NCERT NEET BIOLOGY CLASS 12 REPRODUCTION IN ORGANISMS

Reproduction in Organisms

Page Title Reproduction: A characteristic feature of all organisms for continuation of species.

The ability of living organism to give rise to the young ones of its own kind is called reproduction. .

It is an essential life process which not only helps in survival but also helps in continuity of species. .

Reproduction is a process of organic evolution by transmitting advantageous variations to the offsprings. .

The period from birth to the natural death of an organism is called Life span. .

Life span of organism includes four stages: .

i. Juvenility – It is the stage when organism develops capacity to reproduce. .

ii. Maturity – In this stage reproduction starts. .

iii. Ageing and senescence – Progressive deterioration of body of living organism is called ageing. .

The terminal irreversible stage of ageing is senescence, .

iv. Death – In death, there is permanent cessation of all vital activities. .

Modes of reproduction.

There are mainly two types of reproduction: .

i. Sexual reproduction (Amphimixis, gametic) .

ii. Asexual reproduction (Apomixis, non – gametic) .

i. Sexual reproduction (Amphimixis) .

Production of offsprings by formation and fusion of gametes is called sexual reproduction. .

Characteristics: .

a. Two parents are involved, .

b. Gametes are formed, .

c. Fertilization takes place. .

d. Organisms formed by sexual reproduction show characters of both the parents, .

e. As meiosis takes place, offsprings show variations. .

f. It is a slow process. .

g. Sexual reproduction takes place in higher plants and animals. It is also found in some protists .

eg. Monocystis, Plasmodium, Paramoecium. .

Types of Sexual reproduction: .

a. Syngamy.

It involves the complete and permanent fusion of two haploid gametes to form a diploid zygote. .

When both types of reproductive organs (gonad) are present in separate parents, then they arecalled unisexual or dioecious, .

eg. Papaya, Mulberry, etc. .

When reproductive organs are present in same parent, then the parent is called bisexual ormonoecious or hermaphrodite, .

eg. Chara. Maize, etc. .

It may be Endogamy (self fertilization), Exogamy (cross fertilization), Anisogamy or Heterogamy, Isogamy. .

Endogamy: Gametes are derived from same parent, eg. Taenia solium. .

Exogamy: Gametes are derived from different parents, eg. All vertebrates. .

Anisogamy or Heterogamy: Fusion of gametes which are morphologically and physiologically different from each other, .

eg. mammals, frog and fish. .

Isogamy: Fusion of similar gametes

eg. Monocystis. .

b. Conjugation: .

The temporary pairing of parents for exchange of genetic material is called conjugation. .

eg. Paramoecium. .

ii. Asexual reproduction (Apomixis) .

Production of offsprings by a single parent without the formation and fusion of gametes is called asexual reproduction. .

Characteristics: .

a. Single parent is involved. .

b. Gametes are not formed. .

c. Fertilization does not take place. .

d. Only mitosis takes place. .

e. Organisms reproduced through asexual reproduction are genetically identical to their parent. .

f. Multiplication is rapid. .

g. As new organisms are produced from the somatic part of parental organism, it is also called somatogenic reproduction, .

h. Asexual reproduction takes place in lower organisms like monera, protists, sponges,coelenterates and in some plants. It does not take place in vertebrates and higher invertebrates. .


Asexual reproduction takes place by following modes: .

i. Fission.

ii.Sporulation.

iii. Budding.

iv. Plasmotomy.

v. Fragmentation .

vi. Regeneration.

vii. Spore formation .

viii. Gemmae formation

i. Fission:Parent body divides into two or more daughter individuals. .

Modes of fission: .

a. Binary fission.

Division of parental body into two nearly equal daughter individuals during favourable condition. .

Modes of binary fission: .

1. Simple or irregular fission: .

Cytokinesis takes place through any direction, but it is always perpendicular to that of karyokinesis. .

eg. Amoeba.

2.Longitudinal fission:

Cytokinesis takes place along longitudinal axis,

eg. Euglena

3. Transverse fission:

Cytokinesis takes place along transverse axis,

eg. Paramoecium

b. Multiple fission

The parental body divides into many daughter cells simultaneously during unfavourable conditions to increase the chances of survival of daughter cells,

eg. Plasmodium, Amoeba, Monocystis

ii. Sporulation:

During unfavourable condition, Amoeba forms a hard covering or cyst around itself, it is known as encystation.

During favourable condition, the encysted Amoeba divides by multiple fission and produces many minute amoebae. Cyst wall burst out and spores are liberated. This phenomenon is termed as sporulation.

iii. Budding:

A small projection or bud is formed on parent body.

New individual is formed by mitosis.

