How Do Organisms Reproduce Class 10 Notes Biology Science
Chapter 8

Introduction

All organisms produce offspring and thereby maintain their kind. The means by which organisms reproduce vary remarkably between species.

Reproduction is an orchestrated process. It needs the coordinated preparation of many tissues in female starting from maturation of ovum, its release its transportation, fertilisation and also the preparation of uterine wall to receive developing egg and its implantation.

The preparation for reproduction are cyclic in mammals. The change in structural and functional characteristics of both male and female reproductive tissue occur as a prelude to reproduce.

These are mediated by hormones. It is also concerned with the asexual development of the embryo from structures other than egg and also with the regeneration of lost or damaged parts and repair of defects.

Need of Reproduction: Reproduction is aimed at multiplication and perpetuation (stability) of the species. In other words it provides group immortality by replacing the dead individuals with new ones.

Basic Features of Reproduction: The modes of reproduction vary in different organisms. However all of these have certain common basic features. These are –

replication of DNA
cell division
formation of reproductive bodies or units
development of reproductive bodies into offsprings

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

It is more primitive type of reproduction. It ensures rapid increase in number.

Characteristic

It involves only one parent and no formation of gametes.
The offspring produced is direct replica of the parent leaving no scope for variation.
This is also called as vegetative or somatic reproduction and common in plants and lower animals
Occurs by budding, binary fission, multiple fission, sporulation and also the regeneration.
Protozoans, sponges and coelenterates reproduce mainly by this way.

Types Of Asexual Reproduction

(i) Fission:

It is the simplest form of reproduction in which unicellular organism either divides into two or many organisms.

It is also divided into two types:

Binary fission:
Binary fission, meaning "division in half", refers to a method of asexual reproduction. It is the most common form of reproduction in prokaryotes and occurs in some single-celled eukaryotes. After replicating its genetic material, the cell divides into two equal sized daughter cells. The genetic material is also equally partitioned, therefore, the daughter cells are genetically identical (unless a mutation occurred during replication) to each other and the parent cell. Transverse binary fission divides the cell across the short axis (e.g., most bacilli-shaped bacteria), longitudinal binary fission across the long axis (e.g.,Trypanosoma), and random binary fission across no defined axis (e.g.,Amoeba). Some biologist use this term for multi-cellular organisms that asexually reproduce by dividing into two (e.g., some star fish). This is also known as fragmentation. Spirogyra, a type of algae also reproduces by binary fission.

Multiple fission:
Sometimes the nucleus divides several times, into many daughter nuclei. The daughter nuclei arrange at the periphery of the parent cell, and a bit of cytoplasm around each daughter nuclei is present. Nucleus develops an outer membrane. Finally the multi-nucleated body divides into many daughter cells. e.g. Plasmodium.

(ii) Budding: Formation of a daughter individual from a small projection which is called as bud, arising on the parent body is called as budding.

Budding is also of two types:
Exogenous budding: [External budding]

In this, bud arises from the surface of parent body. e.g., Hydra.

Budding in Hydra: A bulge appears on the body as a result of repeated mitotic division in the cells resulting in the formation of lateral out-growth called bud. This bud enlarges in size by further division of cells and attains the shape of parent. It then separates from the parent body and starts behaving as new Hydra.

Endogenous budding: [Internal budding]

In this, bud arises inside or within the parent body. e.g., Sponges.

Budding in yeast: A small bud like out-growth appears at one end of the parent cell which gradually enlarges in size. The nucleus also enlarges and divides into two daughter nuclei. One nucleus remains in the parent cell and other goes to the daughter. When the bud attains almost similar size like parent, a constriction appears at the base of the bud separating it from the parent. However, sometimes a chain of buds can also be seen.

(iii)Fragmentation :
This is a type of asexual reproduction in fungi which is similar to the vegetative reproduction in some plants. In this process, a detached fragment of the fungal hyphae gives rise to a new individual under suitable conditions.

(iv) Sporulation :

In fungi, other than yeasts, the most common type of asexual reproduction is sporulation. The process of formation of spores is called sporulation. The spores formed in sporangia are called sporangiospores which are formed by several mitotic divisions. A sporangiospores germinate to develop into an organism with many vegetative hyphae.eg: Mucor

Conidia :

In this process the fungal hyphae differentiate into structures like sporangia, conidiophores etc., which will give rise spores or conidia by mitotic division. eg: Aspergillus

Spore formation:

A spore is a single or several-celled reproductive structure that detaches from the parent and gives rise, directly or indirectly to a new individual.

Spore formation takes place mostly in bacteria and fungi.

