1. Introduction
• All living organisms have an instinct to live their own lives and then survive through their generations.
• Living things take birth, grow old and die. Before death individuals give birth to the off springs (children) to continue their race.
• Giving birth to off springs, which are similar to parents, is known as reproduction.
Importance of Reproduction
• Reproduction is one of the most important characteristics of life. Reproduction is the creation of new life of its own kind.
• Reproduction is essential for the continuation of species.
• A species will disappear from the face of the earth if it does not reproduce.
• Reproduction takes place in both, plants and animals.
2 Asexual Reproduction
• In asexual reproduction, a single parent gives rise to new individuals.
• The production of a new organism from a single parent without the involvement of sex cells for gametes) is called asexual reproduction.
• It is called asexual reproduction because it does not use special cells called ‘sex cells’ (or gametes) for producing a new organism.
• In asexual reproduction, a part of the parent organism separates off and grows into a new organism.
Examples :
• Binary fission in Amoeba, Budding in Hydra, Spore formation in Rhizopus, Regeneration in Planaria, Fragmentation in Spirogyra, Vegetative propagation in plants
3. Fission
• In the asexual reproduction method, certain body cells of the parent organism undergo repeated mitotic cell divisions to form two (or more) new organisms of the same kind.
• Fission is the process of reproduction in unicellular organisms such as protozoa.
• In the process of fission, a unicellular organism splits (or divides) to form two (or more) new organisms.
• Fission is of two types : Binary fission and Multiple fission
4. Binary Fission
• In Binary Fission, the parent organism splits to form two new organisms
Examples : Amoeba, Paramecium, Leishmania
Binary Fission in Amoeba
• Amoeba reproduces by binary fission by dividing its body into two parts.
• When the Amoeba cell has reached its maximum size of growth, then first the nucleus of Amoeba lengthens and divides into two parts.
• After that the cytoplasm of Amoeba divides into two parts, one part around each nucleus.
• In this way, one parent Amoeba divides to form two smaller Amoebae (called daughter Amoebae). And we say that one Amoeba produces two Amoebae
• The two daughter Amoebae produced here grow to their full size by eating food and then divide again to produce four Amoebae, and so on. In the unicellular organisms such as Amoeba, the splitting of the parent cell during fission (or cell division) can take place in any plane.
Binary Fission in Paramecium
• Paramecium is a unicellular animal having short thread-like structures called cilia over its surface. Paramecium also reproduces by the method of Binary Fission.
• A fully grown Paramecium divides its body into two parts to form two smaller Paramecia.
Binary Fission in Leishmania
• Leishmania is a unicellular protozoan.
• It is a parasite which causes the disease known as kala-azar (or black fever).
• Kala-azar is also known as Leishmaniasis. Leishmania has a greater degree of organisation in its body, having a whip-like structure called flagellum at its one end.
• In Leishmania, the splitting of parent cell during fission (or cell division) takes place in a definite plane (longitudinally) with respect to flagellum at its end.
5 Multiple Fission
• In Multiple fission, the parent organism splits ( or divides) to form many new organism at the same.
• Sometimes, a cyst or protective wall is formed around the cell of a single-celled organism (like that of Plasmodium).
• Inside the cyst, the nucleus of cell splits several times to form many smaller nuclei called daughter nuclei.
• Cytoplasm collects around each daughter nuclei and thin membranes are formed around them. Many new daughter cells are formed from a single parent cell within the cyst.
• When the favourable conditions arrive, the cyst breaks open and the many daughter cells present in it are released, each forming a new organism
• Plasmodium is a protozoan which reproduces by the asexual method of multiple fission.
6. Budding
• In budding, a small part of the body of the parent organism grows out as a ‘bud’ which then detaches and becomes a new organism.
The asexual reproduction by budding is observed in Hydra and yeast.
Budding In Hydra
1. Hydra is a multicellular organism with a soft cylindrical body.
2. It bears tentacles surrounding its mouth towards the top.
3. For reproduction by budding, one or more buds bulge from its lateral sides.
4. Each bud grows into a baby Hydra.
5. Baby Hydra separates out from the parent forming an independent Hydra.
Budding In Yeast
1. Yeast is tiny, unicellular, non-green plant (which is a fungus).
2. Yeast reproduces by budding.
3. The Figure below shows a parent yeast cell (which is a complete plant). In yeast, first a bud appears on the outside of the cell wall
4. The nucleus of parent yeast cell then divides into two parts and one part of the nucleus moves into the bud.
5. Ultimately, the bud separates off from the parent yeast cell and forms a new yeast cell (or new yeast plant).
7. Spore Formation
• In spore formation, the parent plant produces hundreds of microscopic reproductive units called ‘spores’.
• When the spore case of the plant bursts, then the spores spread into air.
• When these air-borne spores land on food under favourable conditions, they germinate and produce new plants.
Spore Formation in Rhizopus
• The common bread mold plant consists of fine, threadlike projections called hyphae and thin stems having knob- like structures called sporangia.
• Each knob- like structure (or sporangium) contains hundreds of minute spores enclosed in a spore case. When the spore case bursts, the tiny spores are dispersed in air.
• These spores are the asexual reproductive units which can produce more bread mold plants under suitable conditions.
