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Part VII : Reproduction in Flowering Plants

Fruits and Seeds
Other Methods of Dispersal
Conditions necessary for Germination
Development of the Embryo
Seeds Summary (useful for revision)
Seeds : Questions
Laboratory work relating to Seeds
and Seed Germination


Flowers and Reproduction
The Structure of Insect Pollinated Flowers
Pollinating Mechanisms
Wind-pollinated flowers

Topic Chapters Index

 

Half haricot bean with Iodine solution applied as seen under binocular microscope © Shirley Burchill

 

Labelled diagram L.S. through Haricot Bean © Shirley Burchill

REPRODUCTION IN PLANTS

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Seed Germination

Plants produce many fruits and therefore many seeds. This is because only a few of these seeds will find suitable places and conditions in which to germinate. The majority of the seeds will be wasted; some wind-dispersed seeds could fall in water or on stones, for example.

When fruits and seeds are dispersed the seeds have very little water in them. The outer covering, the testa, is hard and sometimes rough and creased. Although the seeds seem dry and inactive they are very much alive. They are said to be in a dormant state and some seeds can remain like this for a long period of time. Inside the testa there is just enough water to keep the embryo (the baby plant) living until the external conditions are correct for the seed to germinate.

 

The Structure of a Haricot Bean Seed

Labelled drawing haricot bean without testa, front view © Shirley Burchill

Labelled drawing haricot bean without testa, side view © Shirley Burchill

 

The outer covering of a seed is called the testa. It is usually hard and protects the softer parts of the seed. There is a tiny hole in the testa called the micropyle. When the seed is ready to germinate, water is taken in through the micropyle. The first root, called the radicle, will grow out of the seed through the micropyle. On the surface of the testa there is a scar which shows the place where the seed was attached to the fruit.

A dried seed is difficult to study. If the seed is soaked in water for twelve hours it takes in some of the water and swells. Peas and beans are useful seeds to study after they have been soaked because they are big. The testa can be removed easily and the inside of the seed can be examined under the binocular microscope.

 

Fact File No.37

Seeds are important because they provide us with food. The cereal plants such as wheat, rice and maize contain starch. Starch is a substance which we eat to give us energy. Starch is found in the endosperm of cereal seeds. This makes cereals important crops and they are grown all over the world.

Wheat endosperms are ground into a powder called flour. Flour is then used to make bread, cakes and pasta. Maize and rice can also provide flour, although both types of seed are often cooked and eaten whole.

Haricots, lentils, peas and nuts are important sources of protein. Protein is important in our diet to help us grow as well as to give us energy.

Other crops are grown because the seeds contain oil in their endosperms. Oil is also an energy-giving food. The oil can be taken out of the seeds and sold as cooking oil. Sunflower, olive and palm seeds are all between 20% to 50% oil. Maize seeds contain oil as well as starch.

 

Inside the Seed

Inside both the pea and the bean there are two large food-storing structures called cotyledons. The food contained in the cotyledons gives energy to the embryo when the seed germinates. The embryo is found between the two cotyledons. It is made up of two parts; the radicle, or first root, which points toward the micropyle, and the plumule, or first shoot. Under the binocular microscope the plumule can be seen as two very tiny leaves.

Some types of seeds have only one small cotyledon. Most cereal grains, such as wheat, are like this. The embryo is surrounded by a structure called the endosperm which is also a food storage area. Both the cotyledon and the endosperm will provide energy to the embryo during germination.

 

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