24-2 Seed Development
and Germination
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24-2 Seed Development and
Germination
Seed and Fruit Development
Seed and Fruit Development
Once fertilization is complete, nutrients flow into the
flower tissue and support the development of the
growing embryo within the seed.
As angiosperm seeds mature, the ovary walls thicken
to form a fruit that encloses the developing seeds.
A fruit is a ripened ovary that contains angiosperm
seeds.
Biologically, a fruit is any seed that is enclosed within
its embryo wall. This includes apples and grapes, as
well as peas, corn, beans, rice, cucumbers, and
tomatoes, which we commonly call vegetables.
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24-2 Seed Development and
Germination
Seed Dispersal
Seed Dispersal
Seeds are dispersed by animals, wind, and water.
Seeds of many plants are eaten by animals.
These seeds are covered with tough coatings that
protect them from digestive chemicals, allowing them
to pass through a digestive system unharmed.
The seeds then sprout in the feces eliminated from
the animal.
Seeds dispersed by animals are typically contained in
fleshy, nutritious fruits.
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24-2 Seed Development and
Germination
Seed Dispersal
Seeds dispersed by wind or water are typically
lightweight, allowing them to be carried in the air or to
float on the surface of the water.
Some seeds are encased in winglike structures that
spin and twirl, helping to glide a considerable distance.
A coconut is a seed that contains a liquid endosperm
(“milk”) and is buoyant enough to float in seawater
within its protective coating for many weeks.
Tumbleweed plants break off at their roots and scatter
their seeds as they are blown by the wind.
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24-2 Seed Development and
Germination
Seed Dormancy
Seed Dormancy
Many seeds will not grow when they first mature.
These seeds enter a period of dormancy, during
which the embryo is alive but not growing.
The length of dormancy varies in different plant
species.
Environmental factors such as temperature and
moisture can cause a seed to end dormancy and
germinate.
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24-2 Seed Development and
Germination
Seed Dormancy
Seed dormancy can be adaptive in several ways:
• allows for long-distance dispersal
• allows seeds to germinate under ideal growth
conditions
Other environmental conditions can end dormancy:
• Some pine seeds produced in sealed cones
remain dormant until high temperatures
generated by forest fires cause the cones to
open.
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24-2 Seed Development and
Germination
Seed Germination
Seed Germination
Seed germination is the early growth stage of the
plant embryo.
When seeds germinate, they absorb water which
causes food-storing tissues to swell and crack
open the seed coat.
The young root grows through the cracked seed
coat.
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24-2 Seed Development and
Germination
Seed Germination
In most monocots, the cotyledon remains underground.
The growing shoot emerges while protected by a
sheath.
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24-2 Seed Development and
Germination
Seed Germination
In dicots, germination takes place in one of two ways.
•
In some species, the cotyledons emerge above
ground, protecting the stem and first foliage
leaves. The cotyledons may then wither and
drop off the plant or become photosynthetic.
•
In other species, the cotyledons stay
underground and provide a food source for the
growing seedling. The young stem grows
longer and forms an arch that protects the
delicate shoot tip.
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24-2 Seed Development and
Germination
Young
shoot
Seed Germination
Foliage
leaves
Cotyledons
Cotyledons
Seed coat
Germinating
seed
Primary
root
Bean (dicot)
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