• These spores are the asexual reproductive units which can produce more bread mold plants under suitable conditions.
• If we remove one sporangium from the bread mold, keep it on a slide, put a cover slip over it and observe this slide through a microscope, we can see the spores.
8. Regeneration
• In some organisms (plants as well as animals) small cut parts of their body can grow (or regenerate) to form whole new organisms complete in all respects.
• The simple animals like Hydra and Planaria show regeneration. This means that in these organisms, whole new organisms can be reproduced from their cut body parts.
• In other words, if Hydra or Planaria somehow get cut into a number of pieces, then each body piece can grow into a complete organism.
Regeneration in Planaria
• Planaria is a flatworm which is found in freshwater ponds and slow-moving streams.
• Planaria possesses great power of regeneration.
• If the body of Planaria somehow gets cut into a number of pieces, then each body piece can regenerate into a complete Planaria by growing all the missing parts.
• Planaria is a flatworm which is found in freshwater ponds and slow-moving streams. Planaria possesses great power of regeneration.
Regeneration in Hydra
• If the body of a Hydra gets cut into a number of pieces, then each body piece of Hydra can grow into a complete Hydra.
How does regeneration occur?
• The cells of cut body part of the organism divide rapidly to make a ‘ball of cells’.
• The cells present in the ball of cells move to their proper places within the ball where they have to form various organs and body parts of the organism.
• The cells then change their shapes to form different types of tissues.
• These different tissues form various organs and body parts of the organism. In this way a complete organism is regenerated.
Why regeneration is not possible in multicellular organisms?
In complex multicellular organisms,
As complex multicellular organisms have a very high degree of organisation in their body, they cannot be reproduced from their cut body parts by the process of regeneration. A dog is a complex multicellular organism which cannot be regenerated from its cut body part say, a cut tail. This is because the cells present in the cut tail of a dog cannot produce dog’s organs like heart, brain, lungs, stomach, intestines and limbs, etc, needed for the making of a complete dog. The complex multicellular organisms need more complex ways of reproduction like sexual reproduction.
9. Fragmentation
• The breaking up of the body of a simple multi cellular organism into two for more) pieces on maturing, each of which subsequently grows to form a complete new organism is called fragmentation
Examples : Spirogyra and sea anemones reproduces by fragmentation.
Fragmentation In Spirogyra
• Spirogyra is a green, filamentous alga plant which is found in ponds, lakes and slow moving streams.
• Spirogyra filament simply breaks into two or more fragments on maturation, and each fragment then grows into a new Spirogyra.
• This breakup of the filament of a mature Spirogyra on its own brings about asexual reproduction. Thus, Spirogyra reproduces by the asexual method of fragmentation.
10. Vegetative propagation
In vegetative propagation, new plants are obtained from the parts of old plants (like stems, roots and leaves), without the help of any reproductive organs.
• Vegetative propagation is an asexual method of reproduction.
• The reproduction by vegetative propagation occurs only in plants.
• Vegetative propagation usually involves the growth and development of one (or more) buds present on the old part of the plant to form a new plant.
• These buds are in the dormant state (inactive state) in the old part of the plant.
• When provided suitable conditions (like moisture, warmth, etc.), these buds grow to form new plants.
• Please note that vegetative propagation is also called vegetative reproduction.
Vegetative Propagation In Grass
It is a common observation that green grass plants spring up in dry fields after the rains. This happens due to vegetative propagation as follows
• The fields have dry stems of the old grass plants all over them.
• These dry stems have buds which are in the inactive state.
• By getting rain water, the buds present on dry grass stems get activated and grow to produce new grass plants.
• Thus, the green grass grows in the fields after rains from the dry, old stems of grass plants present in the fields, by the method of vegetative propagation.
Vegetative Propagation In Bryophyllum
• Buds are present on the stems as well as the leaves of the Bryophyllum plant which can develop into new plants.
• Bryophyllum plants can be reproduced by vegetative propagation by using either a piece of its stem or its leaves.
• If a broken piece of the stem of a Bryophyllum plant in the ground, we will get a new Bryophyllum plant growing from it in a week’s time.
• Even the leaves of a Bryophyllum plant can produce new plants.
• The leaves of a Bryophyllum plant have special type of buds in their margins (or edges).
• These buds may get detached from the leaves, fall to the ground and then grow to produce new Bryophyllum plants.
• The buds can also drop to the ground together with the leaf and then grow to produce new plants.
• Sometimes even before a leaf drops off from a Bryophyllum plant, we can see new plantlets already growing on it.
• When such a mature leaf of the Bryophyllum plant falls on the ground, then each plantlet can grow into a new plant.
• Thus, the leaves of Bryophyllum plant can produce new plants.
• Another plant called Begonia also reproduces by vegetative propagation through its leaves.
Vegetative Propagation in Money Plant
• Cut a piece of stem of money plant in such a way that it contains at least one leaf on it
Note : The point on stem where a leaf is attached is called a node.
• Dip one end of this stem in water.
• After a few days we will find that new roots appear at the point where leaf was attached.
• The piece of stem will gradually grow into a new money plant.
• Note: If we cut the stem of money plant in between two leaves, then it will not grow into a new plant. This is because it does not have a growing point (here a node) in it.
