Transcript 24-2 PowerPoint

Lesson Overview
24.2 Fruits and Seeds
Lesson Overview
Fruits and Seeds
Seed and Fruit Development
Once fertilization of an angiosperm is complete, nutrients
flow into the flower tissue and support the development of
the growing embryo within the seed.
A fruit is a matured angiosperm ovary, usually containing
seeds.
Fruits vary in their structure.
Lesson Overview
Fruits and Seeds
Seed and Fruit Development
The term fruit applies to the sweet things we usually think
of as fruits, such as apples and strawberries. However,
foods such as peas, corn, rice, and tomatoes, which we
commonly call vegetables, are also fruits.
Lesson Overview
Fruits and Seeds
Seed and Fruit Development
The ovary wall surrounding a fruit may be fleshy, as it is in
grapes and tomatoes, or tough and dry, like the shell that
surrounds peanuts. (The peanuts themselves are the
seeds.)
Lesson Overview
Fruits and Seeds
Dispersal by Animals
The seeds of many plants, especially those encased in
sweet, fleshy fruits, are often eaten by animals.
The seeds are covered with tough coatings, allowing them
to pass through an animal’s digestive system unharmed.
Lesson Overview
Fruits and Seeds
Dispersal by Animals
The seeds then sprout in the feces eliminated from the
animal.
These fruits provide nutrition for the animal and also help
the plant disperse its seeds—often to areas where there is
less competition with the parent plants.
Lesson Overview
Fruits and Seeds
Dispersal by Animals
Animals also disperse many dry fruits, but not
necessarily by eating them.
Dry fruits sometimes have burrs or hooks that catch in
an animal’s fur, enabling them to be carried many
miles from the parent plant.
Lesson Overview
Fruits and Seeds
Dispersal by Wind and Water
Seeds dispersed by wind or water are typically contained
in lightweight fruits that allow them to be carried in the air
or in buoyant fruits that allow them to float on the surface
of the water.
A dandelion seed, for example, is attached to a dry fruit
that has a parachute-like structure, allowing the seed to
glide considerable distances away from the parent plant.
Lesson Overview
Fruits and Seeds
Dispersal by Wind and Water
Some seeds, like a coconut, are dispersed by water.
Coconut fruits are buoyant enough to float in seawater for
many weeks, enabling the seeds to reach and colonize
even remote islands.
Lesson Overview
Fruits and Seeds
Seed Dormancy and Germination
Many seeds will not grow when they first mature. Instead,
these seeds enter a period of dormancy, during which the
embryo is alive but not growing.
Germination is the resumption of growth of the plant
embryo.
Lesson Overview
Fruits and Seeds
Seed Dormancy and Germination
Environmental factors such as temperature and moisture
can cause a seed to end dormancy and germinate. The
effect of temperature on the germination of Arisaema
seeds is shown in the graph.
Lesson Overview
Fruits and Seeds
How Seeds Germinate
Before germinating, seeds absorb water, which causes
food-storing tissues to swell and crack open the seed coat.
Through the cracked seed coat, the young root emerges
and begins to grow.
The shoot—the part of the plant that will emerge above
ground—emerges next, as seen in the germinating corn
seed.
Lesson Overview
Fruits and Seeds
The Role of Cotyledons
Cotyledons are a flowering plant’s first leaves. They store
nutrients and then transfer them to the growing embryo as
the seed germinates.
Lesson Overview
Fruits and Seeds
The Role of Cotyledons
Monocots have a single cotyledon, which usually remains
underground while it passes nutrients to the young plant.
The growing monocot shoot emerges from the soil
protected by a sheath.
Lesson Overview
Fruits and Seeds
The Role of Cotyledons
In dicots, which have two cotyledons, there is no sheath to
protect the tip of the young plant.
Instead, the upper end of the shoot bends to form a hook
that forces its way through the soil.
This protects the delicate tip of the plant, which straightens
as it emerges into the sunlight.
Lesson Overview
Fruits and Seeds
Advantages of Dormancy
Seed dormancy can allow for long-distance dispersal, and
for seeds to germinate under ideal growth conditions.
For some species, a period of cold temperatures during
which the seeds are dormant is required before growth can
begin.
The period of cold that is required is long enough that
seeds will not germinate until the dangerous winter season
has passed.
Lesson Overview
Fruits and Seeds
Advantages of Dormancy
Sometimes, only extreme environmental conditions can
end seed dormancy.
Some pine trees, for example, produce seeds in cones
that remain sealed until the high temperatures generated
by forest fires cause the cones to open.
The high temperature both activates and releases the
seeds, allowing the plants to reclaim the forest quickly
after a fire.