In fungi e.g. Rhizopus, Mucor, Aspergillus, Penicillium etc., spores are formed in a sac-like structure called sporangium at the tips of fungal hyphae. The nucleus divides inside the sporangium and gets surrounded by a small mass of cytoplasm forming a spore. After attaining maturity, the sporangial wall ruptures releasing the spores. The spores are covered with thick walls that protect them until they come in contact with another moist surface and can begin to grow.

Regeneration

Regeneration is defined as a natural ability of some simple multicellular organisms to replace worn out parts, to repair damaged parts or to regrow cast off organs. It takes place mostly in Hydra, Planaria, sponges, etc. If sometimes the individual is somehow cut or broken into many pieces, many of these pieces grow into separate individuals. But regeneration is not same as reproduction since most organisms would not normally depend on being cut up to be able to reproduce.

Regeneration is performed by specialised cells. These specialised cells proliferate and make large number of cells. Different cells from the mass of cells undergo change to become various cell types and tissues. These changes takes place in an organized sequence referred to as development. Complex multicellular organisms cannot give rise to new individuals by regeneration. This is because they have complex body organization.

Vegetative propagation

Vegetative propagation: It is a form of asexual reproduction seen in plants. Plant undergoing vegetative reproduction propagates by a part of their body other than a seed. This part is called propagule. Vegetative reproduction is of different types.

Natural method: There are many plants which propagate naturally. Some plants propagate by roots e.g. sweet potato, guava, etc.; some by stems like ginger, banana, potato, strawberry; etc.; some by leaves like Bryophyllum, Begonia, etc.

Artificial method: There are some plants which propagates artificially by following methods :

Cutting: This is the very common method of vegetative propagation practised by the gardeners all over the world. It is the process in which a vegetative portion from plant is taken and is rooted in the soil to form a new plant. e.g. Grapes, Sugarcane etc.

Layering: In this process the development of adventitious roots is induced on a stem before it gets detached from parent plant, e.g., Mango, roses etc

Mound layering: In the process of layering the lower stem branch of plant is used. Leaves are removed from this stem. Then it is bent close to the ground, pegged and covered with the moist soil in such a way that it's growing tip remains above the soil surface. This pegged down branch is called as layer. After a few days the covered portion of stem develops roots. This stem is then detached from the parent plant and is grown separately into a new individual. e.g. Jasmine

Air layering: It is adopted in those plants where stem cannot be bent to the ground. In this process the stem is girdled (i.e. ring of the bark is removed). Then it is covered with moist moss or cotton and wrapped with a polythene sheet to preserve the moisture. After few weeks adventitious roots develop from the injured part. The branch along with roots is then separated from the parent plant and planted to grow into a new plant. e.g. Orange, Pomegranate etc.

Grafting: The process of joining together of two different plants in such a manner that they live as one plants is called as grafting. Out of the two plants one is rooted in the soil and is known as the stock. The other part consists of a small shoot bearing one or more buds it is known as scion. Their union is carried out in such a way that their cambium must overlap each other. e.g. Mango, roses etc.

Methods of grafting.
There are four methods of grafting. They are

Approach grafting
Cleft grafting (or) Wedge grafting
Tongue grafting
Bud grafting

Approach grafting : Ex: Sapota, Mango, Guava etc.

In this method both scion and stock remain rooted. A small slice is cut off from the stem of scion and stock.
Scion is bent towards the stock. The two cut surfaces of the scion and stock are brought together and tied with a tape.
In course of time the two stems get united. Then the top of the stock and the base of the scion are cut off.

Cleft grafting (or) wedge grafting

The stem of the stock is cut across. A 'V' shaped insertion is made at the end of the stock.
The scion is cut in shape. The scion is inserted into the 'V' shaped root stock. The point of union is held in position and a waxed tape is put around the junction.

Tongue grafting : This method is used on stocks that are relatively small.

Top of the stock is cut diagonally and in an upward direction. Scion is cut diagonally in downward direction.
A second cut is given from above downwards, which forms the tongue, such that the notch or tongue of the scion closely fits with that of stock.
Union is tied with twine and covered with a waxed tape.

Bud grafting : Ex. Apple, orange, rose etc.

A 'T' shaped incision is made on the bark of the stock.
A bark is removed on either side of vertical cut.
A single bud scion with a little wood is placed in the incision of below the bark and held in position applying tape.

Advantage of vegetative propagation.