Vegetative Propagation in Tubers
• A tuber is the thickened, underground stem (or root) of a plant which is swollen with stored food.
• The tuber has a number of ‘buds’ (called ‘eyes’).
• Each bud of the tuber grows into a new plant when the old tuber is planted in the soil in the next growing season.
• There are two types of tubers : stem tubers and root tubers.
• Potato is a stem tuber whereas sweet potato is a root tuber.
• Potato tuber is an underground stem of the potato plant.
• Each potato tuber can produce more than one plant.
• A potato tuber has many buds (called eyes) on its body.
• When a potato tuber is planted in the soil, then the various buds of the potato tuber start growing to form new potato plants.
Vegetative Propagation in Guava Plant
• The roots of a guava plant have buds which can develop into new guava plants.
• In fact, a large number of plants can be reproduced by the method of vegetative propagation.
Other examples
Potato, Onion, Banana, Garlic, Water hyacinth, Tulip, Mint, Strawberry and Lily.
11. Artificial propagation of plants
• The process of growing many plants from one plant by man-made methods is called artificial propagation of plants.
• The three common methods for the artificial propagation of slants are:
1. Cuttings
2. Layering
3. Grafting.
12. Cuttings
A small part of a plant which is removed by making a cut with a sharp knife is called a cutting.
Method
• A cutting of the parent plant part (say, of stem or shoot) having some buds on it is taken.
• Its lower part is buried in the moist soil.
• After a few days, the cutting develops roots and shoot, and grows into a new plant.
• Cuttings are a means of asexual reproduction in plants.
• The new plant formed from a cutting is exactly similar to the parent plant.
Examples
Bougainvillea, Chrysanthemum, grapes, sugarcane, bananas, and cactus, etc., can be grown by means of cuttings.
Propagation of rose Plants by Cutting
• Rose plants are propagated (or reproduced) by means of cuttings from stems (or shoots).
• A piece of stem (or side shoot) having bud is cut from an existing rose plant with the help of a knife.
• The lower part of this cutting is buried in moist soil.
• After a few days, the end of cutting buried in soil develops roots and later on grows to become a new rose plant.
Note: An advantage of cuttings method is that by using this method we can produce many new plants from just one plant quickly, without waiting for flowers and seeds.
13. Layering
In layering, a branch of the plant ispulled towards the ground and a pat of it is covered with moist soil leaving the tip of the branch exposed above the ground.
• After some time, new roots develop from the part of the branch buried in the soil.
• The branch is then cut off from the parent plant.
• The part of the branch which has developed roots grows to become a new plant.
Layering in Jasmine Plant
• Jasmine plant (chameli) is propagated or produced by the layering method.
• One left side branch and one right side branch of the parent jasmine plant have been buried in moist soil.
• The parts of branches which are buried in soil grow their own roots.
• When this happens, the branches of the parent plant connecting the newly formed plants are cut off so that the newly formed plants may grow on their own and develop into mature plants (like the parent plant).
• Many plants like strawberry and raspberry are propagated by the natural layering method.
• The natural layering occurs because these plants form runners (which are soft horizontal stems running above the -round).
• Wherever the ends of such runners touch the ground, new plants are formed at those places. In this way, many more strawberry or raspberry plants are formed from the parent plant in a natural way.
Examples
Jasmine, Strawberry, Raspberry, Lemon, Guava, Hibiscus (China rose), Bougainvillea and many slender ornamental plants.
14. Grafting
Grafting is a method in which the cut stems of two different plants (one with roots and other without roots) are joined together in such a way that the two stems join and grow as a single plant.
This new plant will have the characteristics of both the original plants.
• The cut stem of a plant (or tree) having roots (and fixed in soil) is called stock.
Stock is the lower part of a plant (or tree) having the roots.
• The cut stem of another plant (without roots) is called scion.
Scion is the upper part of a plant which may have leaves on it (but no roots).
Method
• In carrying out grafting, two plants (or trees) are chosen which are to be used as scion and stock.
• First, the stem (or branch) is removed from the plant chosen to be made scion (for its desirable characteristics) by making a slanting cut.
• This gives us the scion with a slanting cut.
• The stem of second Plant (or tree) to be used in grafting is also cut in a slanting way.
• The lower part of this plant (or tree) is stock. It has also a slanting cut. The scion is placed over the stock
• The cut surfaces of the scion and stock are fitted together and bound tightly with a piece of cloth and covered properly with polythene sheet (so as to prevent harmful infection by bacteria or fungus, and loss of water and plant sap from the cut and joined ends of stock and scion).
• While joining the scion to the stock, care should be taken to make sure that the cambium layer of scion is in contact with the cambium layer of stock (because the cambium layer in the stem is responsible for growth).
• The cut soon heals and the stock and scion of two plants grow together and become one plant.
• The scion continues to produce its original leaves, flowers and fruits but it gets water and minerals for making food from the chosen stock. So, the fruits will have the characteristics o: both the plants (from which scion and stock have come).
• Grafting is used to breed fruit trees and flowering bushes. Apple, peach, apricot and pear trees are often grafted.
Advantages of Grafting
(i) Grafting enables us to combine the most desirable characteristics of the two plants (scion and stock in its flowers and fruits.
