Reproduction and responses in plants (pg. 51, 58-61)

• The stamen is the male reproductive part: it consists of the anther and filament.
• The carpel is the female reproductive part: it consists of the ovary, style and stigma.
• The anther contains the male gametes inside pollen grains.
• The filament is the stalk that supports the anther.
• The stigma is the end bit that the pollen grains attach to.
• The style is the rod-like section that supports the stigma.
• The ovary contains the female gametes inside ovules.
• Pollination is the transfer of pollen from an anther to a stigma, so that the male gametes can fertilise the female gametes in sexual reproduction.
• Cross-pollination is a type of sexual reproduction where pollen is transferred from the anther of one plant to the stigma of another.
• Plants that cross-pollinate rely on things like insects or the wind to help them pollinate.
• Plants are adapted for insect pollination:

  1. They have brightly coloured petals to attract insects.
  2. They have scented flowers and nectaries to attract insects.
  3. They make big, sticky pollen grains so that they stick to insects.
  4. The stigma is sticky so that any pollen picked up by insects will stick to it.
• Plants are adapted for wind pollination:

  1. Small, dull petals on the flower (no need to attract insects).
  2. No nectaries or strong scents.
  3. A lot of pollen grains - they are small and light so that they can be easily carried by the wind.
  4. Long filaments that hang the anthers outside the flower so that a lot of pollen gets carried away by the wind.
  5. A large and feathery stigma to catch pollen as it's carried by the wind. The stigma often hangs outside the flower too.
• Parts of a plant diagram:

  A) petal

  B) stigma

  C) style

  D) ovary

  E) ovule

  F) carpel

  G) anther

  H) filament

  I) stamen

  J) sepal

  K) stem

  
• Fertilisation in plants:

  1. A pollen grain lands on a stigma.
  2. A pollen tube grows out of the pollen grain and down through the style to the ovary and into the ovule.
  3. A nucleus from the male gamete moves down the tube to join with a female gamete in the ovule.
  4. Fertilisation is when the two nuclei fuse together to make a zygote. This is divided by mitosis to form an embryo.
  5. Each fertilised female gamete forms a seed. The ovary develops into a fruit around the seed.
• Flowering plants can only be fertilised by pollen grains from the same species (or a closely related species).
• A seed will lie dormant until the conditions around it are right for germination.
• Germination is when seeds start to grow.
• Conditions for seeds to start germinating:

  1. Water - to activate the enzymes that break down the food reserves in the seed.
  2. Oxygen - for respiration, which transfers the energy from food for growth.
  3. A suitable temperature - for the enzymes inside the seed to work. This depends on what type of seed it is.
• Process of germination:

  1. The seed takes in water and starts to grow using its store of energy.
  2. The first root starts to grow down into the soil.
  3. The shoot grows up.
  4. Finally, extra roots grow and the first green leaves appear.
• A developed seed contains an embryo and a store of food reserves, wrapped in a hard seed coat. When a seed starts to germinate, it gets glucose for respiration from its own food store. This transfers the energy it needs to grow. Once the plant has grown enough to produce green leaves, it can get its own food for energy from photosynthesis.
• Germination Investigation:

  1. Take four boiling tubes and put some cotton wool at the bottom of each one.
  2. Put 10 seeds on top of the cotton wool in each boiling tube.
  3. Set up each boiling tube as follows:

  A) seeds

  B) moist cotton wool

  C) dry cotton wool

  D) cold temperature

  E) moist cotton wool

  F) layer of oil

  G) boiled and cooled water

  
• You have to boil water and then cool in the germination experiment because it doesn't contain any dissolved oxygen as most of it has evaporated.
• In the germination experiment, you should only see germination happening in Tube A. This is because all of the conditions needed for germination are present.
• Plants can reproduce asexually using natural methods or we can clone them using artificial methods.
• Natural asexual reproduction in plants (for example, a strawberry):

  1. The parent strawberry plant sends out runners - fast-growing stems that grow out sideways, just above the ground.
  2. The runners take root at various points (a short distance away) and new plants start to grow.
  3. The new plants are clones of the parent strawberry plant, so there's no genetic variation between them.
• Artificial asexual reproduction in plants:

  1. Gardeners can take cuttings - each with a new bud on - from good parent plants, and then plant them to produce genetically identical copies (clones) of the parent plant.
  2. These plants can be produced quickly cheaply.
• Plants are more likely to survive if...

  1. They respond to changes in their environment.
  2. They respond to the presence of predators to avoid being eaten.
  3. They respond to abiotic stress.
• Auxins are plant hormones which control growth at the tips of shoots and roots. They move through the plant in solution (dissolved in water). They are involved in the growth responses of plants to light (phototropism) and gravity (geotropism).
• Auxin is produced in the tips and diffuses backwards to stimulate the cell elongation process which occurs in the cells just behind the tips.
• Auxin promotes growth in the shoot, but inhibits growth in the root.
• Shoots are positively phototropic (grow towards light).

  1. When a shoot tip is exposed to light, it accumulates more auxin on the side that's in the shade than the side that's in the light.
  2. This makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light.
• Shoots are negatively geotropic (grow away from gravity)

  1. When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
  2. This causes the lower side to grow faster, bending the shoot upwards.
• Roots are positively geotropic (grow towards gravity).

  1. A root growing sideways will have more auxin on its lower side.
  2. However, in a root, the auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards.
• Roots are negatively phototropic (grow away from light).

  1. If a root starts being exposed to some light, more auxin accumulates on the more shaded side.
  2. The auxin inhibits cell elongation on the shaded side, so the root bends downwards, back into the ground.
• Roots that are underground grow downwards due to positive gravitropism.