Transcript Seeds, - Ms Atkinson's Leaving Certificate Page

Seeds,
Dispersal, Dormancy and
Germination
Seed Formation
• A seed consists of
– A dormant plant embryo
– A food reserve
– A protective coat
• Each fertilised ovule develops into a seed
• Endospermous seeds
– the food reserve is outside the embryo as triploid
endosperm e.g. rice, maize
• Non-endospermous seeds
– the food reserve is within the leaves of the plant embryo
e.g. broad beans, peas
Seed Formation (contd)
• Embryo sac and ovule grow ~ 500 times bigger
• Triploid endosperm nucleus divides by mitosis
to form endosperm
• The endosperm (3n) is a food rich tissue which
nourishes the developing plant embryo
• The diploid plant embryo is formed by mitosis
and cell division of the diploid zygote
Seed Formation (contd)
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Embryo stops growing and becomes dormant
It loses up to 90% of its water
The micropyle closes
Hard protective coat (testa) forms from the
integuments
• A seed has formed
NB: The male gamete doesn’t supply any mitochondria
or chloroplasts – extrachromosomal inheritance is
via the egg cell
Seed Structure
• A seeds consists of a plant embryo and a food
reserve contained within a protective coat
(the testa).
• The food reserve may be
a) In the endosperm (as in endospermous seeds)
e.g. Maize, Castor Beans.
a) In the cotyledon (as in non-endospermous
seeds)
e.g. Broad Beans.
Non-endospermous Seed
Endospermous Seed
Functions of the Parts of the Seeds
Part of the Seed
Function
Embryo
Whole dormant immature plant inside the seed
Radicle
Part of the embryo which will become the roots of the
plant after germination
Plumule
Part of the embryo which will become the shoots (stems,
leaves etc.) of the plant after germination
Cotyledon
This is a “leaf” of the embryo which can
• be a food reserve (in non-endospermous seeds)
or
• absorb nutrients from the endosperm for growth of the
during germination (in endospermous seeds)
Testa
Protective coating of the seed, formed from the
integuments of the ovule.
Fruit formation
• A fruit is a fertilised, ripened, ovary of a flower
which contains the seeds.
• The fruit protects the seed and helps in
dispersal.
• A tomato is a true fruit, so is a grape.
• The inner area of an apple or a pear is a fruit,
which is contained within the outer sweet
flesh of the pear
• This outer area is a “false fruit” or swollen
receptacle.
Seedless Fruit
• Some are seedless varieties, and this is due to
genetics
– oranges: pollination occurs but fertilisation
doesn’t occur
– bananas: fertilisation occurs but seeds don’t
develop
• Spraying with plant growth regulators
– Auxin can stimulate fruit formation without
fertilisation
Seed Dispersal
• Dispersal is the transfer of a seed away from
the parent plant
• Reasons for dispersal
– Avoids competition with each other and parents
– Finding new areas to colonise
– Increases chances of survival of larger numbers of
plants
Wind Dispersal
• Tiny light seeds
– e.g. Orchids
• `Parachutes’ for increased air travel time
– e.g. Dandelion, Thistle
• Fruit with wings
– e.g. Sycamore
Water dispersal
• Air-filled fruits which can float
• Large distances can be covered
• Seeds can travel by river and streams
– e.g. Water lilies
• Seeds can even travel on the open sea to
other land masses
– e.g sea-beans, coconuts
Animal Dispersal
• Animals can travel long distances
• Animals often live in places where seeds can
germinate (e.g. underground)
• 2 types of Fruits
a) Hooked or barbed fruits e.g. Burdock,
goose grass
b) Edible fruits e.g. Tomato, Blackberry,
Acorns
Self Dispersal
• Explosive structures, catapults etc.
• Examples:
– Peas
– Gorse
– Poppies
Dormancy
• Dormancy is a resting period when seeds
undergo no growth and have reduced
cell activity or metabolism, even though
the environmental conditions are
suitable for growth.
Causes of Dormancy
• Growth inhibitors in the outer parts of the
seed
• Testa is too thick to allow water or oxygen in.
• Growing embryo can’t get out during
germination because the testa is too tough.
• A lack of growth regulators promoters
(auxins).
Breaking Dormancy
• Sometimes dormancy is broken by soaking or
scraping
– softens or breaks the testa
• Often a cold period is needed to break dormancy
– breaks down the growth inhibitors causing the
dormancy
– stimulates the production of growth promoters
• Light or warmth (e.g. in spring) may stimulate
production of growth promoters
Advantages of Dormancy
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Allows the embryo time to develop
Allows time for seed dispersal
Helps plant avoid winter conditions
The plant grows during the spring and
summer months
– optimum conditions for growth
• Duration of dormancy varies
– Staggered germinations allows seed banks to
develop in the soil
Dormancy in Agriculture
• Some seeds may need to be treated in a
certain way to break dormancy.
• Delayed and staggered germination is a
disadvantage in horticulture.
Germination
• Germination is the start of the process of an
embryo developing into a adult plant.
• Germination is the regrowth of the embryo,
after a period of dormancy, if the
environmental conditions are suitable.
• Dormancy causes the embryo to halt its
growth.
• Germination means that this growth resumes.
Conditions for Germination
• Water
– necessary for enzymes to work
• Oxygen
– necessary for respiration
• Suitable temperature
– necessary for enzymes to work
Dormancy must be finished and the need for light
or darkness varies from plant to plant.
Steps in Germination
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Preparation
Digestion
Respiration
Growth
Photosynthesis
Steps in Germination I
• Water and oxygen is absorbed through the
micropyle.
• This activates enzymes which digest
Fats
Starch →
Proteins
→
Glycerol and Fatty Acids
Sugars
→
Peptides and amino acids
• The products of digestion are moved to the
embryo.
Steps in Germination II
• Embryo undergoes rapid metabolism
fuelled by the products of digestion.
• Aerobic respiration results in the food
reserve getting used up.
• Mass of the seed drops as carbon dioxide
is produced and diffuses out of the cell.
Stages in Germination III
• Cell growth and division
• Differentiation of cells into tissues and organs
– Root system from the radicle
– Shoot system from the plumule
• Photosynthesis begins as shoots and leaves
develop
• Finally, the mass of the seedling increases
when
rate of photosynthesis > rate of respiration