(ii) By grafting method, a very young scion (shoot part of a plant) can be made to flower and produce fruits quite fast when it is grafted to the stock (stem having roots) of a mature plant.
(iii) It enables us to obtain flowers and fruits having different desired characteristics by grafting scions from different varieties of plants on the same stock. Many varieties of mango have been produced by grafting method.
(iv) Grafting can be used to produce varieties of seedless fruits.
Note
Artificial vegetative propagation is usually used for the propagation (or reproduction) of those plants which produce either very few seeds or do not produce viable seeds.
Examples
Banana, Pineapple, Orange, Grape, Rose, etc.
15. Advantages of Artificial Vegetative Propagation
Some of the important advantages of the artificial vegetative propagation of plants are given below:
1. The new plants produced by artificial vegetative propagation will be exactly like the parent plants. Any desirable features of the parent plant will be replicated in the new plants.
2. The fruit trees grown from seeds may take many years before they start to bear fruit. But the fruit trees grown from cuttings or by grafting start to bear fruits much earlier (only after a few growing seasons).
3. The plants grown by vegetative propagation usually need less attention in their early years than the plants grown from seeds.
4. Many plants can be grown from just one parent plant by artificial propagation.
5. We can also get seedless plants by artificial propagation.
The cuttings, layering and grafting are the traditional methods for the artificial propagation of plants.
16. Tissue Culture
The growth medium (or culture solution) used for growing plant tissues is very important in this process because it contains various plant nutrients in the form of jelly (called agar) and plant hormones which are necessary for plant growth.
Process
1. A small piece of plant tissue is taken from the growing point of the plant (tip of the plant) and placed on a sterile jelly which contains nutrients and plant hormones.
2. The hormones make the cells in the plant tissue divide rapidly producing many cells which form a shapeless lump of mass called callus.
3. The callus is then transferred to another jelly containing suitable plant hormones which stimulate the callus to develop roots.
4. The callus with developed roots is then put on a yet another jelly containing different hormones which stimulate the development of shoots.
5. The callus having roots and shoots separates into tiny plantlets.
6. In this way, many tiny plantlets are produced from just a few original plant cells (or tissue).
7. The plantlets thus produced are transplanted into pots or soil where they can grow to form mature plants.
Examples
Orchids, dahlia, carnation, Chrysanthemum, etc. The production of plants by the method of tissue culture is also known as micro propagation (due to the extremely small amount of plant material used).
Advantages of Tissue Culture
1. Tissue culture is a very fast technique. Thousands of plantlets can be produced in a few weeks’ time from a small amount of plant tissue.
2. The new plants produced by tissue culture are disease free.
3. Tissue culture can grow plants round the year, irrespective of weather or season.
4. Very little space is needed for developing new plants by tissue culture.
17. Do Organisms Create Exact Copies of Themselves in Asexual Re production
Asexual reproduction usually results in the production of genetically identical offsprings, the only genetic variation arises as a result of occasional inaccuracies in DNA replication (or DNA copying) at the time of cell division.
Why are the off springs identical?
• The material which carries genetic information from the parents to the offsprings is DNA-Deoxyribo Nucleic Acid (which is present in the form of chromosomes in the nuclei of all the cells).
• The basis of asexual reproduction is mitosis. This is the division of a nucleus into two identical daughter nuclei.
• Each daughter nucleus has the same genetic make up because of the replication of DNA (or copying of DNA) of the parent cell.
• After the division of the nucleus, the rest of parent cell divides to form two genetically identical daughter cells. The daughter cells can then form two offsprings. From this conclude that all the offsprings produced by one parent as a result of asexual reproduction are usually genetically identical.
Clones
• The new organisms (or offsprings) produced by one parent through asexual reproduction (which are genetically identical to the parent) are called clones.
• The clones possess exact copies of the DNA (or genes) of their parent and hence show remarkable similarity to the parent and to one another.
• The clones of plants can be produced by the asexual methods of reproduction such as cutting -layering, grafting, tissue culture, etc.
• These days techniques have been developed to clone even in animals. Dolly, the sheep hit the headlines in 1997 as the first successfully produced animal clone.
• The process of producing genetically identical new organisms (or offsprings) by asexual reproduction methods is called cloning.
Variations are introduced in asexual reproduction
• The replication (or copying) of DNA in the cell is done by certain biochemical reactions which synthesize more of genetic material. No biochemical reaction can reproduce 100 per cent same results.
• When the DNA already present in the nucleus of the parent cell is replicated (or copied) by making more DNA at the time of asexual reproduction, then slight variations come in the two copies formed.
• Due to this, the two DNA molecules formed by replication will be similar but may not be exactly identical to the original DNA. These slight variations in the replication of DNA molecule will lead to slight variations in the offsprings produced by asexual reproduction.
• Although the offsprings produced by asexual reproduction are said to be genetically the same as the parent organism, but still they have occasional variations. This means that the organisms do not always create exact copies of themselves in asexual reproduction.
• It is to be noted that the importance of DNA copying in asexual reproduction is that the characteristics of the parent organisms are transmitted to its offsprings and at the same time some occasional variations are also produced in the offsprings.
Importance of variations introduced in reproduction
• The importance of variations in organisms introduced during reproduction is that it helps the species of various organisms to survive and flourish even in adverse environment.
