CHAPTER 14: SEXUAL REPRODUCTION IN PLANTS
In this chapter, you will learn the following objectives:
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Identify flowers as reproductive structures; structures and functions of the parts of flowers
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•
State differences between wind and insect pollinated flowers; self and cross pollination; identify agents of pollination
Describe a detailed account of the process of fertilization; development of embryo and cotyledon; fruit and seed formation
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•
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Outline the process of fertilization; fruit and seed dispersal by water, animals, explosive and wind
Define germination
Describe the process of germination of a named dicotyledonous and named monocotyledonous seed
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•
Describe environmental and external factors necessary for germination
Explain the importance of enzymes in the release of energy from stored nutrients for the germinating seed
The flower is the sexual reproductive organ in flowering plants.
STRUCTURE OF THE FLOWER
STRUCTURE
DESCRIPTION
Carpel
Female part of the flower. It consists of the stigma, style and ovary.
Stamen
Male part of the flower. It consists of the anther and filament.
Petals
Sometimes provide color and scent to attract insects and birds for pollination
Sepal
Small leaf like structure that houses unopened buds and support opened flowers
Receptacle
Holds flower parts together, houses the ovary, and holds nectary which to attract insects and birds.
Stalk
Holds flower in best position for pollination
Anther
Produce pollen grains
i Filament
Holds up the anther in best position for pollination
!
Stigma
Catch pollen due to sticky outside
Style
Supports stigma, houses pollen tube, connects stigma and ovary
Ovule
Contains embryosac which contains egg cell, after fertilization becomes the seed
Micropyle
Smail hole in wall of ovule for male nuclei to enter
Ovary
Contains ovule, after fertilization become fruit
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POLLINATION
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Pollination is the transfer of pollen from the anther to the stigma.
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Self pollination: pollen is transferred to the stigma of the same flower or other flowers of the same plant.
Cross pollination: pollen is transferred to the stigmas of flowers on different plants of the same species. This results in
variety in the species.
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Cross pollination is brought about by several agents mainly wind and insects. Flowers are adapted for a particular type of
pollination.
STRUCTURE
WIND-POLLINATED FLOWERS
Pollen transported in the wind
INSECT-POLLINATED FLOWER
Pollen sticks to the body of insects as they feed on
the nectar of the flower.
FLOWER
Small and inconspicuous
Large and conspicuous
PETALS
Small, green, no scent, no nectarines, no
honey guides: no pollinating agent to attract
Larger, brightly colored, scented, have nectarines
and honey guides: need to attract insects
POLLEN GRAINS
Large number produced. The more produced
increases chances for reaching another flower.
Small, smooth, light: easily blown in the wind.
Smaller number produced. Large, sticky or spiky:
stick on insects
STAMENS
Long filaments with large, loosely attached
anthers which hang outside the flower:
pollen easily blown away by wind.
Short, stiff filaments 12183272 attached anthers
which are inside the flower: insect has to brush past
them to get nectar - picks up pollen
STIGMAS
Large, branched and feathery, hang outside Flat or lobed and sticky, situated inside the flower;
the flower: greater chance of catching pollen insect has to brush past to get nectar - deposit pollen
grains
EXAMPLES
Hibiscus, grass flower
Bauhinia
Fertilization in a Fl
FERTILIZATION
After the pollination:
1. A pollen tube grows out of the pollen grain.
2. The pollen tube with the help of enzymes grows into the
stigma and down the style to the ovary. It is attracted by the
sugar in the stigma and nourished by substances in the
tissues of the style.
3. The pollen tube grows into the ovule, usually through the
micropyle, and releases a male nucleus into the
embyrosac.
4. The male nucleus fuses with the egg cell, fertilization,
thus forming a fertilized egg called a zygote.
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-iter fertilization:
The zygote divides forming one or two cotelydons and an embryo, which consists of a plumule (young shoot), a radicle
(young root).
; The embryo becomes surrounded by endosperm tissue which acts as a food store. If the seed is a monocotyledon e.g. corn the
endosperm remains if it is a dicotyledon the endosperm is absorbed by the cotyledon e.g. legumes such as peas and beans.
The ovule develops into the seed; the wall of the ovule becomes the seed coat with the micropyle still there.
-, The ovary develops into the fruit.
: The seed dries out, becoming dormant.