Vegetative propagation helps to maintain fixed qualities and characteristic features of the parent plant.
For Example in ornamental plants with attractive flowers and leaves or any other interest in variation in one plant can be continued in next generation through vegetative propagation.
Such plants can be produced in large numbers in a short time.
Vegetative reproduction offers uniform root stock for budding or grafting.
This root stock can be selected from a plant that is immune to diseases.
In this method it would be possible to develop new varieties with useful characters.
It can be used to grow other varieties by means of grafting.
Vegetative propagation is a very useful method of reproduction in plants that rarely produce flowers.

Micro Propagation

Micro propagation: It has now become possible due to recent techniques to produce a large number of plantlets from a small piece of tissue taken from the shoot tip or other suitable plant parts. This method of propagation is called as micro-propagation. It involves the process of tissue culture. e.g., Orchids. ornamental plants etc.

Mechanism of micro propagation:

A small piece of plant tissue placed in a culture medium divides rapidly to form a shapeless lump called ‘callus’.
The callus is then placed in different culture media to stimulate the development of root and shoot.
Tiny plantlets are formed from just few cells which are transplanted into pots or soil where they can grow to form mature plants.

Advantages of micro propagation

It is a fast technique producing many plantlets from a small plant tissue in few weeks and using very little space. In other words, it is quite economical.
The plants produced by tissue culture are disease free.

Sexual Reproduction In A Flowering Plant

Sexual reproduction in flowering plants involves transformation of diploid sporophytic cells into haploid gametophytic cells by meiosis and subsequent fusion of haploid gametes of opposite sex to form diploid zygote. The zygote then develops into an embryo which ultimately forms a diploid plant body. In flowering plants, all these steps of sexual reproduction occur within specialized reproductive organs, called the flowers.

Structure of the flower: Morphologically flower is a modified shoot meant for sexual reproduction of the plant. Typically, it is a condensed branch in which internodes have become condensed, bringing nodes very close to one another, and the leaves are modified to form floral whorl that directly or indirectly participate in the process of reproduction.

The flower is commonly borne on short or long stalk called the pedicel. It has an upper swollen region known as receptacle (thalamus or torus).

Parts of a flower: A typical angiospermic flower consists of four whorls of floral appendages attached on the receptacle : calyx, corolla, androecium and gynoecium. Of these, the two lower whorls (i.e., calyx and corolla) are sterile and considered as nonessential, accessory or helping whorls. The two upper whorls (i.e., androecium and gynoecium) are fertile and considered as essential or reproductive whorls.

Calyx : It is the outermost whorl of the flower. It is composed of leaf like green sepals. The sepals are essentially green in colour but in some cases they are coloured like petals. Such a condition of calyx is called petaloid. Sepals enclose the bud and protect the delicate part within. They prevent rapid transpiration from the inner parts of the flower.
Corolla :This is the second whorl of the flower and consists of a number of petals. Petals are generally brightly coloured and sometimes fragrant which make the flower to become attractive. Petals usually attract the insect pollinators and helps in pollination. The petals and sepals together form the floral envelope called perianth
Androecium : It is the third whorl of flower and is the male reproductive organ consisting of stamens. Each stamen is made of filament and anther. The filament supports anther at its tip. Usually anthers are bilobed and contain four microsporangia (or pollen sacs), but sometimes they have only one lobe and two microsporangia. The portion of stamen which connects the anther and the filament is known as connective.

(A) Filament: It forms the stalk that bears more or less cylindrical or ovoid anther.

(B) Connective: it connects anther to filament.

(C) Anther: It is present on the top of filament. Each anther consists of two lobes that is why it is called as bi-lobed. Each anther lobe has two pollen sacs which contain millions of tiny microscopic pollen grains, called as microspores. The pollen grains are like yellow dusty powder in appearance.

Gynoecium :This is the last and the fourth whorl of flower and is the female reproductive organ of the flower. It occupies the central position on the receptacle and composed of ovary, style and stigma and the component parts are called carpels.Ovary encloses the ovules. Stigma is the receptive spots which lodges the pollen grains. Style is the connection between stigma and ovary.

Each pistil usually consists of three distinct parts - ovary, style and stigma.

Ovary: It is a basal, swollen part of the pistil. The ovary has one or more chambers called the loculi which are distributed in a special cushion like parenchymatous tissue called the placenta, from which the ovule develops.
Style: From the top of the ovary arises a long, elongated structure called as style.
Stigma: The terminal end of style is called as stigma. It is rough, hairy or sticky to hold pollen grains during pollination process.

Types of flower based on reproductive organs

Unisexual: In such flower, only one reproductive part is present, either male (stamen) or female (pistil) e.g. cucurbits, mulberry, papaya, watermelon, etc.
Bisexual: When stamens and carpel are found in the same flower, it is called hermaphrodite or bisexual, e.g. Hibiscus (china rose), mustard, rose, pea, cotton, etc.

