaging. Would you rather eat a pine cone or an
apple? So would a lot of animals. When they do, they are able
to spread the seeds across wide areas. After the animal poops
the seeds out of course.
• Reproductive organs in flowering plants. The stamens
are the male parts of the plant. Each consists of a
stalklike filament topped by an anther. The anther
contains four pollen sacs which burst to release tiny
grains of pollen, the male sex cells. The carpels are the
female reproductive parts. Each carpel has a stigma
which catches the pollen grain. The style connects the
stigma to the ovary. The ovary contains one or more
ovules, the female sex cells. Buttercups have many
ovaries; the lupin has only one.
Flowers in Angiosperms
•
•
The flower is the main reproductive organ that holds the sexual organs
of the plant. Each part of the flower has a specific function during the
different stages of sexual reproduction; namely pollination,
fertilisation and fruit formation with seeds. The flower is arranged in
whorls (rings) of modified leaves each performing a specific function.
These whorls are arranged on an enlarged base of the flower stalk
called the receptacle.
See diagram of flower
Pollination and Fertilization in
Angiosperms
• Pollination occurs when pollen is transferred from the ripe
anther to the ripe stigma. Minute pollen grains become exposed
when the anther splits open and are carried away by insects,
birds, the wind and sometimes water. Flowers must be
pollinated to ensure that the egg cells in the ovary can be
fertilised.
• If the pollen reaches the ripe stigma of the same flower it is
called self-pollination. If the pollen reaches the ripe stigma of a
different flower of the same species, then it is called crosspollination. The latter method of pollination produces stronger
and healthier seeds and therefore plants.
• Do the following activity to differentiate between self and cross –
pollination.
Seed Development in
Angiosperms
A seed consists of an embryo, food storage
tissue, and a seed coat. The embryo contains
the parts that form a new plant. It also has
one or more cotyledons, which absorb and
digest food from the food storage tissue. The
seed coat protects the seed from injury,
insects, and loss of water.
Gymnosperms
• Were the first of the two groups of seed
plants to appear in the fossil record.
• Almost all conifers are evergreens with
needle-shaped leaves and are among the
tallest, largest, and oldest living organisms.
• The cone is the distinguishing feature of
conifers; in the pine, two different types of
cones produce male and female
gametophytes on the sporophyte tree
• Pollen grains released as immature male
gametophytes land on female cones housing
immature females gametophytes inside
complex ovules.
• After a period of gametophyte maturation,
fertilization occurs.
• The zygote develops into an embryo, which is
packaged into a winged seed that disperses
by the wind from exposed scales of the
mature female cone.
Gymnosperms
• Seeds let you send you offspring out into the
world. Seeds provide a protective coat so that
the embryo plant can develop when it finds a
nice piece of soil. But remember this... no
flowers. Flowers are an evolutionary
advancement after seeds. So if you have a
vascular system, seeds and no flowers what
are you? A GYMNOSPERM!
Seed Dispersal in Angiosperms
Mechanism of dispersal
Seed Dispersal
• Seeds have adapted so as to prevent them
falling close to the parent plant. Failure to
disperse would result in overcrowding and
competition for the same resources. The role
of the fruit is to help scatter or disperse the
seed far and wide so that they can grow in
turn into healthy new plants. The main agents
of dispersal are wind, water and animals.
• Try this activity to see to better understand
the different methods of seed dispersal.
Wind Dispersal
Examples: poppy, dandelion, sycamore, tumble weed, agapanthus, pine
tree, maple, orchid, oats
•
The seeds are light and small so that they can be blown along
by a light breeze.
•
They are also smooth so they can roll along the ground easily.
•
Some seeds have papery wings, parachutes or long hairs to
carry them on air currents.
•
Some plants are able break off when dry; the wind then rolls
them along the ground scattering the seeds as they roll.
Try this activity to check your understanding
Animal Dispersal
Examples: many berries, many fleshy fruits and vegetables, protea,
mistletoe
•
Some seeds have hooked fruits that catch in the fur of passing
mammals or in people's clothing. They then fall off some
distance from the parent plant.
Succulent and brightly coloured fruits attract birds, insects and
animals that eat the fleshy part, while the seed passes
undigested through the alimentary canal, to be dropped some
distance from the parent plant.
•
Other seeds in large fruits may be carried away from the
parent tree by animals that eat the flesh and discard the seed.
Try this activity to check your understanding