The following discussion make it clear
• There may be some drastic changes like excessive heat or cold or shortage of water (drought), etc., in the habitat of a species of organisms.
• If all the organisms of a population living in that habitat are exactly identical, then there is danger that all of them may die and no one would survive under those conditions.
• This will eliminate the species from that habitat completely. However, if some variations are present in some individual organisms to tolerate excessive heat or cold or survive on meagre water supply, then there is a chance for them to survive and fluorish even in adverse environment.
• In this way, the introduction of variations during reproduction provides stability to the populations of various species by preventing them from getting wiped out during adverse conditions.
• For example, if there is a population of certain bacteria living in temperate water (which is neither very hot nor very cold) and the temperature of water increases too much due to global warming, then most of these bacteria will not be able to tolerate excessive heat and hence die.
• But some bacteria which had variations to resist heat would survive and grow further.
18. sexual reproduction
• Sexual reproduction takes place by the combination of special reproductive cells called sex cells.
• Sex cells are of two types : male sex cells and female sex cells. The sex cells are commonly known as gametes or germ cells.
• Gametes are of two types: male gametes and female gametes.
• In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called zygote.
19. sexual reproduction in plants
• The plants in which the sex organs are carried within the flowers and the seeds are enclosed in a fruit are called angiosperms. Angiosperms are commonly known as flowering plants.
• The sex organs (or reproductive organs) of a plant are in its flowers.
• The majority of plants are bisexual having the male and female reproductive organs in the same plant (or same flower).
The sexual reproduction in plants takes place in the following steps
1. The male organ of flower called ‘stamen’ makes the male gametes (male sex cells) of the plant. These male gametes are present in pollen grains.
2. The female organ of a flower called ‘carpel’ makes the female gametes (female sex cells) of the plant. These female gametes are present in ovules. The female gametes present in ovules are also called ‘ova’, ‘egg cells’ or just ‘eggs’.
3. The male gametes present in pollen grains fertilise the female gametes or egg cells present in ovules.
4 The fertilised egg cells grow within ovules and become seeds.
5. The seeds produce new plants on germination (under suitable conditions of water, warmth, air and light, etc.).
20. Parts of a Flower
The main parts of a flower are: Receptacle, Sepals, Petals, Stamen and Carpel. These main parts of a flower are shown in Figure.
1. Receptacle
• The base of a flower to which all the parts of a flower are attached is called receptacle.
2. Sepals
• The green, leaf-like parts in the outermost circle of a flower are called sepals.
• All the sepals taken together are called calyx.
• The function of sepals (or calyx) is to protect the flower in its initial stages when it is in the form of a bud.
3. Petals
• The colourful parts of a flower are called petals.
• The petals lie inside the sepals. All the petals taken together are called corolla.
• The petals are usually scented.
• The function of petals (or corolla) is to attract insects (for pollination) and to protect the reproductive organs which are at the centre of the flower.
4. Stamen
• The little stalks with swollen tops just inside the ring of petals in a flower are called stamens.
• Stamen is the male reproductive organ of the plant.
• Stamen produces pollen grains.
• The stamen is made of two parts: a filament and an anther.
• The stalk of stamen is called filament and the swollen top of stamen is called anther.
• It is actually the anther of a stamen which makes the pollen grains and stores them
• Pollen grains contain the male gametes (or male sex cells) of the plant.
5. Carpel
• Carpel is the here is a flask-shaped organ, female reproductive organ of the plant.
• A carpel is made of three parts: stigma, style and ovary.
• The top part of carpel is called stigma. Stigma is for receiving the pollen grains from the anther of stamen (during pollination). Stigma is sticky so that pollen can stick to it.
• The middle part of carpel is called style. Style is a tube which connects stigma to the ovary.
• The swollen part at the bottom of a carpel is called ovary. The ovary makes ovules and stores them.
• Ovules contain the female gametes (or female sex cells) of the plant. There are usually many ovules the ovary.
• Each ovule contains only one female gamete of the plant.
• The female gamete -T female sex cell) of the plant which is present inside the ovule is called ‘ovum’ or ‘egg’.
Note:
• Another point to be noted is that the female organ called carpel is surrounded by a number of male organs called stamens in the flower.
• The flowers which contain only one sex organ, either stamens or carpels, are called unisexual flowers. The flowers of papaya and watermelon plants are unisexual flowers.
• The flowers which contain both the sex organs, stamens as well as carpel, are called bisexual flowers. The flowers of Hibiscus and mustard plants are bisexual flowers.
21. Pollination
• The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination.
• Pollination can occur in two ways: self-pollination and cross-pollination.
• When the pollen grains from the anther of a flower are transferred to the stigma of the same flower (or another flower on the same plant), it is called self-pollination.
• When the pollen grains from the anther of a flower on one plant are transferred to the stigma of a flower on another similar plant, it is called cross-pollination.
• Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination.
• Anemophily (anemos = wind, philein = to love) is a mode of pollination or transfer of pollen grains from anther to stigma through the agency of wind. This type of pollination is mostly seen in grasses.
• Hydrophily (hydro = water, philein = to love) is a mode of pollination or transfer of pollen grains from anther to stigma through the agency of water.
