• 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:

1. Investigate the effect of water, oxygen and temperature on germination.

2. Use of starch agar or skimmed milk plates to show digestive activity during germination.

Seeds - Dispersal and Germination

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.