SEXUAL REPRODUCTION IN PLANTS
What is sexual reproduction?
Ans) Sexual reproduction is a process involving fusion of nuclei of two gametes
to form a zygote and the production of offspring that are genetically different
from each other.
Discuss the advantages and disadvantages of sexual reproduction of
plants.
Ans) Advantages
I.
II.
III.
IV.
V.
VI.
New varieties can be created, which may have more resistance to diseases.
Variation increases adaptation to a changing or new environment of
offspring.
Produce seeds which, which allow dispersal, away from the parent plant
and reduce competition.
Slower than asexual reproduction, therefore, it does not lead overcrowding
problem.
Seed dispersals allow them to colonize new areas.
It plays a key role in evolution by genetic recombination and variation.
Disadvantages
I.
II.
III.
IV.
Offspring are genetically different from there parents. So, excellent
individuals cannot give identical offspring.
It is a slow process and produce few offspring.
Growth of a new plant from a seed is slow.
This process leads a lot of energy.
Write the differences between asexual and sexual reproduction.
Asexual reproduction
i.
ii.
iii.
iv.
One parent is involved.
No gametes are formed.
No zygotes are produced.
It is a rapid force.
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v.
vi.
No variation and offspring are identical to parents.
Useful for maintaining the purity of crop characteristics and vegetative
propagation of economically important crops.
Sexual reproduction
I.
II.
III.
IV.
V.
VI.
Two parents are involved.
Gametes are involved.
Diploid cells called zygote are formed from haploid gametes.
It is a slow process.
Offspring show variations and are not identical to parents.
Use in plant breeding and crop improvement, plays a key role in evolution
by genetic recombination and variation.
Describe the functions of different parts of a flower.
Petals (Corolla): Petals are usually bright colored and sometimes scented.
Functions
i.
ii.
iii.
Protect the reproductive parts of the flower.
Bright color and scented petals attract insect for pollination.
Some petals have guideline, to guide the insect to the nectary gland.
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Sepals (Calyx): Outside of the petals there is a ring of green leaves called sepals or
calyx.
Functions
i.
Protects the flower parts when it was a bud.
Stamen: Stamen are the male reproductive part. Each stamen has a stalk called
filament with an anther on the end.
Functions
i.
Produces pollen grains.
Pistil: Pistil are the female reproductive part. Each pistil consists of an ovary,
bearing a style and stigma.
Functions
a) Stigma
i. Stigma has a sticky surface and receives pollen grains from the same or
different plant flower during pollination.
ii. It produces nutritive materials to the pollen and stimulate to form a pollen
tube.
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a) Style: Holds the stigma to be exposed for pollinating agent like wind, water,
insects, animals, humans etc.
b) Ovary: Contains different numbers of ovules, which contain the female
gametes.
Receptacle: It holds the flower parts.
What is pollination?
Ans) Pollination is the transfer of pollen grain from anther to stigma within the
species. It can be of two types:
a) Self-Pollination: It is the transfer of pollen grains from anther to the stigma
of the same flower or another flower on the same plant.
b) Cross Pollination: It is the transfer of the pollen grains from an anther of
one plant to the stigma of another flower of the same species.
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Write the characteristics of self-pollinated and cross-pollinated
flowers.
Ans) characteristics of self-pollinated flower
i.
ii.
Anther and stigma are matured at the same time.
Stamens are longer than carpels.
Characteristics of cross-pollinated flower
i.
ii.
The anther and stigma are matured at different time.
Stamens are shorter than carpels.
Write the advantages and disadvantages of self-pollination.
Advantages
i.
ii.
iii.
iv.
v.
It preserves the parental characteristics.
It does not need any external agency.
It is necessary for species which are at great distant from each other.
It is advantageous to those plant which do not produce nectar.
Maintain the purity from one generation to another as no variation takes
place.
Disadvantages
i.
ii.
iii.
iv.
The useful characteristics cannot be introduced.
The undesirable characters do not eliminate.
Continuous self-pollution can lead to death of species.
No variation is produced as a result no evolution takes place.
Write the advantages and disadvantages of cross-pollination.
Advantages
i.
ii.
iii.
Offspring may produce desirable qualities from both parents.
More varieties of offspring can be produced due to genetic recombination.
Variation increases the chance of survival of the species during changes in
the environment.
Disadvantages
i.
Waste of pollen is high.
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ii.
iii.
Pollination will not be successful for species which are great distant from
each other.
Consume more energy.
Describe the structural adaptations of wind-pollinated and insectpollinated flower.
Adaptations of wind-pollinated flower
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Small dull colored flower usually has no petals, but some have small
inconspicuous petals.
Not scented.
No nectar.
Light and smooth pollen grains.
Large feathery stigma hanging outside.
Large anther which often have long filaments. So, that they hang well
outside the flower.
Produce large quantities of pollen.
High pollen wastage.
Adaptations of insect-pollinated flower
i.
ii.
iii.
Flowers are large, often has large colored conspicuous petals with
guidelines.
Nectar is present.
Scented.
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iv.
v.
vi.
vii.
viii.
Small sticky stigma inside the flower.
Produce relatively less quantity of pollen.
Less pollen wastage.
Anthers are inside the flower to brush against the insect.
Sticky or spikey pollen grain, which stick to insects.
Write the anthers and stigmas of wind-pollinated glower.
Stigmas of wind-pollinated flower
Stigmas are large and feathery and dangling outside the flower. It acts like a net,
catching pollen as it floats through the air.
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Anthers of wind-pollinated flower
Large anthers which often have long filaments. So, that they can hang well
outside the flower. They are loosely attached so that they can sway and shake out
pollen even when in lightest breeze.