• Entomophily is a mode of pollination or transfer of pollen grains from anther to stigma through the agency of insects. The most common insect pollinators are – moths, flies, butterflies, wasps, bees, beetles, etc.
• Ornithophily (Gk. ornis = bird, philein = to love) is a mode of pollination performed by birds.
• Chiropterophily (Gk. cheir = hand, pteros = wing, philein = to love) is a mode of pollination performed by bats.
22. Fertilisation
• After a pollen grain falls on the stigma, the next step is fertilisation.
• Fertilisation occurs when the ma gamete present in pollen grain joins with the female gamete (or egg) present in ovule.
• When a pollen grain falls on the stigma of the carpel, it bursts open and grows a pollen downwards through the style towards the female gamete in the ovary.
• A male gamete moves down the pollen tube. The pollen tube enters the ovule in the ovary.
• The tip of pollen tube bursts open and male gamete comes out of pollen tube.
• In ovary, the male gamete of pollen combines with the nucleus of female gamete or egg present in ovule to form a fertilised egg (called zygote).
Formation of Fruits and Seeds
• The fertilised egg (or zygote) divides several times to form an embryo within the ovule.
• The ovule develops a tough coat around it and is gradually converted into a seed (containing the baby plant).
• The ovary of flower develops and becomes a fruit (with seeds inside it).
• The other parts of flower like sepals, petals, stamens, stigma and style dry up and fall off.
Only the ovary is left behind.
• A seed is the reproductive unit of a plant (which can be used to grow a new plant). The seed contains a baby plant (or embryo) and food for the baby plant.
• The part of baby plant in seed which develops into shoot with leaves is called plumule and the part which develops into root is called radicle.
• The part of seed which contains stored food for the baby plant is called cotyledon
• When the seed gets suitable conditions like water, air and warmth, etc., it germinates and a new plant grows out of the seed.
Germination of Seeds
• The seeds obtained from a plant are usually very dry.• Germination begins when the seed absorbs water, swells and bursts through the seed coat.
• The water helps the enzymes to function in the seed. The enzymes digest the stored rood in cotyledons and make it soluble.
• This soluble food makes the radicle and plumule present in the seed to grow. The germination of a bean seed is shown in Figure. The radical of the seed grows first to form the root.
• The root pushes down into the soil and begins to absorb water and minerals from the soil. After this the plumage grows upwards to form the shoot.
• The shoot and root grow further. When the shoot comes up above the ground, it develops green leaves.
• The green leaves of the shoot begin to synthesis own food in the presence of sunlight.
• This seedling grows gradually and ultimately becomes a new plant.
23. Sexual Reproduction in Animals
Gametes
• Gametes are the sexual reproductive cells (or sex cells).
• The male gamete in animals is called ‘sperm’ and the female gamete in animals is called ‘ovum’ or ‘egg’.
• The important part of ovum or egg is its nucleus.
• The sperm cell is hundreds or even thousands of times smaller than the ovum or egg and it has a long tail.
• The sperms are motile which can move independently with the help of their tails.
• The nuclei of the sperm and ovum contain chromosomes which carry the genes.
• The cell which is formed by the fusion of a male gamete and a female gamete is called zygote.In most simple words, zygote is a ‘fertilised ovum’ or ‘fertilised egg’.
• All the multi-cellular animals start their life from a single cell called zygote through sexual reproduction.
Fertilisation
• The process of fusion of gametes is called fertilisation.
• The zygote grows and develops to form a new baby.
• The unborn baby in the uterus in the early stages of development is called an embryo and in the later stages of development is called a foetus.
• The fertilisation which occurs inside the female body is called internal fertilisation. In mammals, birds and reptiles, the fertilisation occurs inside the female body.
• This transfer of sperms from the testes c: the male animal into female animal’s body occurs at the time of copulation.
• The fertilisation which occurs outside the female body is called external fertilisation. It takes place in amphibians and fishes.
Advantages of Sexual Reproduction
• In asexual reproduction, the off springs are almost identical to their parent because they have the same genes as their parent.
• The species obtained through sexual reproduction can adapt more quickly to changes in its surroundings.
Why the amount of DNA does not get doubled during sexual reproduction
• Gametes contain only half the amount of DNA as compared to the normal body cells of an organism.
• When a male gamete combines with a female gamete, the ‘zygote’ will have the normal amount of DNA.
• For example, the human sperm has 23 chromosomes and the human egg has also 23 chromosomes. So, when a sperm and an egg fuse together during fertilisation, then the zygote formed will have 23 + 23 = 46 chromosomes, which is the normal number of chromosomes.
How sexual reproduction in animals takes place
1. The male parent produces male gametes called sperms. The sperm is a small cell with a long tail for
movement.
2. The female parent produces female gametes called ova. The ovum is a much bigger cell than the sperm, having a lot of cytoplasm.
3. The sperm enters into the ovum and fuses with it to form a new cell called ‘zygote’. This process is called fertilisation. So, the zygote is a fertilised ovum.
4. The zygote then divides again and again to form a large number of cells (all of which remain together And ultimately zygote grows and develops to become a new baby. From the above discussion we conclude that the whole process of sexual reproduction in animals involves the formation of sperms and eggs; joining together of sperm and egg to form a zygote; and then the growth and development of zygote to form a baby animal.
