Plants

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Plant groups  Photosynthesis    Sexual reproduction   Asexual reproduction

Sexual Reproduction in flowering plants  

Structure   Pollination   Fertilization    Seed dispersal    Germination

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).

The structure of a flower

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)

Sexual reproduction in a flowering plant has two main stages:
Pollination and Fertilization.    
After the seed is formed it has to be dispersed and then needs to germinate     

Pollination can occur two main ways:

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.

Fertilization

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)


Seed dispersal

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

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.

 

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.

 

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