Seed Dispersal & Germination (HL IB Biology)

Seed dispersal

• Seed dispersal is then required in order to distribute the seeds away from the parent plant and reduce competition between the offspring and the parent plant
  • Methods of seed dispersal include
    • Wind or water
      • Parachute or wing shaped lightweight seeds will travel on the wind or float in water
    • Animals
      • Fleshy fruit is eaten by animals and seeds distributed through egestion 
      • Sticky or hooked seeds catch on to the fur or feathers of passing animals
    • Explosions
      • Some pods explode propelling the seeds away from the parent plant
• Seed dispersal can often be confused with pollination
  • Pollination is the transfer of pollen from anther to stigma, while seed dispersal is the distribution of mature seeds. Both processes can involve wind, water, or animals.

Seed germination

• Once a seed has formed within the ovary of a flower they undergo a period of dormancy 
• When conditions become favourable the seed may germinate
  • Germination is the start of growth in the seed

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 which take place are controlled by enzymes, which cannot function effectively when temperatures are too low

The process of germination

• A seed contains a plant embryo and food reserves for its growth
• The food reserves contain endosperm tissue which are transferred to the embryo through early leaf structures called cotyledons
• Seeds needs to replenish water lost during dormancy and does so through a process called imbibition which activates the biochemistry of the embryo
• The rate of respiration and protein synthesis increases and the embryo can prepare to emerge through the seed coat
• A structure called the radicle is the first to emerge and forms the initial root structure which responds to gravity and grows downward into the soil
• The first structure to appear above ground is called the hypocotyl, this is a curved portion of the plant shoot found below the cotyledons and grows upwards
• As the shoot grows the first leaves begin to appear from the cotyledon and photosynthesis can begin
• The root structure is also established and full plant growth can occur

Stages of germination diagram

The process of germination in flowering plant showing the development of roots and shoots