Form 4 Biology – GROWTH AND DEVELOPMENT 2

GROWTH AND DEVELOPMENT IN FLOWERING PLANTS

Development are changes that occur in an organism from the time of fertilization to the formation of an adult body.

– In flowering plants, development starts with the growth of zygote into an embryo.

– In most flowering plants growth starts when the seed begin to germinate.

– Germination is the process by which the seed develops into seedling (young plants).

A summary of changes which occur during seed germinating

• The seed absorbs water through micropyle and enlarge.
• Later on the testa bursts and the radicle emerges. Radicle continues to elongate and gives rise to many roots.
• Then the plumule emerges.at this stage the young plant is called a seedling.

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CONDITION NECESSARY FOR GERMINATION

1. WATER

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• It is a suitable medium of enzymes to break down the stored food into suitable form.
• It hydrolyze food substance into glucose
• It is used to transport food materials between the cell/ to where they are used as source of energy.
• It softens the seed coat/testa so that it ruptures (bursts easily)

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2. OXYGEN/ AIR:

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In the dormant condition the seeds respiratory rate is very low and so oxygen is required in very small quantities. But for germination, oxygen is needed in large quantities. The seeds obtain oxygen that is dissolved in water and from the air contained in the soil. If soil conditions are too wet, an anaerobic condition persists, and seeds may not be able to germinate

3. TEMPERATURE: 

Germination can take place over a wide range of temperature and is specific to individual crop types, and can be specific to varieties. The optimum for most crops is between 65-75°F, but exceptions do apply. For example lettuce germinates best at 65°F and can be inhibited at temperatures over 68°F while peppers and eggplants prefer warmer temperatures around 80°F and will not germinate well at cooler temperatures.  If your soil is too cold or too hot, your seeds may not sprout. Check your seed packet to find the best temperature needed for your seeds.

4. LIGHT:

Light has varied effects on germinating seeds of different plants. Some seeds need light for germination, while in some seeds germination is hindered by light. Most wild species of flowers and herbs prefer darkness for germination and should be planted deep in the soil while most modern vegetable crops prefer light or are not affected by it, and are planted shallowly to allow small amounts of light to filter through the soil.

 

TYPES OF GERMINATION

1. EPIGEAL GERMINATION

• In epigeal germination the hypoctyl elongates first, plumule and cotyledons are brought above the soil surface.
• Seeds showing epigeal germination generally have small cotyledons, which once exposed to light develops chlorophyll and start to photosynthesis e.g beans and sunflower.

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                    epigeal germination diagram

 

2. HYPOGEAL GERMINATION

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In hypogeal germination the epicotyl elongates first and plumule is pushed upwards out of the ground.

• These seeds have large food reserved in their cotyledons
• Example of the seeds are maize and wheat

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             hypogeal germination diagram

 

NOTE:

Germination occurs only in a seed which is viable (whose embryo is alive)

 

MERISTEMS

• Meristerms are regions in plants where growth and development takes place.
• Main meristems are located at the tip of shoot and at the tip of roots.
• Active cell division and cell elongation takes place in meristems.
• The growing region of the radicle and hence the root has three main regions, including regions of cell division, elongation and differentiation.

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Types of Meristems

1. Apical meristems
2. Lateral meristems

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• Apical meristems bring about an increase in length and height of roots and shoots.
• Lateral meristems give rise to branches of shoot. Lateral meristems bring about the increase in growth of the shoots and roots.

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SEED DORMANCY

Seed dormancy is an inhibition of growth of an organism or part of it.

Some viable seeds will not germinate even when supplied with proper amounts of heat, water and oxygen. This is the period of reduce activity during which growth does not occur.

Factors which bring about seed dormancy

• Nature of the testa
• Presence of certain chemicals
• After ripening
• Food and water
• Climate
• Embryo may not be full develop

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• Lack of moisture and oxygen

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1. NATURE OF THE TESTA

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Some seeds the testa may be impermeable to oxygen and water. In such seeds time is required before the testa becomes permeable. The testa may also be hard – preventing the radicle and plumule form emerging.

2, Presence of certain chemicals

Certain chemicals present in either the seed or fruit are known to prevent seed germination. These chemicals are removed by leaching.

3. 
   AFTER – RIPENING

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Some seeds will not germinate immediately after harvest. It has been found out that at that time embryos are not fully developed. Such seeds have to go to be stored for some time so that the embryo develop fully. The period is called after-ripening.

WAYS OF BREAKING SEED DORMANCY

• Provide water
• Provide air
• Provide suitable temperature
• Ensure seed embryo is mature

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IMPORTANCE OF SEED DORMANCY.

• Seed dormancy has a survival value to plants seeds can survive for a long period especially in adverse environmental conditions such as drought, extreme temperature and shortage of food.
• If all seeds germinate at the same time in such conditions all seedling would perish.
• Seed dormancy helps an organism to with stand unfavourable conditions such as cold and drought shortage of water etc

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It allows time for dispersal of seeds by agents such as water and wind

Seed viability

The seed which retains its capability to germinate are known as viable and the ability is known as viability

FACTORS AFFECT THE VIABILITY OF SEEDS

1. Seed maturity.
2. Environmental condition.
3. Nature of a testa
4. Availability of moisture.
5. Temperature.
6. Light intensity.
7. Storage condition.

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                   GROWTH REGIONS OF A SEEDLING

– The growth of the radical and the plumule causes the elongation of the seedling. The rate of the growth can be measured at the tip of the root and shoot cells at the root and shoot tips have high capacity to divide.

– The dividing cells are known as meristematic cells, these cells make a tissue known as apical meristem. These cells rapidly undergo mitosis to provide growth which increases in size of the shoot and root.

– Growth in the tip of the root and shoot is known as primary growth.

– Widening of the plant to form the trunk and hard wood is called secondary growth.

NOTE: Meristems are regions of the plant in which plant cells divide.