9.3 Reproduction in Angiospermophytes


9.3.1  Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower

Structure of a Flower

9.3.2  Distinguish between pollination, fertilisation and seed dispersal

Pollination:  The transfer of pollen grains from the anther to the stigma (usually of another plant), often facilitated by animals, wind or water movement

Fertilisation:  Fusion of the male gamete nuclei (in the pollen grain) with the female gamete (in the ovule) to form a zygote

Seed Dispersal:  Fertilised ovules form seeds which move away from the parental plant before germination, reducing competition for resources

  • There are a variety of seed dispersal mechanisms, including fruit, wind, water and animals

9.3.3  Draw and label a diagram showing the external and internal structures of a named dicotyledonous seed

Structure of a Pea Seed  (Pisum sativum)

9.3.4  Explain the conditions needed for the germination of a typical seed

Germination is the process by which a seed emerges from a period of dormancy and starts to sprout

For germination to occur, a seed requires a combination of:

  • Oxygen:  For aerobic respiration (need ATP in order to grow)
  • Water:  To metabolically activate the cells
  • Temperature:  For the optimal function of enzymes

In addition, particular seed species may require other specialised conditions, such as:

• Fire            • Light or darkness            • Freezing            • Prior animal digestion            • Erosion of the seed coat            • Washing (to remove inhibitors)

9.3.5  Outline the metabolic processes during germination of a starchy seed

  • The first step in the germination process is the absorption of water, which causes gibberellin - or gibberellic acid (GA) - to be produced
  • Gibberellin causes the synthesis of amylase, which breaks down starch into maltose
  • Maltose is transported to the embryo, where it is either hydrolysed to glucose (for energy) or polymerised to cellulose (for cell wall formation)
  • Stored proteins and lipids will also be hydrolysed by the addition of water to form enzymes, triglycerides and phospholipids
  • Germination uses the food stored in cotyledons as an energy source until the developing shoot reaches the light and can begin to photosynthesise

9.3.6  Explain how flowering is controlled in long day and short day plants, including the role of phytochrome

  • Flowering is controlled by phytochrome, which is affected by light (photoperiodicity)
  • Phytochrome exists in two forms:
    •  A red (Pr) form absorbs red light (~660 nm) and is converted into a far red form (Pfr)
    • A far red (Pfr) form absorbs far red light (~730 nm) and is converted into a red form (Pr)
  • The Pfr form is the active form of phytochrome, while the Pr form is the inactive form of phytochrome
  • Sunlight contains more red light, so the Pfr form is predominant during the day, with the gradual reversion to the Pr form occurring at night
  • In long day plants, the active Pfr form is a promoter of flowering and so flowering is induced when the night period is less than a critical length and Pfr levels are high
  • In short day plants, the active Pfr form is an inhibitor of flowering and so flowering is induced when the night period is greater than a critical length and Pfr levels are low

The Control of Flowering in Plants