Pollination

The process of transfer of pollen grains, from an anther to the stigma of the same flower or of different flower.

It is of two types :

Self pollination
Cross pollination

Self pollination :This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. It is of two types :
1. Autogamy :It is a kind of pollination in which the pollen from the anthers of a flower are transferred to the stigma of the same flower.
2. Geitonogamy :It is an kind of pollination in which the pollen from the anthers of one flower are transferred to the stigma of another flower borne on the same plant. It usually occurs in plants which show monoecious condition (unisexual, male and female flowers are borne on the same plant). Geitonogamy involves two flowers but these belong to the same parent plant.
  Merits
  (a) Pollen grains are not wasted.
  (b) The purity of the generation is maintained.
  Demerits
  (a) New and healthier varieties are not formed.
  (b) It results in weaker progeny, producing weaker seeds and plants.
  Contrivances for self pollination :The major contrivances or adaptations which favours self pollination are :
  - Bisexuality :Flowers should be bisexual or hermophrodite.
  - Homogamy :Anthers and stigma of the bisexual flowers of some plants mature at the same time. They are brought close to each other by growth, bending or folding to ensure self pollination. This condition is called homogamy. e.g., Mirabilis (Four O, clock), Catharanthus (Vinca), Potato, Sunflower, Wheat, Rice, etc.
  - Cleistogamy :Some plants never open to ensure complete self-pollination. This condition is called cleistogamy, e.g., Commelina bengalensis, Oxalis, Viola, etc. The cleistogamous flowers are bisexual small, inconspicious, colourless and do not secrete nectar.
Cross pollination :Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. It is also called xenogamy.

Merits
(a) Seeds are more and viable
(b) Progenies are healthier.
(c) Adaptability is better.
(d) New varieties can be produced.
Demerits
(a) The process is not definite because plants depend on agencies.
(b) Large amount of pollen grains are wasted.

Contrivances for cross pollination: Nature favours cross pollination. All unisexual flowers and a large number of bisexual flowers are naturally cross pollinated.

The main contrivances ensuring cross pollination are as follows :

Diclincy or Unisexuality: In unisexual flowers stamens and carpels are found in different flowers. Unisexuality can be of two types :
Monoecious plant : When male and female flowers are borne on the same plant. e.g., Maize, Cucurbits, Castor.
Dioecious plant: When male and female flowers are borne on different plants. e.g., Carica papaya, Cannabis.
Dichogamy: In bisexual flowers, when two sexes mature at different intervals and thus avoid self pollination is known as dichogamy. When stamens mature earlier than the stigma, it is known as protandry and the flowers are called protandrouse.g. Coriander, Jasmine, Sunflower, Lady’s finger, etc. When stigma matures earlier than the stamens, it is known as protogynyand the flowers are called protogynous. e.g., Rose, Tobacco, Crucifers, etc.
Heterostyly: The plants of some species in which flowers are dimorphic. Thus, facilitate cross pollination. Some of them possess a long style but short stamens and are known as pin-eyed while others have short style and long stamens. These are known as thrum-eyed. e.g., Oxalis.
Herkogamy :In some bisexual flowers where the stigma and anthers mature at the same time, self pollination is avoided by some sort of barrier. The flowers show following contrivances :
The male and female sex organs lie at some distance from each other.
In some flowers corolla has peculiar forms which act as barrier in self pollination. e.g. sAristolochia.
In some other flowers, the pollens are held together to form pollinia which can only be carried away by insects. e.g. Orchids and Calotropis.
Self sterility or Incompatibility : When pollen grain of an anther do not germinate on the stigma of the same flower, then such flower is called self sterile or incompatibility and this condition of flower is called self sterility, intraspecific incompatibility or self incompatibility. In these flowers cross pollination is the only means for fertilisation and production of seeds.
Agents for cross pollination :Cross pollination involves external agents for the transfer of pollen grains of one flower to the stigma of another flower. There are two main groups of agents :
1. Abiotic agents like wind and water
2. Biotic agents which include animals of different types such as insects, birds, bats, snails, etc.
  1. Abiotic agents
    - Anemophily :When flowers are pollinated by wind agency, the phenomenon is known as anemophily. Wind pollinated flowers produce very large amount of pollen grains to compensate the wastage. Pollen grains of such plants are small, light, dry, and smooth. The female flowers have large feathery or brush like stigmas to catch the pollen grains. Anemophilous flowers are small and inconspicuous with long and versatile stamens. e.g. Sugarcane, Maize, Wheat, Bamboo, Pinus, Papaya, Grasses, Typha, Datepalm, Coconut, Mulberry, Chenopodium, etc. This type of pollination mainly observed in Graminae
    - Hydrophily :When the pollination takes place through the agency of water, it is known as hydrophily. All aquatic plants are not hydrophilous some are anemophilouse.g.Potamogeton, Myriophyllum or Entomophilous .eg. Alisma, Lotus. Hydrophily is of two types :
    - Hypohydrophily :Plants which are pollinated inside the water e.g.Zostera, Ceratophyllum, Najas, etc.
    - Epihydrophily :Plants which are pollinated outside the water. e.g.Vallisneria (Ribbon weed).
  2. Biotic agents
    - Entomophily : When pollination is brought about by the agency of insects, it is known as entomophily or insect pollination. About 80% pollination occurs by insects like moths, bettles, butterflies, wasp, etc. All the flowers pollinated by insects are brightly coloured, have a sweet smell and produce nectar. Entomophilous flowers produce a small amount of pollen which has a spinous and sticky exine due to presence of pollenkitt. The stigmas of such flowers are long rough and sticky. The insects visit the flower for nectar, edible pollen grain and shelter. Bees obtain both nectar and pollen grains from the flowers and have basket for collecting pollen. Salvia is excellent example of insect pollination is which pollination occurs by lever or turn pipe mechanism. Other examples of insect plants A Flower pollinated by Honey bee and Butterfly.