It is a common method of asexual reproduction observed in some protozoans and lower animals suchas sponges (Scyphd), coelenterates (Hydra}, annelids (Chaetopterus) and tunicates (Salpa)

There are two types of budding.

a. Exogenous budding

In exogenous budding, bud is formed on the outer surface of parental body. It is observed in Scypha and Hydra.

b. Endogenous budding:

In endogenous budding, buds called gemmules are formed on the inner surface of parental body. It is observed in Spongilla. Each gemmule of Spongilla consists of mass of undifferentiated cells called archaeocytes. Gemmule helps in perennation and dispersal.

iv. Plasmotomy:

Multinucleate protozoan divides into several small, multinucleate daughters without nuclear division.

It occurs in Opalina and Pelomyxa.

v. Fragmentation:

Parent body breaks into two or more fragments. Each fragment develops into an organism,

eg. algae, fiingi, bryophytes, pteridophytes

.vi. Regeneration:

It was first observed in Hydra by Abraham Trembley.

Regeneration means formation of the whole body of organism from a small fragment or there placement of lost part.

eg. Planaria, Hydra.

Two types;

a. Reparative regeneration – damaged tissues can be regenerated

b. Restorative regeneration – several body parts can be redeveloped

vii. Spore formation:

Spores are minute, single celled structures. Spore formation takes place in algae, fungi, monera and protista.

Types of spores.

a. Zoospores: Flagellated motile spores produced by algae,

eg. Chlamydomonas.

b. Conidia: Non motile spores which are produced exogenously on special hyphal branches called conidiophore.

eg. Penicillium

c. Chlamydospores: Thick walled spores capable of withstanding long unfavourable conditions

eg. Rhizopus.

d. Sporangiospores: Non motile spores produced within sporangia,

eg. Mucor

e. Oidia: In fungi, the hyphae break up into small fragments known as oidia.

viii. Gemmae formation:

These are specialised structures which are green, multicellular, asexual buds which develop in small receptacles called gemma cups located on thalli. Gemmae formation is found in liverworts

, eg. Marchantia

Vegetative propagation in plants

Vegetative propagation:

It is a process of multiplication in which a portion of fragment of plant body functions as propagule and develops into new individual.

Two types of vegetative propagation:

i. Natural method

ii. Artificial method

Natural methods

a. Root Tuber: eg. Sweet Potato (Ipomoea batatas)

1. It is a modification of root for vegetative reproduction.

2. These roots develop from the nodes of stem.

3. They become tuberous and fleshy for storage of food.

4. In addition to the storage of food, these roots also develop adventitious buds on their surface which sprout under favourable conditions to produce ‘leafy shoot’ and adventitious roots.

5. Under suitable environmental conditions, these leafy shoots separate and develop into new plants.

6. The tuberous roots are adventitious and when produce singly are called as simple tuberous roots

eg. sweet potato

when produce in groups or cluster are called as fasciculated tuberous roots

eg. Asparagus, Dahlia.

7. The other examples of tuberous roots are Dalbergia, Guava, Murraya, Albizzia.

b. Stem tuber: eg. Potato (Solarium tuberosum)

1. Stem tuber is a modified stem for vegetative reproduction.

2. In case of potato, some lateral branches are produced from under ground part of stem which grow down in the soil.

3. The tip of these branches store food and hence are swollen.

4. These tubers show nodes, internodes, scale leaves and axillary buds.

5. A stem tuber has many notches on its surface called ‘eye’.

6. The eyes that are seen on the surface of the tuber represent nodes.

7. Each ‘eye’ is actually a node and is made up of one or more axillary buds subtended by a leaf scar.

8. Under favourable conditions, axillary buds develop into new plants.

9. Rhizome reproduce by vegetative reproduction eg. Banana, Ginger, Turmeric

10. Corm, an underground stem reproduce vegetatively eg. Colocasia, Crocus.

11. A bulb bears number of buds eg. Garlic, Onion.

12. Suckers have aerial shoots at the base which help in vegetative propagation,

eg. Chrysanthemum

c.Runner

1. Runner is a slender, prostrate, subaerial branch with short or long internode and creeps horizontally on the soil.

2. Runner develops from the axillary bud in the axil of the lowest leaf.

3. After creeping some distance, away from the parent plant, it produces shoot from upper side and roots from lower side of the nodes.

4. Many runners are produced by the parent plant which may get detached from parent plant to develop new plants, eg. Cynodon

Vegetative propagation by leaf:

1. In some plants leaves also take part in vegetative reproduction.

2. In Bryophyllum, leaves are fleshy and notched along the margin.

3. Adventitious buds or epiphyllous buds are developed on their leaves. These buds usually remain dormant, till the leaf is attached with plants.

