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• The fruit develops from the following layers:
• Fruit wall from ovary wall.
• Seed from ovule.
Lorraine Kuun, July 2011
A SEED …• A seed consists
of a1. Seed coat –
outer layer of ovule (pericarp).
2. Embryo – from fertilisized egg cell (zygote undergoes mitosis).
Lorraine Kuun, July 2011
A SEED …Endosperm – result of double fertilization. Endosperm is food for embryo – also why we eat seeds for food.
Lorraine Kuun, July 2011
When a seed germinates • The cotyledons
contain food for the embryo in some seeds.
• In beans the cotyledons are responsible for photosynthesis after germination before first leaves are formed.
Lorraine Kuun, July 2011
• Resistant to unfavourable conditions as they have seed coat.
• Can be dispersed effectively (see later).• Can remain viable in dormant state for long
periods.• Seeds have stored food reserve in endosperm or
cotyledons; includes starch, oils and or protein.• Important to man as they are cheap form of plant
propagation, way to store plants and are a store of food.
Lorraine Kuun, July 2011
Significant features of seeds
• Dormancy is a state of rest. Embryo inactive, seed will not germinate.
• Some plants have obligatory period of dormancy – seed will not germinate even if conditions are favourable.
• Dormancy prevents seeds from hatching in wrong season when seedlings would be exposed to unfavourable conditions. Allows seeds to survive unfavourable conditions.
• Allows for seed dispersal agents to act.
Lorraine Kuun, July 2011
Why is dormancy important?
• Wind – seeds are light with plumes or wings.• Animals – hooks and thorns – cling to wool,
stick in paws.• Animals – edible fruit – seeds egested in
different position.• Water – seeds contain oil or air bubbles – float
away.• Self-dispersed – fruit dry, dehiscent.
Lorraine Kuun, July 2011
Agents for seed dispersal
SEEDS NEED TO BE DISTRIBUTED BY DIFFERENT AGENTS
COCO DE MER drifts along ocean currents
Lorraine Kuun, July 2011
• Most important plant source of food for humans.• Practical form of food – easy to transport and
store for long periods of time.• Grains – wheat, maize (mealies), sorghum, rice,
oats; mainly starch• Pulses (legumes) – beans, peas, soy beans,
peanuts, lentils, plant proteins• Nuts – oily seeds in hard shells e.g. walnuts,
cashews, pecans etc.
Lorraine Kuun, July 2011
Seeds as a source of food
• Many plant species under threat.• Seed bank stores seeds of wild plants and crops.• UK – conserves seeds of about 10% of wild plant
species at Kew – Millennium Seed Bank Project.• Swedish International Seed Vault – reinforced
concrete tunnel – 4,5 million seed samples – will remain viable for 1000’s of years.
• MSBP working with SA National Biodiversity Institute – contributing 2500 indigenous species – endangered, endemic, over-exploited
Lorraine Kuun, July 2011
Use of seed banks to maintain biodiversity
• Offer protection against loss of species in wild due to:
1.Habitat loss – agriculture, development of cities, building of dams, large-scale ecological disasters etc.
2.Climate change3.Over-exploitation of certain species
Lorraine Kuun, July 2011
How do seed banks contribute to maintain biodiversity?
Can be used to 1. re-establish damaged, lost habitats and
ecosystems2. re-introduce extinct, endangered or
threatened species3. provide research material
Lorraine Kuun, July 2011
SEED BANKS ...