• Flowers are the?reproductive organ?of the plant
• They usually contain both male and female reproductive parts
• Plants produce?pollen?which contains a nucleus inside that is the?male gamete
• Unlike the male gamete in humans (sperm), pollen is?not capable of locomotion?(moving from one place to another)
• This means plants have to have mechanisms in place to?transfer pollen from the anther to the stigma
• This process is known as?pollination?and there are two main mechanisms by which it occurs: transferred by?insects?(or other animals like birds) or transferred by?wind
• The structure of insect and wind-pollinated flowers are slightly different as each is adapted for their specific function

Parts of the flower

General flower structure

Structure of a flower

Features of an insect-pollinated flower

Features of a wind-pollinated flower

• The pollen produced by insect and wind-pollinated flowers is also different:
  • Insect pollinated flowers produce?smaller amounts?of?larger, heavier?pollen grains that often contain?spikes or hooks?on the outside so they are better able to stick to insects
  • Wind pollinated flowers produce?large amounts?of?small, lightweight?pollen grains that are usually?smooth?

   

Here is an example of a multiple-choice question asking students to use their knowledge to identify types of pollen grain:

A?multiple-choice question asking students to use their knowledge to identify types of pollen grain

Pollination

• Pollination is discussed above as the ways in which pollen grains can be transferred from an anther to a stigma

Fertilisation

• Fertilisation occurs when a?pollen nucleus fuses with an ovum nucleus?in the ovule
• As the pollen has no ‘tail’ to swim to the ovary of a plant, in order to reach the ‘female’ nucleus in the ovary it has to grow a?pollen tube
• This only happens if the pollen grain has landed on the right kind of stigma (i.e. of the same species as the flower the pollen came from)
• The nucleus inside the pollen grain?slips down the tube?as it grows down the style towards the ovary
• The ovary contains one or more?ovules?which each contain an ovum with a female nucleus that a male pollen nucleus can fuse with
• Once the nuclei (pl) have joined together, that ovule has been fertilised and a?zygote?has been formed
• The zygote will start to divide and eventually form a?seed?within the ovule
• As different plants have different numbers of ovules, this explains why different fruits (which develop from the ovary) have different numbers of seeds (which develop from the ovules)

Growth of a pollen tube

?

Fertilisation in a flowering plant

• Germination?is the?start of growth in the seed
• Three factors are required for successful germination:
  • Water?- allows the seed to swell up and the enzymes in the embryo to start working so that growth can occur
  • Oxygen?- so that energy can be released for germination
  • Warmth?- germination improves as temperature rises (up to a maximum) as the reactions which take place are controlled by enzymes

   

• As carbon dioxide is not necessary for germination but also does not inhibit it, it makes no difference whether it is present or not

Investigating Germination

• Set up 4 boiling tubes each containing 10 cress seeds on cotton wool
• Set each test tube as shown in diagram below
• Leave tubes in set environment for a period of time: A, B and C incubated at 20°C; D placed in a fridge at 4°C
• Compare results and see which tube has the greatest number of germinated seeds

Conditions required for germination

Conditions required for germination - results:

Self- & Cross-Pollination: Extended

• Cross-pollination occurs when the?pollen from one plant?is transferred to the stigma of?another plant of the same species
• This is the way most plants carry out pollination as it?improves genetic variation
• Occasionally, the pollen from a flower can?land on its own stigma?or on the?stigma of another flower on the same plant?- this is known as self-pollination
• Self-pollination?reduces genetic variety of the offspring?as all the gametes come from the same parent (and are therefore genetically identical)
• Lack of variation in the offspring is a disadvantage if environmental conditions change, as it is?less likely that any offspring will have adaptations that suit the new conditions?well
• On the other hand, cross-pollination relies completely on the presence of?pollinators?and this can be a problem if those pollinators are?missing?(eg the reduction in?bee?numbers is of great importance to humans as bees pollinate a large number of food crops) - this doesn’t apply to wind-pollinated plants

Growth of the pollen tube and its entry into the ovule followed by fertilisation