SEEDS

SEEDS

A seed has:

An embryo which consists of an embryonic root or radicle and an embryonic shoot or plumule.

A testa or seed coat which protects the seed.

A micropyle for absorbing water and air during germination.

A food reserve which stores food and this is usually a cotyledon or seed leaf.

A hilum or scar left by the placenta.

Dicotyledons have two cotyledons while monocotyledons have one cotyledon.

At times food is stored in the endosperm.

Most monocotyledons seeds are endospermic for example maize while some are non-endospermic for example the water plantain.

Most dicotyledons are non-endospermic for example beans while some are endospermic for example castor oil.

Structure of a bean seed:

External view:

           

Longitudinal section:

                    

Testa removed:

 

                             

Structure of a maize grain:

The maize grain is a fruit because:

-          It has two scars: one left by the attachment to the cob and the other left by the style.

-          Its testa is fused with the pericarp.

External front view:

                                             

Longitudinal section:

                                   

GERMINATION

This is the beginning of growth.  In seeds germination involves the emergence of an embryo from a seed.

There are two types of germination:

Hypogeal germination where the cotyledons remain below the ground.  This is brought about by the epicotyl growing faster than the hypocotyl. Examples in monocotyledons like maize and in dicotyledons like castor oil.

Epigeal germination where the cotyledons appear above the ground. This is brought about by the hypocotyls growing faster than the epicotyl. Examples in dicotyledons like beans and in monocotyledons like onions.

Germination of the bean seed:

  • The seed absorbs oxygen and water through the micropyle then swells.
  • The stored starch is converted to sugars by an enzyme called diastase.

  • The seed uses up stored food of the cotyledons for respiration thus decreasing in dry weight.
  • The radicle emerges.
  • The hypocotyl grows faster than the epicotyl.
  • Root hairs appear on the radicle.
  • The hypocotyl bends pulling the cotyledons out of the ground (epigeal germination).
  • The testa falls off.
  • Lateral roots appear.
  • Two foliage leaves appear.
  • Cotyledons turn green and assist in photosynthesis.
  • Foliage leaves appear and carry out photosynthesis.
  • Cotyledons fall off.

 

Germination of a maize grain (hypogeal):

  • The grain absorbs water and swells.
  • The stored starch is converted to sugars by an enzyme called diastase.
  • The coleohirza (radicle sheath) bursts out through the pericarp.
  • The radicle then bursts out through the coleohirza.
  • Root hairs develop.
  • The coleoptile (plumule sheath) bursts out through the pericarp.
  • The epicotyl grows faster than the hypocotyl (hypogeal germination).

  • Adventitious roots appear below the coleoptile.
  • The plumule then bursts out through the coleoptile.
  • Lateral roots appear.
  • A foliage leaf appears and manufactures food.
  • Prop roots appear to give extra support.
  • The cotyledon rots under the ground.

 

Dormancy:

This is the state in which viable seeds will not germinate under normal conditions.

Causes of dormancy:

There are four major causes of dormancy in seeds:

Environment: This refers to the absence of an environmental factor other than water, air and warmth for example light, pH.

Such dormancy caused by lack of an environment factor can be broken by providing the missing environmental factor for example light, pH.

Seed coat structure: The testa may be hard.

Such dormancy can be broken by:

-          Microbial activities of fungi, bacteria in the soil which soften the testa.

-          Passage through guts of birds, bats and monkeys for example guava seeds.

-          Crackling due to heat and cold diurnal changes.

-          Artificially by churning in concentrated acid.

Immaturity of the seed embryo: Some seeds are dispersed when the embryo is not fully mature.

Such dormancy is broken by allowing time for the embryo to reach maturity.

Germination inhibitors: These may be in the seed or in the soil.  Inhibitors stop cell division, enzyme reaction or overall growth.  An example of an inhibitor is abscissic acid.

Such dormancy can be broken by use of growth promoters.

Advantages of seed dormancy:

-         It allows seeds to germinate only during favourable conditions.

