GCSE Environmental and Land-Based Science

Plant Cultivation and Small Animal Care

Seeds

We will learn:

  • about the structure of a seed
  • about the functions of seed structures
  • about techniques for breaking seed dormancy

A seed contains everything needed for a new plant to grow. It is made up of a protective covering containing the embryo plant and enough food to allow the plant to develop to the stage where it can make its own food.

Seed structure

Together the plumule and the radicle (shaded structures) form the plant embryo

Questions

1. / What is the name of the protective covering around the seed?
2. / What two structures make up the plant embryo?

Functions of seed structures

Each part of the seed has its own role.

The testaprotects the seed from attack by insects, fungi and bacteria. The micropyle is a small hole in the testa. This lets water into the seed, causing it to swell and split the testa so that germination can occur.

The cotyledons form the first two leaves of the developing plant. They contain enough food reserves to allow the young plant to develop to the stage where it can start making its own food by photosynthesis.

Together the plumule and radicle form the embryo plant. The radicle emerges first from the germinating seed and forms a root. The plumule develops to form the shoot of the seedling.

Germination

To germinate and develop into a new plant, seeds need oxygen, water and the right temperature.

The seed needs oxygen for respiration to provide the energy needed for growth. It can get the oxygen it needs from air trapped in spaces in the soil.

Water in the soil surrounding the seed softens the testa. To grow, the seed absorbs water through the micropyle and swells, splitting the testa and allowing the radicle to push out of the seed. Water activates enzymes in the seed that convert the food, which is stored in the cotyledons in the form of starch, to sugars that the growing plant can use.

Seeds will not germinate if it is too cold. They need a warm temperature to grow. Warmer temperatures also speed up enzyme reactions that convert the storage starch to sugars.

Questions

3. / What are cotyledons?
4. / How does water help the seed to germinate?

Seed dormancy

It is important that seeds germinate at just the right time for the young plant to develop and grow under the best conditions. Most seeds are produced in late summer and autumn. If they germinated straight away the delicate young seedlings would be exposed to the low light levels, cold temperatures and harsh conditions of winter. This is why seeds don't begin to grow until spring: they remain dormant.

Did you know?

If they are stored in cool, dry conditions wheat grains can remain dormant for 15 years. When they are given the right conditions for growth their dormancy is broken and they begin to grow.

The seed needs a trigger to break its dormancy when conditions are right for it to grow. In some species dormancy is broken by light. Increasing light levels and longer days in spring activate enzymes in the seed, prompting it to germinate.

Some seeds, such as those of conifers, need a period of cold before they will germinate. This is called vernalisation.

Dormancy is controlled in some seeds by chemicals such as tannin in the testa -soaking the seeds to wash away these inhibitors can help to break the dormancy.

Different ways of breaking dormancy

Commercial growers need to control germination so that it becomes more predictable, ideally so all the seeds germinate at the same time. This can be done by mimicking natural conditions. Techniques include:

  • Cold treatment (seed has been through a ‘winter’)
  • Heat treatment (seed germinates after a forest fire)
  • Acid treatment (seed passes through animal digestive system)
  • Abrasion of seed cost (seed needs testa to degrade)
  • Soaking of seed (rain leaches out germination inhibitor chemical)
  • Light treatment (small seeds may need light as if on soil surface)

Questions

5. / Why is dormancy important?
6. / Briefly describe two ways in which dormancy may be broken.

... seed structure ... GCSE conditions for germination ... seed dormancy