6. The stigma, style, petals and stamens wither away and drop off. The sepals may or may not remain.
-RUIT AND SEED DISPERSAL
”he function of the fruit is protecting the seed and dispersing it. Dispersal of seed is important to prevent overcrowding and thus
competition and to allow colonization of new areas.
’here are two main types of fruits:
Fleshy (succulent) fruits one or more soft and juicy e.g. mango, tomato, orange. Fruit is eaten and seeds left behind or thrown
away. Sometimes seeds are eaten and are passed out in faeces. Their hard coats protect them from being digested.
2. Dry fruits are dry and hard e.g. pods. They are dehiscent fruits which split open when fully dry and scatters the seeds
(mechanical dispersal). Other dry fruits that do not split open to release seeds are called indehiscent. There are various methods
of dispersal:
a. Animals and other moving things: Fruits with hooks that cling to animals fur and people’s clothes e.g. goosegrass,
. shepherd’s needle, ‘prickles’ and burdocks
b. Wind and air currents: Fruits are light and have 'parachutes' or ‘wings’ to be carried by the wind e.g. dandelions.
Also occurs with spores.
c. Water: Floating seeds are usually large with air spaces e.g. coconut
d. Being eaten or collected for food: Animals eat fruits and seeds are passed out in faeces. Nuts dispersed by
animals that collect them and lose some.
e. Explosive (mechanical) mechanism: Fruit splits open and scatters seeds or spores over a wide area e.g. peas
f.
Pepper-pot mechanism: Case shaken by wind and seeds or spores scattered about.
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REVIEW #1 [42]
1. What is the function of the flower? [1 ]
2. Define pollination. [1]
3. List and explain the two types of pollination. [4]
a. Which type results in variety among the species?
4.
5.
6.
[1]
List the differences between wind-pollinated flowers and
insect-pollinated flowers in the following areas: [6]
a. Flower
b. Petals
c. Pollen grains
d. Stamen
e. Stigma
f.
Name an example of a wind-pollinated flowering
plant. [1]
g. Name an example of an insect-pollinated flowering
plant (both f and g should be examples mentioned
in class). [1]
Give a brief description of fertilization in plants. [5]
In plants, what is a fertilized egg called? [1]
a. Name the part of the seed that becomes the shoot.
7.
b.
Name the part of the seed that becomes the root.
c.
[1]
What is the function of the endosperm in the seed'
d.
[1]
In what type of seed does the endosperm remain?
[1]
What becomes of the following structures after fertilization?
[4]
a. Ovule
b. The wall of the ovule
c. Ovary
d. Petals
8. What is a fruit? [1]
a. What is the function of the fruit? [2]
9. Why is dispersal of the seed so important? [2]
10. Name the two types of fruit and give an example of each. [4‘
11. For the examples name the method of dispersal for each. [¿
a. Coconut
b. Dandelions
c. Nuts
d. Shepherd’s needle
[1]
ADVANTAGES AND DISADVANTAGES OF SEXUAL REPRODUCTION
Advantages
Disadvantage
If environment changes offspring may be better adapted.
Some offspring may not be adapted to the environment.
Offspring normally spread over a wide area which prevents
overcrowding and competition.
Slower, requires more time and energy consuming. A mate must be
found.
Genetic variability of the species is increased
Offspring are not produced continuously
The species may be able to colonize new areas
Fewer offspring may be produced
If parents are both of poor quality, offspring can still be of
better quality
If parents are of good quality, the offspring can be of poor quality
COMPARISON OF SEXUAL AND ASEXUAL REPRODUCTION
ASEXUAL
SEXUAL
One parent
Two parents
Offspring inherit one set of genes from one parent thus identical to
parents
Offspring inherit two sets of genes by fertilization
Offspring identical to parents and each other
Offspring not identical to parents or each other (unless identical
twins)
No variation - therefore unable to adapt to a changed environment
Variation due to fertilization; new and useful characteristics can
emerge which enable them to adapt to a changed environment
Fast - population grows rapidly (do not have to find mate)
Slow - population grows slowly (have to find a mate)
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GERMINATION
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Germination is the development of a seed into a seedling.
Environmental conditions for germination:
1. Water: to activate enzymes, for swelling and bursting of the seed, movement of food reserves and for growth of the shoot and
root.
