1. Dispersal, Dormancy and Germination
Seeds,
Dispersal, Dormancy and Germination

2. 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 2

3. 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 3

4. Seed Formation (contd)
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 4

5. 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
In the endosperm (as in endospermous seeds)
e.g. Maize, Castor Beans.
In the cotyledon (as in non-endospermous seeds)
e.g. Broad Beans. 5

6. Non-endospermous Seed Endospermous Seed6

7. 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. 7

8. 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. 8

9. 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 9

10. 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 10

11. Wind Dispersal Tiny light seeds
e.g. Orchids
`Parachutes’ for increased air travel time
e.g. Dandelion, Thistle
Fruit with wings
e.g. Sycamore 11

12. 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 12

13. Animal Dispersal Animals can travel long distances
Animals often live in places where seeds can germinate (e.g. underground)
2 types of Fruits
Hooked or barbed fruits e.g. Burdock, goose grass
Edible fruits e.g. Tomato, Blackberry, Acorns 13

14. Self Dispersal Explosive structures, catapults etc. Examples: Peas
Gorse
Poppies 14

15. 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. 15

16. 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). 16

17. 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 17

18. Advantages of Dormancy
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 18

19. 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.

20. 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. 20

21. Conditions for Germination
Water
necessary for enzymes to work
Oxygen
necessary for respiration
Suitable temperature
Dormancy must be finished and the need for light
or darkness varies from plant to plant. 21

22. Steps in Germination Preparation Digestion Respiration Growth
Photosynthesis

23. Steps in Germination I
Water and oxygen is absorbed through the micropyle.
This activates enzymes which digest
Fats → Glycerol and Fatty Acids
Starch → Sugars
Proteins → Peptides and amino acids
The products of digestion are moved to the embryo. 23

24. 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.

25. 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

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