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)• 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 formedNB: 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.

Endospermous Seed

Non-endospermous Seed

Part of the Part of the SeedSeed

FunctionFunction

EmbryoEmbryo Whole dormant immature plant inside the seedWhole dormant immature plant inside the seed

RadicleRadicle Part of the embryo which will become the roots of Part of the embryo which will become the roots of the plant after germinationthe plant after germination

PlumulePlumule Part of the embryo which will become the shoots Part of the embryo which will become the shoots (stems, leaves etc.) of the plant after germination(stems, leaves etc.) of the plant after germination

CotyledonCotyledon This is a “leaf” of the embryo which can This is a “leaf” of the embryo which can • be a food reserve (in be a food reserve (in non-endospermous non-endospermous seeds) seeds) oror • absorb nutrients from the endosperm for absorb nutrients from the endosperm for growth of the during germination (in growth of the during germination (in endospermousendospermous seeds) seeds)

TestaTesta Protective coating of the seed, formed from the Protective coating of the seed, formed from the integuments of the ovule.integuments of the ovule.

Functions of the Parts of the Functions of the Parts of the SeedsSeeds

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• 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 lightor darkness varies from plant to plant.

Steps in Germination

• Preparation• Digestion• Respiration• Growth • Photosynthesis

Steps in Germination I

• Water and oxygen is absorbed through the micropyle.

• This activates enzymes which digest Fats → Glycerol and Fatty AcidsStarch → SugarsProteins → 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