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HOW DO ORGANISMS REPRODUCE
1. Define reproduction.
The production of new organisms from the existing organisms of the same species is called
reproduction.
2. Is reproduction required for survival?
No, unlike breathing and nutrition, reproduction is not a necessity to live.
3. Why is reproduction essential?
Reproduction is one of the important characteristic features of living organisms.
Reproduction is essential for the survival of species on the surface of earth. It ensures the
continuity of life on earth.
Reproduction is the process thro which variations can happen in the population leading to
evolution.
4. What is the importance of DNA copying in reproduction?
Reproducing organisms create new individuals like themselves, and this is possible only if the
same blueprint of the body design is created. Thus, basic event in reproduction is making copies
of the DNA in reproducing cells so that two similar individuals can be produced.
5. How do variations occur in a population?
The bio chemical reactions inside the cell for DNA copying cannot be absolutely reliable. So,
resulting DNA copies can be similar but not identical. The process of DNA copying will have some
variations, resulting in individuals with similarities but not identical.
5. Why is variation beneficial to the species and not necessarily to the individual?
If the niche suitable for a population drastically changes, the population can get wiped out. But, if
there are some variations in the population, which can help few organisms to survive, it prevents
the species from getting eliminated from earth, hence helping in the survival of a species. Thus
variations are beneficial to the species and not necessarily to an individual.
7. What are the types of reproduction?
The two types of reproduction are:
i.
ii.
Asexual reproduction: it is the method of reproduction in which a new individual is
produced by a single parent and no special cells or gametes or sex cells are formed. Eg.
Binary fission in amoeba, regeneration in Planaria, fragmentation in spirogyra, etc.
Sexual reproduction: it is a method of reproduction in which, a new organism is produced
by two parents, involving the formation and fusion of gametes or sex cells by these
individuals.
Eg. Fishes, frogs, humans, flowering plants.
8. What are the different types of asexual reproduction? Give examples.
The different types of asexual reproduction are:
1. Fission ……a) Binary fission…………eg. Amoeba, paramecium, eug
b) Multiple fission………..eg. Plasmodium
2. Budding……………………………………….eg. Yeast, Hydra
3. Spore formation………………………………eg. Rhizopus
4. Regeneration …………………………..……..eg Planaria
5. Fragmentation………………………………..eg. Spirogyra
6. Vegetative propagation…….a) Natural ………..i) leaf…….eg. Bryophyllum
ii) stem…..eg. potato tuber
iii)
Root……….eg. sweet potato
b) Artificial………i) cutting………..eg. Rose
ii) layering………...eg. guava
iii) grafting ………..eg. Mango
Fission: this is a method of asexual reproduction in which an organism, on achieving optimum
growth divides into two or more individuals simultaneously and each of them grows into an adult.
Fission can be of two types:
i) Binary fission: the parent cell on achieving maximum growth, divides / splits into two new
individuals. The parent ceases to exist. Each parent gives rise to two daughter cells. Unicellular
organisms such as Amoeba, Paramoecium, Leishmania reproduce by binary fission. ( explain
binary fission in amoeba with diagrams)
ii) Multiple fission: in this method of asexual reproduction, the nucleus of the parent cell divides
simultaneously into several nuclei, each of them gets surrounded by a bit of cytoplasm and plasma
membrane. This parent cell, with several daughter cells, remains dormant by developing a cyst
wall around itself during unfavorable conditions. When the conditions are favorable, the cyst wall
ruptures to release these daughter cells, which grow into new individuals. This method of
reproduction is seen in certain parasitic protozoans. Eg. Plasmodium. (malarial parasite)( explain
multiple fission in Plasmodium with diagrams)
Budding: in this method, a small part of the body of the organism grows a ‘bud’ which then
detaches to grow into a new individual. Eg. Hydra, yeast.
In Hydra, certain regenerative cells grow and develop into a ‘bud’ on the body of the parent hydra.
This bud grows gradually developing a mouth and tentacles. A tiny hydra is formed. The tiny hydra
then detaches itself from the parent body, drifts and fixes itself to a substratum, and becomes an
independent individual. Thus hydra reproduces asexually by budding.
(Figures to be drawn)
Spore formation: This method of asexual reproduction is seen in simple multicellular plants such
as Rhizopus, mucor, aspergillus etc.. The parent plant produces hundreds of microscopic asexual
reproductive bodies called ‘spores’. The spores are produced in balloon like spore case called
sporangium which is borne on the upright sporangiophores. The spores are released when the
sporangium ruptures. These spores are very light and microscopic. Each of these spores has a
thick wall which helps them to tide over adverse climatic conditions. Under favourable conditions
these spores germinate to give rise to new individual. ( figure of Rhizopus with sporangia).
Regeneration: is a method of asexual reproduction in which an organism which is accidentally
cut into two or more parts can regenerate the lost parts of its body to form whole new individuals,
complete in all respects. Simple animals like hydra and planaria show regeneration.
The regeneration of an organism from its cut body part occurs by the process of growth and
development. The regenerative cells of the cut body part of the organism divide rapidly to form a
ball of cells which move to proper places inside the body and develop into the lost body parts.