2. Oxygen: for respiration to provide energy.
3. Suitable temperature: 5-40°C, for enzyme functioning.
■ Internal conditions are mainly controlled by enzymes:
•
Water absorbed through the micropyle activates enzymes to breakdown stored food in the cotyledon or the endosperm. The
endosperm is usually used up during germination:
Enzyme
Function (breaks down)
Use of end product by embryo
oteases
Protein to amino acids
Amino acids: growth
Carbohydrases
Starches to glucose
Glucose: respiration (energy) and to make
cellulose cell walls
joases
Fats to fatty acids and glycerol
Fatty acids and glycerol: respiration
’HE PROCESS OF GERMINATION
The dry seed absorbs water through the micropyle and expands.
: This expansion causes the seed coat to burst.
The radicle (young root) emerges and grows downwards.
- Then the plumule (young shoot) emerges and grows upwards.
Root hairs develop which increase the surface area for absorption of water and minerals salts.
Once it reaches above the soil, the shoot develops green leaves which start photosynthesizing.
As the plumule develops under the soil it is bent for protection or the coleoptile (a sheath).
Also the roots are protected by the root tip (slimy mass of loosely packed cells).
nere are two types of germination:
Epigeal (‘epi’-above; ‘geal’ ■ earth): cotyledons come above soil. Once it has reached the surface, the cotelydons turn green and
photosynthesize until leaves develop.
Hypogeal (‘hypo’ ■ below): cotyledons stay below the soil.
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Hypocotyl elongates and arches
upwards pulling cotyledons and
plumule out of sori
Radicle grows
downwards into soil
Stages in the germination of a red bean; an example of epigeal germination
Stages in the germination of a gungo pea (pigeon pea); an example of hypogeal -germination
■M
M»
Experiment
Watching bean seeds and corn seeds grow:
1. Soak seeds in water overnight.
2. Then put some water in the bottom of two beakers.
3. Roll up a two pieces of wet blotting paper and put one it into each beaker.
4. Remove seeds from water.
5. In one beaker place beans seeds between the blotting paper and the jar.
6. Do the same with the corn seeds in the next beaker.
7. Observe at intervals over the next ten days.
USES OF SEEDS BY HUMANS
1. Food e.g. peas and beans
2. Flour from wheat grain: testa and bran (fruit wall) used in whole-wheat flour and endosperm for white flour
3. Malt sugar for brewing beer from barley grains
4. Oils e.g. sunflower seed oil
5. Growing of crops for agriculture
M
REVIEW #2 [29]
1.
2.
3.
4.
5.
6.
List and explain an advantage and a disadvantage of sexual reproduction. [2]
List and explain three (3) differences between sexual reproduction and asexual reproduction. [6]
Define germination. [1]
Name the three conditions necessary for germination. Explain why they are needed. [7]
Name the enzymes involved in germination and describe the role they play. [9]
Distinguish between hypogeal and epigeal germination and give an example of both. [4]
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VOCABULARY
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Anther: produce pollen grains
•
•
•
•
Carpel : female part of the flower
Cross pollination: pollen is transferred to the stigmas of flowers on different plants of the same species
Embryosac: found in ovules and contains eggs
Epigeal germination: germination where the cotyledon comes above the soil
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Fertilization: male nucleus fuses with egg cell
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Filament: holds up the anther in best position for pollination
•
•
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Flower: sexual reproductive organ in flowering plants.
Fruit: organ that protects and disperses seeds
Hypogeal germination: germination where the cotyledon comes below the soil
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Micropyle of flower: small hole in wall of ovule for male nuclei to enter
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Nectary: produces nectar
Ovary: contains ovule, after fertilization become fruit
•
•
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Ovule: contains egg and after fertilization becomes seed
Petals: sometimes provide color and scent to attract insects and birds for pollination
Pollination: transfer of pollen from the anther to the stigma.
•
•
Receptacle: holds flower parts together, houses the ovary, and holds nectary
Self pollination: pollen is transferred to the stigma of the same flower or other flowers of the same plant.
•
•
Sepal: small leaf like structure that houses unopened buds and support opened flowers
Stalk: holds flower in best position for pollination
•
•
•
Stamen: male part of the flower
Stigma: catch pollen
Style: supports stigma, houses pollen tube, connects stigma and ovary
•
Zygote: fertilized egg
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