Plants can reproduce asexually (see cells)
however in order to make a seed a plant needs to reproduce sexually and this
requires fertilization (see cells).
Petals: Brightly coloured structures used to attract insects by
their bright colour and scent.
Sepals: Green leaves around the outside of
the flower. Sepals are usually smaller than the petals,. Used to
protect the flower while it is still in bud.
Stamens: Male part of the flower. Consist of
two parts: the filament (a thin stalk) and the anther
( a swelling at the top of the stalk). Pollen, which contains the
male gamete, is formed on the anther.
(Anther + filament = stamen)
Carpel: Female part of the flower. Contains the ovary, stigma and
style. The ovules, which contain the female gamete, are found in the
ovary.
(Stigma + style + ovary = carpel)
1. By insects: which visit the flower to obtain nectar. Some of
the pollen grains from the stamen stick to the body of the insect. When the
insect visits another flower some pollen grains fall off its body and stick to
the stigma. These flowers often have features to attract insects eg brightly
coloured petals or a pleasant smell. eg rose
2. By wind. Pollen is easily blown from the male part
of one flower to the female part of another plant Eg in a hazel.
Usually pollen moves from one plant to another plant of the same species.
This is called cross pollination.
Sometimes pollen land on the stigma of the same plant. This is called self-pollination.
After pollination fertilization takes place. This is when the male
nucleus and female nucleus join.
After fertilization the fertilized egg cell (zygote) divides many times and
forms an embryo. The embryo forms a hard wall around it and is then known
as a seed.
The seed also contains a food store, usually starch.
The part of the flower surrounding the seed is known as the fruit.
This usually formed from the ovary.
After fertilization the petals and stamens wither and die. The ovary (which
forms the fruit) swells up, sometimes considerably. (ie as in the apple)
The job of the fruit is to carry the seeds as far as possible from the parent
plant so the new plants have room to grow and do not compete for resources such
as light, water and nutrients in the soil.
This process is called seed dispersal.
Fruits disperse their seeds four main ways:
1. The fruit is eaten by animals such as birds but are not digested. The
seeds pass out the animal along with its droppings eg cherry, blackberry. These
fruits look and taste nice.
2. The fruit splits open. sometimes this happens with a lot of force and the
seeds are shot out. eg beans. the pod is the fruit and the beans
are the seeds.
3. The fruits have little hooks. these hooks stick to the fur of animals. eg
burdock.
4. The fruits have wings or hairs and this lets them get carried by the
wind,. eg sycamore trees have winged fruits.
Germination is the process by which the dormant seed actually
starts to grow.
(Dormant means sleeping or resting.
By becoming dormant it helps the seed survive adverse conditions - eg the cold
winter.)
Conditions needed for germination The seed needs the following three conditions to germinate:
water, oxygen
and warmth. When the seed gets damp it
absorbs water, ofter doubling in mass. Providing that the seed has oxygen and
water it will start to germinate.
The germination period is time between planting and starting to
germinate.
Experiment to show how warmth effects the
germination period
Three small pots had damp cotton wool placed
in the base, with three bean seeds placed on each piece of cotton wool.
The pots were placed at the following
locations: A warm, brightly lit window-sill, a closed refrigerator and a warm
dark airing cupboard. The seeds were inspected and watered daily.
The time taken to germinate was recorded in each case.
Conditions:
Window-sill: warmth, light and
water
Airing cupboard: warm, no light and water
Refrigerator: cold, no light and
water;
NOTES: This type of experiment is called a controlled
experiment.
The seeds on the window-sill are the control, where the conditions are
normal. To make it a 'fair test' only ONE condition is altered between each
experiment, the others are kept the same. Several seeds are used in case of any
fail to grow, and the average time to germinate recorded.
Results The seeds in the refrigerator took longer to germinate showing that warmth
is necessary for germination.
The other two sets of seed germinated in a similar time showing that light is
NOT needed for germination
Notes:
Fertilization is when two gametes join.
In animals: male gamete is in the sperm, female gamete is in the egg (ovum)
In plants: male gamete is in the pollen female
gamete is in the ovum
Do not muddle up
fertilization, pollination and seed dispersal.
Do not muddle up wind blowing seeds
around (seed dispersal) with wind blowing pollen around (wind pollination)
Remember: only insect pollinated flowering
plants have petals. Grass IS a flowering plant but it is WIND pollinated.
