Seed Dispersal
Seed dispersal is the scattering of offspring away from each
other and from the parent plant.
Advantages of Dispersal
Improved chance of success by reducing competition and
overcrowding.
Enables colonisation of new suitable habitats — increased
chance of species survival.
Methods of Seed Dispersal
Wind
light weight seeds, e.g., orchid
high air resistance, e.g., ‘parachute’ of dandelion, ‘wings’ of
sycamore
Water
buoyant fruit, e.g., sedge
buoyant seed, e.g., water lily
Animal
passive, e.g., burdock
active — the animal seeks the fruit as a food source, e.g.,
tomato.
Mechanical
pea — the drying pod ‘flicks’ out the seeds.
Adaptations of Seeds as Dispersal Agents
Can survive a long period.
Large food reserve — improved chance of successful
establishment on germination.
Early growth accomplished in parent plant before dispersal —
improved the chance of successful seedling establishment on germination.
Seed Dormancy
Seed dormancy means that the living plant embryo will not begin
active growth even if the external environmental conditions are favourable.
Advantages of Seed Dormancy
Survival of plant embryo during long adverse growing conditions
of winter.
Germination is timed at the beginning of a long growing period
improving seedling success.
Allows more time for more widespread dispersal.
Dormancy varies - one year’s seeds do not germinate the same
year – improves species survival.
Dormancy in Agriculture and Horticulture
Seed dormancy results in many seeds germinating later or not at
all in the present growing season. It is advantageous to have as all the seeds
of crop plants germinating at the same suitable time.
Techniques to overcome the problem of dormancy
Use growth promoter to stimulate seed germination, e.g.,
gibberellins.
Soften the testa of the seeds by special treatment, e.g., soak
seeds in alcohol.
Store seeds in dry air.
Storage in moist cold air can break the dormancy of some seeds.
Seed Germination
Seed germination is the restart of growth by the plant embryo
using the food stored in the seed.
Water, oxygen and a suitable temperature are the major factors
for successful germination.
Water
Plant cells are 90% water.
Water is essential for increase in cell number during growth.
Water is needed for food reserve digestion and transport of
nutrients to the growing points.
Oxygen
Oxygen is needed for efficient ATP production from the reserve
food.
Fermentation is nineteen times less efficient than aerobic
respiration.
Suitable Temperature
Required for optimum enzyme activity and so for optimum growth.
Some of the enzymes are involved in the digestion of the
complex food reserve.
Other enzymes are involved in growth and ‘housekeeping’
activities.
Stages of Seedling Growth
Textbook Diagram: seed germination
Conditions for germination are suitable and dormancy is over.
Water and oxygen enter the seed.
Metabolism speeds up.
Enzymes digest the complex food reserve (starch, protein,
lipid).
The digested food is transported to the growing areas of the
embryo.
The food is used for growth.
Most of the food is used in aerobic respiration resulting in
loss of dry weight.
The radicle grows out forming a rooting system.
The plumule emerges forming a shoot system.
Photosynthesis begins.
Dry weight increases and seedling is independent when
photosynthesis is greater than respiration.
Mandatory Activities
To Investigate the Effect of Water, Oxygen and Temperature on
Seed Germination
Textbook Diagram: set up of the apparatus
Four small clear glass jars kept in darkness.
Jar A: water-soaked seeds on soaked cotton wool, open to air at
20ºC.
Jar B: dry seeds on dry cotton wool, open to the air at 20ºC.
Jar C: 4°C – water-soaked seeds on soaked cotton wool (in a
fridge) open to the air.
Jar D: water-soaked seeds at 20°C in a sealed jar that does not
have oxygen in the air – oxygen removed by pyrogallol or wet iron filings on
filter paper.
One week later check the results.
Results
A: water, oxygen and suitable temperature together
present / germination.
B: no water / no germination.
C: low temperature / no germination.
D: no oxygen / no germination.
Conclusion: water, suitable temperature and oxygen are together
required for seed germination.
To Show Digestive Activity During Seed Germination
Textbook Diagram: set up of apparatus
Soak maize seeds in water for two at 20°C – stimulates
germination.
Place half of the seeds in boiling water to kill them.
Cut the seeds in half so the entire plant embryo is only in one
half.
Sterilise the seed halves in disinfectant solution – no
digestive activity of micro-organisms.
Rinse seed haves in sterilised water.
Sterilise forceps by flaming.
Use forceps to place seed halves with live embryos into the
sterile starch agar in a petri dish.
Repeat for seed halves with dead embryo in a second sterile
starch agar plate – control plate.
A third unopened starch agar plate is used as a second control.
Minimum opening of plates when placing the seed halves.
Sterilise the forceps after transferring the seed halves.
Incubate all plates upside down for 3 days at 20°C.
Open plates and remove the seed halves.
‘Flood’ the plates with iodine solution.
Let soak for 2 minutes and pour off the iodine solution.
Results:
Unopened plate: uniformly blue-black – no starch breakdown.
Dead Embryos: uniformly blue-black – no starch breakdown.
Live Embryos: yellow-brown areas at seed sites – starch
breakdown.
Conclusion: starch digestion occurs during germination.
Repeat to verify the results.
The Syllabus Also Requires You To Know:
Definition and advantages of “dormancy”.
Mention of dormancy in agricultural and horticultural
practices.
Germination: definition, factors necessary, role of digestion
and respiration.
Stages of seedling growth.
Vegetative reproduction.
Practical Activities:
Investigate the effect of water, oxygen and temperature on
germination.
Use of starch agar or skimmed milk plates to show digestive
activity during germination.
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