4. These plantlets fall off from parent plant to continue their growth in the soil, eg. Bryophyllum, Kalanchoe, Begonia, Adiantum, Saintpaulia

e. Bulbil:

Bulbils are modified vegetative or floral buds, propagative in function. Bulbils on maturation, get detached from the plant and fall on the ground. Under favourable condition, it develops into new plant.

Artificial methods

Artificial methods of vegetative propagation or Horticultural methods:

These are the methods used by man in which a portion of plant body is separated from the parent plant mechanically (by special technique) to propagate new plants.

This method is used by horticulturists for quick production and also for combining good qualities of two different varieties.

a. Cutting:

It is defined as a process by which a plant is produced by cutting a vegetative portion from the parent plant and growing it in a suitable medium under favourable condition. Some of the common cuttings are :

1. Root cutting :

eg. Apple, Tamarind, Lemon

2. Stem cutting :

eg. Sugarcane, Rose, Grapes etc.

3. Leaf cutting :

eg. Peperomia, Bryophyllum, Sansevieria.

Special growth hormones like IAA (Indole acetic acid), IBA (Indole butyric acid), NAA Naphthalene acetic acid) are applied in dilute quantities to the lower end of stem cuttings. Such application of hormones leads to the quick formation of adventitious roots.

b. Grafting:

Grafting is an art of joining parts of two plants in such a manner that they unite and continue their growth as one plant (composite plant).

1. This is the characteristic feature of plants which have cambium for secondary growth, eg. Dicotyledons.

2. As monocots do not have inter or intrafascicular cambium and do not show secondary growth, grafting is not possible in monocots.

3. Part of the rooted plant on which grafting is done is called stock (root stock).

4. The part which is inserted on stock is called scion (graft).

5. The stock and scion should be mutually compatible.

6. The success of grafting depends upon matching of cambium between stock and scion which results in organic connection between them.

7. The stock provides root while scion becomes shoot of new plant.

8. The stock usually has strong root system while scion has strong desired characters of flowers and fruits.

9. All shoots sprouting from the stock should be removed otherwise they will not permit the scion to grow.

10. The stock and scion are given slanting cut.

11. Cut surface should be held together tightly by wrapping, nailing etc.

12. Common methods of grafting are Tongue (whip) grafting, Wedge grafting and Crown grafting, etc.

Types of grafting:

1. Tongue grafting (Whip):

i. In this, stock and scion are of same diameter.

ii. The stock and scion are given oblique cuts followed by a notch to get a tongue like structure in each.

2. Wedge grafting:

i. In this, stock and scion are of same diameter.

ii. V – shaped notch is given in the stock and scion is cut like a wedge.

3. Crown grafting:

i. In this, diameter of stock is many times more than scion,

ii. In this case many scions are grafted on to a single stock.

4. Lateral grafting:

i. Only one scion having wedge – shaped cut is inserted in a stock in a lateral slit,

ii. Only one scion or one branch is inserted in a stock.

c. Budding:

1. Budding is a type of grafting in which bud is a scion.

2. In this method instead of a branch, a single bud along with a piece of bark is used as a scion.

3. In the stock, T’ or T shaped incision is made upto the bark.

4. A single bud with little bark is then inserted in the slit of the stock.

5. Both are tied by polythene or by plantain fibre.

6. Bud germinates after 3 weeks and a new plant is produced.

eg. Rose, Mulberry, Rubber.

d. Layering:

In layering, stem branch is chosen for propagation.

It is then pegged out around the parent plant till it produces adventitious roots.

This method is used for plants which produce runners,

eg. Strawberry, Clematis

. e. Micropropagation:

Micropropagation refers to a technique to obtain new plants by cultivating cells or tissue in culture medium.

Significance of vegetative propagation:

a. Vegetative propagation results into genetically identical offspring (clone) and preserve a stock of desired variety

b. It is an easier, less expensive and a rapid method of desired variety,

c. Disease free plants can be developed in a short time by tissue culture technique.

Additional Information

Parthenogenesis:

Development of an egg into a complete individual without fertilization is known parthenogenesis.

It is found in many non-vertebrates such as aphids, bees and crustaceans.

Parthenogenesis is of two types:

1. Natural

2. Artificial

1. Natural parthenogenesis:

In this, parthenogenesis is a regular and characteristic feature of life cycle,

eg. honey bee.

It is of 3 types.

L Arrhenotoky: In mis, only males are produced parthenogenetically.

eg. honey bee, wasp, mite.

ii. Thelytoky: In this, only females are parthenogenetically produced,

eg. Lacerta saxicola armeniaca (belongs to lizard family in which absence of male lizard is seen).

iii. Amphitoky; this, parthenogenetic eggs may develop into any sex.

2. Artificial parthenogenesis:

In this, eggs of some sexually reproducing animals can be induced by artificial means to develop parthenogenetically.