Puberty
• The age at which the sex hormones begin to be produced and the boy and girl become sexually mature is called puberty.
• Boys attain puberty at the age of 13 to 14 years while girls reach puberty at a comparatively lower age of 10 to 12 years.
• On attaining puberty, the male gonads called testes start producing male gametes called sperms and the female gonads called ovaries start producing female gametes called ova.And male and female gonads also produce and secrete sex hormones with the onset of puberty.
• The testes produce the male sex hormone called testosterone, and the ovaries produce two female sex hormones, oestrogen and progesterone.
• Changes which occur in boys at puberty are:
• Hair grows under armpits and in pubic regions between the thighs.
• Hair also grows on other parts of the body like chest and face.
• Body becomes more muscular due to the development of muscles.
• The voice deepens.
• Chest and shoulders broaden.
• The penis and testes become larger.
• The testes start to make sperms.
• Feelings and sexual drives associated with adulthood begin to develop.
Changes which occur in girls at puberty are :
• Hair grows under armpits and pubic region This change is the same as in boys.
• Mammary glands develop and enlarge.
• The hips broaden.
• Extra fat is deposited in various parts of the body like hips and thighs.
• Fallopian tubes, uterus and vagina enlarge.
• Ovaries start to release eggs.
• Menstruation start.
• Feelings and sexual drives associated with adulthood begin to develop.
24. Human Male Reproductive System
Scrotum
• The scrotum is a sac-like organ made of skin and muscles that houses the testes.
• It is located inferior to the penis in the pubic region.
• The scrotum is made up of 2 side-by-side pouches with a testes located in each pouch.
Testes
• Testes are the oval shaped organs which lie outside the abdominal cavity of a man.
• A man has two testes.
• The function of testes is to make the male sex cells called sperms and also to make the male sex hormone called testosterone.
• The testes of a man lie in small muscular pouch called scrotum.
• The testes are outside the abdominal cavity of the body, because the sperm formation requires a lower temperature than the normal body temperature.
Epididymis and Vas deferens
• The sperms formed in testes come out and go into a coiled tube called epididymis.
• The sperms get stored temporarily in epididymis.
• From epididymis, the sperms are carried by a long tube called vas deferens which joins with another tube called urethra coming from the bladder.
Other Parts
• Along the path of vas deferens, the glands called seminal vesicles and prostate gland add their secretions to sperms so that the sperms are now in a liquid.
• This liquid plus the sperms it contains is called semen.
• The secretions of seminal vesicles and prostate gland provide nutrition to the sperms and also make their further transport easier.
• Urethra forms a common passage for sperms and urine.
• Urethra carries the sperms to an organ called penis which opens outside the body.
25. Female Reproductive System
Ovaries
• Ovaries are the primary reproductive organs in a woman.
• Ovaries are the oval shaped organs which are inside the abdominal cavity of a woman near the kidneys.
• A woman has two ovaries.
• The function of ovaries is to make mature female sex cells called ‘ova’ or ‘eggs’, and also to make the female sex hormones.
Oviducts
• Each ovary is composed of several thousand follicles. At puberty these follicles mature to form the ripe eggs or ripe ova. Just above the ovaries are the tubes called oviducts.
• The oviducts are not directly connected to ovaries but have funneled shaped openings which almost cover the ovaries.
• The ovum released by an ovary goes into the oviduct through its funnel-shaped opening.
• The fertilisation of egg by a sperm takes place in the oviduct.
Uterus
• The two oviducts connect to a bag like organ called uterus at their other ends.
• The growth and development of a fertilised ovum into a baby takes place in the uterus.
Vagina
• The uterus is connected through a narrow opening called cervix to another tube called vagina which opens to the outside of the body.
• Vagina is a tubular structure.
• Vagina is also called ‘birth canal’ because it is through this passage that the baby is born after the completion of development inside the uterus of the mother.
• Vagina receives the penis for putting sperms into the woman’s body.
• In female, the opening for passing out urine and the vaginal opening are separate.
26. Fertilisation and Embryo Development
• In human beings, internal fertilisation takes place.
• The sperms made in the testes of man are introduced into the vagina of the woman through penis during copulation.
• The sperms are highly active and mobile. The sperms move up through cervix into the uterus.
• From uterus, the sperms pass into the oviducts.
• One of the oviducts contains an ovum (or egg cell) released by the ovary during ovulation.
• Only one sperm fuses with the ovum in the oviduct to form a zygote. This is called fertilisation. Thus, the fertilisation of the ovum (or egg) takes place in the oviduct.
Development of Embryo
• When the ovum is fertilised in the oviduct, then a zygote is formed.
• The zygote divides rapidly by mitosis as it moves down slowly in the oviduct and forms a hollow ball of hundreds of cells.
• This hollow ball of cells, now called an embryo, sinks into the soft and thick lining of the uterus and gets embedded in it.
• The embedding of embryo in the thick lining of the uterus is called implantation.
• After implantation, a disc-like special tissue develops between the uterus wall (called uterine wall) and the embryo, which is called placenta.
• It is through the placenta that all the requirements of the developing foetus like nutrition, respiration, and excretion, etc., are met from the mother’s body.
• The time period from the fertilisation up to the birth of the baby is called gestation.