Distinguish between the pollen grains of insect-pollinated and wind
pollinated-flowers.
Pollen grain of insect-pollinated flower
i.
ii.
Sticky or spiky pollen grains, which stick to insects.
Produce smaller number of grains than wind-pollinated flower.
Pollen grain of wind-pollinated flower
i.
ii.
Small, light, and smooth pollen grains, so it can blow in the wind and stop it
clumping together.
Produce larger number of grains than insect-pollinated flower.
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What is fertilization?
Ans) Fertilization is the fusion of the male gamete and female gamete producing
a cell called zygote.
Describe the stages of fertilization.
i.
ii.
iii.
iv.
v.
vi.
When a pollen grain from the same or different plant flower lands on the
stigma, it produces a sticky nutritive fluid.
The pollen grains on the stigma absorb this fluid and stimulates to form a
pollen tube. The pollen tube grows down the style.
The pollen grain nucleus travels down the pollen tube.
The pollen tube continues to grow until it reaches the ovary.
The pollen tube grows toward the ovule till it reaches the microphyte.
The male gamete fuses with the female gamete in ovule and produces a
zygote.
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What happens to each of the following once a flower’s female
gametes have been fertilized?
Before fertilization
After fertilization
Petals
Falls off.
Sepals
May or may not fall off.
Stamens
Falls off.
Zygote
Develops into an embryo plant.
Ovule
Develops into a seed, which
contains the embryo plant.
Integuments of the ovule
Testa
Ovary
Fruit containing seed.
Stigma and style
Fall off.
Describe the structure of a seed.
Ans) After fertilization the zygote divides by mitosis form embryo. The ovule is
called seed and the integuments of the ovule become hard, dry and form Testa.
The embryo consists of a radicle, which will grow into a root, and a plumule,
which will grow into a shoot.
The embryo is attached with leaf or leaves called cotyledon, which stored food.
The Testa has a tiny hole called microphyll. The hilum is the scar, where the seed
was jointed to the pod or ovary.
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Write the importance of water, oxygen, and temperature for
germination of a seed.
i.
ii.
iii.
iv.
v.
Water activates the enzyme in the seed and help to break down stored
starch to sugar and protein to amino acids.
Water transports the sugar in solution from the cotyledons to the growing
regions.
Water maintains the turgor pressure of the cells and help to keep the shoot
upright and to expand leaves.
The oxygen is used in aerobic respiration which provide the energy for
many chemical changes and to make new cytoplasm and cell wall of the
growing seedling.
At optimum temperature enzyme works faster. But high temperature
denatures the enzyme and germination may stop.
Describe the germination process of a seed.
Ans) When a seed is given the right temperature, it absorbs water through the
microphyll. The seed swells as a result and the Testa splits. Water activates the
enzymes which digest the food in the following manner:
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Starch amylase
→ Maltose
Protein protease → Amino acid
Lipids Lipase → Fatty acids + glycerol
The digested food is the absorbed by the embryo. Hence, it starts growing,
producing the radicle outside followed by the plumule. The radicle becomes the
root and the plumule becomes the shoot. As leaf opens, photosynthesis begins.
Definitions
Pericarp: It is the wall of fruit formed by the ovarian wall. It may be fleshy or dry.
The thick pericarp of a fleshy fruit maybe differentiated into three layers.
i.
ii.
iii.
Exocarp or pericarp: The thin outermost layer of the pericarp is called
exocarp.
Mesocarp: The middle fleshy or fibrous layer of the pericarp is called
mesocarp.
Endocarp: The innermost layer of the pericarp is called endocarp.
Investigate the need of water for germination.
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Procedure
• Label three containers A, B, and C and put dry cotton wool in the bottom of
each.
• Place equal number of soaked seeds in all three containers.
• Leave “A” quite dry, add water to “B” and make the cotton wool moist.
• Add water to “C” until all the seeds are covered with water.
• Cover them with lid and leave them at room temperature for a week.
Observation: The seeds in the container “B” will germinate normally. Seeds in the
container “A” will not germinate. The seeds in container “C” may have started to
germinate or died.
Result: Water is necessary for germination but too much water prevent
germination by cutting down the oxygen supply to the seed.
Investigate the necessity of oxygen for germination.
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Procedure
i.
ii.
iii.
iv.
v.
Take two conical flask A and B, keep some pyrogallic acid and sodium
hydroxide solution in flask “A”.
Keep only sodium hydroxide solution in flask “B”.
Take some moist cotton wool and rolled in some cress seeds.
Hang them in the flask with bent pin, and bungs. Make the flask airtight
with bungs.
Leave the flask for a week at room temperature.
Observation: Pyrogallic acid and sodium hydroxide absorb oxygen and carbon
dioxide from the air. So, cress seeds in flask “A” are deprived of oxygen and
carbon dioxide. But sodium hydroxide absorbs carbon dioxide from the air in flask
“B”. As flask “B” has oxygen the seeds germinate.
Result: Oxygen is needed for germination.
Investigate the necessity of temperature for germination.
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Procedure
i.
ii.
iii.
iv.
Soak some maize grains for a day and then roll them up in three strips of
moist blotting paper.
Put the rolls into a plastic bag.
Keep one in refrigerator, one in warm place such as over a radiator or in an
incubator at 30 degree Celsius and keep one at room temperature.
After a week examine the seedlings measure the length of roots and
shoots.
Observation: The grains in the refrigerator will not start to germinate at all. The
seedlings kept at 30 degree Celsius will be more advanced than those at high
temperature.
Result: Seeds will not germinate below a certain temperature. The higher the
temperature, the faster the germination at least up to 35 degree Celsius.
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