Seeds & Germination (Cambridge O Level Biology)

• After fertilisation occurs, the ovules develop into seeds
• The seed contains the zygote (the fertilised egg cell), which divides into cells that then develop into the embryo plant
• When the seed germinates, this embryo begins to grow into the young seedling
• Structures known as cotyledons surround the embryo
  • Some plants have one cotyledon, whereas others have two
  • The cotyledons contain food reserves that supply the young seedling with food (and, therefore, energy for growth) when the seed starts to germinate
  • The cotyledons fulfil this role until the young plant grows its own leaves and becomes capable of making its own food via photosynthesis

• After taking in water, the seed coat (testa) splits
• This leads to the production of the plumule (the first emerging shoot) and radicle (the first emerging root)

Germinating seed structure diagram

The structure of a germinating seed

The process of germination

• Germination is the start of growth in the seed, which will eventually develop into a plant
• If conditions are favourable for germination to occur, the testa splits as the radicle emerges from the seed
• As the radicle grows downward, lateral roots and root hairs will develop
  • Lateral roots will anchor the developing plant, while root hairs are responsible for the absorption of water and mineral ions
• Cotyledons provide protection to the plumule as it grows upwards, until it is above the soil
  • The energy for growth up to this point is supplied by starch stored in the cotyledons
    • The starch is converted into soluble sugars by enzymes and transported to where it is needed in phloem tissue
    • These enzymes are activated at the start of germination
• Once the plumule leaves are above the soil and are green, they can photosynthesise and produce food for the plant

Investigating the requirements for germination

Three factors are required for successful germination:

• Water - allows the seed to swell up, which causes the seed coat (testa) to burst, allowing the growing embryo plant to exit the seed. Water also allows the enzymes in the embryo to start working so that growth can occur (increases metabolic activity)
• Oxygen - required for respiration, so that energy can be released for germination
• Warmth - germination improves as temperature rises (up to a certain point) as the reactions that take place are controlled by enzymes, which cannot function effectively when temperatures are too low
• As carbon dioxide is not necessary for germination but also does not inhibit it, it makes no difference whether it is present or not

Apparatus

• Test tubes
• Test tube holder
• Cress seeds
• Cotton wool
• Fridge

Method

• Set up 4 test tubes, with each containing 10 cress seeds on cotton wool
• Label the test tubes A, B, C and D
  • For test tube A, leave the cotton wool dry
  • For test tube B, add enough water to the cotton wool so that it becomes moist
  • For test tube C, add enough water to cover the cotton wool and seeds, then carefully add a layer of oil on top of the water
  • For test tube D, add enough water to the cotton wool so that it becomes moist

• Leave tubes A, B and C at room temperature or incubated at a specific temperature (e.g. 20°C)
• Place tube D in a fridge (approximately 4°C)
• Leave all tubes for a set period of time (e.g. 3 - 5 days)
  • Ensure the cotton wool in tubes B and D remains moist throughout this time by adding more drops of water if required

• Compare the results and see which tube has the greatest number of germinated seeds

Investigating germination diagram

Test tubes containing different environmental conditions can be used to investigate factors affecting germination

Results and analysis

• The test tubes are set up so that each of the factors required for germination (water, oxygen and warmth) can be tested, by selectively removing each in turn
  • In test tube A, water is removed
  • Test tube B is the control tube, where all factors are present
  • In test tube C, oxygen is removed (oxygen cannot pass through the oil and water layers on top of the seeds)
  • In test tube D, warmth is removed
• As germination cannot occur if the conditions are not right (i.e. if even one of the three factors required is absent), it would be expected that only the seeds in the control tube will germinate  

Conditions required for germination: example results table