• The average gestation period in humans is about nine months.
• During the gestation period, the foetus grows to become a baby.
• Birth begins when the strong muscles in the walls of the uterus start to contract rhythmically.
• The rhythmic contraction of uterus muscles gradually pushes the baby out of the mother’s body through vagina.
Differences Between Zygote, Embryo and Foetus
A zygote is formed after fertilisation. A zygote develops and becomes an embryo. And finally, an embryo develops and becomes a foetus. The main differences between zygote, embryo and foetus are given below:
|
Zygote |
Embryo |
Foetus |
|
1. A zygote is formed by t he fusion of male and female gamet es (sperm and egg) |
1. An embryo is formed by the repeated cell divisi on of a zygote. |
1. A foetus is formed by the growt h and development of an embryo. |
|
2. A zygote is the beginning of the formation of a baby. |
2. An embryo is an unborn baby in the uterus in the early stages of development (up to 8 weeks after fertilisation) |
2. A foetus is an unborn baby in the uterus in the lat er stages of development (aft er 8 weeks till birth). |
|
3. A zygote is a single cell. It is smaller than a full stop (.) |
3. An embryo is multicellular. The body features of growing baby in the embryo are not much developed. |
3. A foetus is also multicellular. The body features of developing baby (like hands, legs, head, eyes and ears, etc.) can be identified. |
27. Sexual Cycle in Females: Menstruation
1. When a girl reaches puberty at the age of about 10 to 12 years, the sex hormones released into he: blood cause some of the ova in her ovaries to become mature.
2. Usually one mature ovum is released from the ovary into the oviduct once every 28 days. This is called ovulation.
3. Before ovulation, the inner lining of uterus becomes thick and spongy, and full of tiny blood vessels, and prepares itself to receive the fertilised ovum or egg.
4. If the ovum does not get fertilised then the thick and soft inner lining of uterus is no longer needed and hence it breaks. So, the thick and soft inner lining of uterus alongwith the blood vessels and the dead ovum comes out of the vagina in the form of a bleeding called menstruation.
5. Menstruation usually occurs 14 days after ovulation and usually lasts for about 3 to 5 days.
6. After menstruation is over, the inner lining of the uterus starts building up again so that it may become ready to receive the next ovum in case it gets fertilised.
7. If the ovum does not get fertilised even now, then menstruation takes place again. This cycle of menstruation is repeated again and again in women after every 28 days. The menstrual cycle is controlled by hormones.
Menarche and Menopause
• The first occurrence (around 10 to 12 years)of menstruation at puberty is called menarche. Menarche is the beginning of the reproductive life of a girl.
• The permanent stoppage of menstruation in a woman is called menopause (45 to 50 years).
28. Birth Control Methods
The prevention of pregnancy in women (by preventing fertilisation) is called contraception.
And any device or chemical (drug) which prevents pregnancy in woman is called a contraceptive.
1. Barrier Methods
• In the barrier methods of preventing pregnancy, the physical devices such as condoms and diaphragm (or cap) are used.
• Condoms are used by males.
• Diaphragm is used by females.
• Condom as well as diaphragm prevent the sperms from meeting the ovum (or egg) by acting as a barrier between them.
• An important benefit in the use of condom is that it protects a person from the sexually transmitted diseases such as gonorrhea, syphilis and AIDS. No other method of contraceptive provides protection against sexually transmitted diseases.
2. Chemical Methods
• The females use two types of pills: oral pills and vaginal pills.
• The oral pills contain hormones which stop the ovaries from releasing ovum into the oviduct.
• Oral pills are also called Oral Contraceptives.
3. Intra-Uterine Contraceptive Device (IUCD)
• The use of intrauterine contraceptive device called Copper-T is also very effective in preventing pregnancy.
• A Copper-T is placed inside the uterus by a doctor or a trained nurse. The IUCD or Copper-T prevents the implantation of fertilised egg in the uterus.
4. Surgical Methods
• The surgical procedure carried out in males is called Vasectomy’.
• In males, a small portion of the sperm duct is removed by surgical operation and both the cut ends are ligated properly. This prevents the sperms from coming out.
• The surgical procedure carried out in females is called tubectomy.
• In females, a small portion of the oviducts is removed by surgical operation and the cut ends are ligated. This prevents the ovum from entering into the oviducts.
29. Sexually Transmitted Diseases (STD)
• The diseases which are spread by sexual contact with an infected person are called sexually transmitted diseases (or STD).
• A healthy person can get STD by making sexual contact with an infected person.
• Examples: (i) Gonorrhoea, (ii) Syphilis, and(iii) AIDS (Acquired Immune Deficiency Syndrome).
• Gonorrhoea and syphilis are caused by bacteria.
• The bacteria which cause these diseases spread through sexual contact with an infected person.
• The most common symptoms of the these sexually transmitted diseases are burning sensation at urination, passing of urethral discharge (containing pus) and sores in the genitals.
• Gonorrhoea and syphilis are curable diseases.
• AIDS disease is caused by a virus called HIV (Human Immunodeficiency Virus).
• AIDS damages the body’s immune system so that the body becomes weak and cannot protect itself against infection.
• AIDS is a very dangerous disease which leads to death.
• No definite cure has been found for the AIDS disease so far.