Thus a complete organism is regenerated. Regeneration is possible only in organisms which have
relatively simple body organization with only a few specialized cells. In complex multi cellular
organisms, as the cells become more and more specialized, the ability to regenerate decreases.
Eg. Planaria and Hydra (figures given in notes)
Fragmentation: the breaking up of the body of a simple multi cellular organism into two or more
pieces on maturing, each of which subsequently grows and develops into a new complete
individual is called fragmentation. It is an asexual method of reproduction seen in simple
multicellular plants as well as animals. Eg. Spirogyra.
Spirogyra is a green filamentous alga found in ponds, lakes and streams. Spirogyra filament
simply breaks into two or more fragments on maturation, and each fragment grows into a new
spirogyra. Thus spirogyra reproduces by the asexual method of fragmentation.
9. What is vegetative propagation?
Vegetative propagation: is an asexual method of reproduction seen only in plants. In vegetative
propagation, new plants are obtained from the parts of old plants such as stems, roots and leaves
without the help of any reproductive organs. Vegetative propagation usually involves the growth
and development of one or more buds present in dormant condition in old parts of plants into new
plants. Vegetative propagation is seen naturally in leaves, stem and roots.
i) Vegetative propagation by leaves: eg. Bryophyllum, begonia.
The leaves of the bryophyllum plant have special type of buds in their margins. These buds
develop into new plantlets when the leaf matures and as the leaf falls on the soil, these plantlets
detach and grow into new plants. Sometimes these plantlets can drop off from the leaf into the soil
and grow independently. Thus leaves of bryophyllum can reproduce vegetatively.
ii) Vegetative propagation by stem: eg. Money plant, potato.
A piece of stem of the money plant with one or two nodes when planted in soil can develop into a
new plant. Modified underground stems such as potato tubers can be vegetatively propagated by
planting the buds of the tuber. Potato tuber has a number of buds / eyes on it, which when planted
along with a portion of potato, can develop into new plants.
iii) Vegetative propagation by roots: eg. Sweet potato
We see vegetative propagation by roots in case of sweet potato. The stem of sweet potato
develops adventitious roots wherever the plant comes in contact with the soil and becomes
swollen to form sweet potato. Each node of the creeper develops adventitious roots and hence,
when separated, each bunch of these adventitious roots develops into a new plant.
10. What are the artificial methods of vegetative propagation?
The process of growing many plants from one plant by manmade methods is called artificial
propagation of plants. Artificial methods of vegetative propagation are: i) cutting ii) layering iii)
grafting
i) Cutting: In this type of propagation, any part of the plant like the root, stem or leaf having some
buds is taken and buried partly in the moist soil. Certain plants like Rose plant, Chrysanthemum,
grapes are propagated by means of cutting.
ii) Layering: In this method, one of the lower branches of the plant is pulled towards the ground
and a part of it is covered with moist soil, leaving the growing tip of the branch exposed above the
ground. The adventitious roots are produced in the branch of the stem covered by the soil. The
plant is then detached from the parent plant. Plants such as Hibiscus, Bougainvillea, Jasmine,
Lemon, and Guava are propagated by this method.
iii) Grafting: Grating is a method of vegetative propagation in which the cut stems of two plants,
one with roots and the other without roots are joined together in such a way that the two stems
join and grow as a single plant. The new plant will have the characteristics of both the plants. The
portion of the plant that is grafted on the other plant is called the scion and the plant on which
grafting is performed is called the stock. This method is applied to improve the variety of fruits
like mango, apple, peas, citrus and guava and flowering plants like roses.
11. What are the advantages of vegetative propagation?
Vegetative propagation is a cheaper, easier and rapid method of propagation in plants than
growing plants from their seeds.
i. The traits or characters of the parent plant are preserved.
ii. Better quality of plants can be maintained by this method.
iii Propagation of plants which do not produce viable seeds.
iv. Plants which produce seeds with long dormancy period can be propagated by vegetative
methods.
v. Plants generated by vegetative means require less time to grow and have the advantage of
growing more uniformly and are genetically similar to the parent stock.
12. What are the disadvantages of vegetative propagation?
i) Vegetative propagation induces overcrowding.
ii) There is no genetic variation, so there is less adaptability of the plant to the
environment.
iii) The plants lose vigor over several generations.
iv) The disease of the parent plant gets transferred to the offspring along with certain beneficial
characters.
13. What is Tissue culture?
i) It is a method of artificial propagation of plants from isolated plant cells that grow on an artificial /
synthetic medium.
ii) It is done as follows: Growing tissues of plants are separated and put in an artificial medium
kept in an aseptic condition.
iii) The plant tissue divides to form small group of cells or callus.
iv) The callus is then transferred to another nutritive medium with hormones for growth and
differentiation.
v) The plantlets produced are now transplanted to small pots or soil and protected in nurseries.
vii) The plants grow to form mature plants.
viii) This technique is also known as micropropagation – in vitro because the young plant is
developed and it grows in an artificial environment.
xi) This technique is used to produce ornamental plants of commercial value such as orchids,
carnation, anthuriums, dahlia etc.