-         It allows seeds to germinate only when the embryo is fully mature in order to attain healthy plants.

Conditions necessary for germination:

Environmental conditions:

¨      Water: this dissolves the food in the endosperm or cotyledon and breaks the dormancy period.

¨      Oxygen: this oxidizes the food in the seed to release energy necessary for germination.

Optimum (suitable) temperature: this should be around 40oC at which temperature enzymes allow metabolic processes to go on.

N.B: Some seeds may require exposure to light before they can germinate for example tobacco seeds.

Internal conditions:

¨      Enzymes for quickening metabolic processes.

¨      Energy for the process of germination.

¨      Viability i.e. ability of a seed to germinate for example lack of embryo due to damage to the seed by rodents, insects and fungi.

Control experiments

To find out whether a factor is necessary for a process, two experiments must be carried out.

For instance, to find out whether oxygen is necessary for germination, two experiments must be carried out: one where there is oxygen provided and the other where oxygen will not be provided.

The second experiment performed is called the control experiment and its aim is to confirm that the factor being investigated is actually responsible for the process concerned.

The two experiments must be almost identical.

Writing experiments:

Write down the title or aim.

List down the materials required including those for the control.

Outline the method in successive steps.  A diagram of the set up may be drawn under the method.

State the length and conditions under which the experiment should be left for example 2 hours in sunlight, 4 days in a moist place.

Write down the observation for both experiments.

Write down the conclusion basing on your title and observations.

An experiment to show that oxygen is necessary for germination:

Materials required:

Two conical flasks                                          sodium hydroxide solution to absorb carbon dioxide.

Soaked seeds                                                  2 corks

2 hooks or threads                                           moist cotton wool

pyrogallic acid to absorb oxygen

Method:

-          To the first flask A add some pyrogallic acid and sodium hydroxide solution.

-          To the second flask B add sodium hydroxide solution alone.

-          Wrap two groups of soaked seeds in moist cotton wool.

-          Suspend the seeds on a hook or thread in each of the two flasks.

-          Cover the flasks with cork.

-          Leave for three days.

Observation:

After three days, the seeds in flask A where oxygen was absent did not germinate while those in flask B which had oxygen germinated.

Conclusion: Oxygen is necessary for germination to take place.

An experiment to show that water is necessary for germination:

Materials required:

Soaked peas                                                    dry peas

4 beakers labelled A-D                                   4 covers

Wet cotton wool                                             dry cotton wool

Water

Method:

-          Label the four beakers A - D.

-          To the first beaker A, put wet cotton wool and soaked seeds.

-          To beaker B, put soaked peas on dry cotton wool.

-          To beaker C, put dry seeds on dry cotton wool.

-          To beaker D, put soaked seeds on wet cotton wool and cover with water.

-          Cover the four beakers.

-          Leave for four days.

                  

Observation:

-          In beaker A, germination took place successfully.

-          In beaker B, germination took place but later the radicles dried up as there was very little water.

-          In beaker C, no germination occurred at all.

-          In beaker D, the seeds rotted.

Conclusion:

Water is necessary for germination.

N.B: Beakers A and C act as the experiment and its control respectively.

In beaker D, too much water excluded air so that no germination occurred.

An experiment to show that germination takes place at optimum temperatures:

Materials:

Soaked seeds                          refrigerator

Wet cotton wool                     oven

Incubator and thermometers.

Method:

-          Wrap the seeds in cotton wool.

-          Divide them into five groups.

-          The first group should be placed in a refrigerator at 4oC.

-          The second group should be placed in an incubator at 20oC, another at 35oC, and another at 60oC.

-          The last group of seeds should be placed in an oven at 100oC.

Observation:

-          There was no germination of seeds in the refrigerator and the oven.

-          Very little germination was observed in the incubator at 20oC or at 60oC.

-          Very successful germination in the incubator at 35oC

Conclusion:

Germination takes place at optimum temperatures for example at 35oC where enzymes work best.

N.B:    Very low temperature inactivates enzymes.

            Very high temperature denatures enzymes.

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