- - Entomophily : When pollination is brought about by the agency of insects, it is known as entomophily or insect pollination. About 80% pollination occurs by insects like moths, bettles, butterflies, wasp, etc. All the flowers pollinated by insects are brightly coloured, have a sweet smell and produce nectar. Entomophilous flowers produce a small amount of pollen which has a spinous and sticky exine due to presence of pollenkitt. The stigmas of such flowers are long rough and sticky. The insects visit the flower for nectar, edible pollen grain and shelter. Bees obtain both nectar and pollen grains from the flowers and have basket for collecting pollen. Salvia is excellent example of insect pollination is which pollination occurs by lever or turn pipe mechanism. Other examples of insect plants A Flower pollinated by Honey bee and Butterfly.

How Do Organisms Reproduce ?

Reproduction is the biological process by which new individual organisms (offspring) are produced from their parents. Reproduction is a fundamental feature of all known life. Each individual organism exists as the result of reproduction.
The inheritance of features from parents can be transferred to their offsprings through chromosomes present in the nucleus of the cell.
Basic event in reproduction is the creation of a DNA copy.
Some changes in DNA copying produces variations in organisms known as mutations.

Vegetative propagation in plants is also an example of asexual reproduction. New plants can be grown from different vegetative parts of the plants like roots, stems, leaves etc.
Some plants like Bryophyllum develops adventitious buds on their leaves which develops into new plants.
Modified tuberous roots like sweet potato can be propagated vegetatively when planted in soil.
Artificial methods of vegetative propagation includes cutting, layering, grafting etc.

The parts of a flower are calyx, corolla, androecium and gynoecium.
Calyx is the outermost whorl of a flower which is a collection of sepals mostly green in colour and they are protective in function.
Corolla is the collection of petals, they are brightly coloured and helps in attraction of insects for pollination.
Androecium is a collection of stamens which are the male reproductive organs of a flower. A stamen consists of anther and filament. Anther contains pollen sacs in which pollen grains are produced. Pollen grains contain male gametes.
Gynoecium is a collection of carpels which are the female reproductive organs of a flower.
A carpel consists of stigma, style and ovary
The ovary contains ovules which contains the female gametes.
Double fertilisation is a complex fertilisation mechanism in flowering plants. This process involves the fusion of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm).
Ovules grow into seeds and ovaries grow into fruits after fertilisation.

Gonads are primary sex organs in humans. Testes are male gonads which produces sperms whereas ovaries are female gonads which produce eggs.
Testes are found within sac like structures called scrotum. Vas deferens or sperm ducts carry sperms from testis.
Ovaries produce eggs. The fertilisation of egg with sperms occurs at oviduct. After fertilisation zygote is formed which develops into embryo that grows in uterus to a full term baby in about 280 days which is called gestation period.
If fertilisation does not occur the egg disintegrates and along with blood and mucus it comes out through the vagina. This cycle occurs every month in females known as menstrual cycle.

To avoid rapid growth of population some preventive measures are taken which is called birth control.
The infectious diseases which spread from an infected person to healthy ones through sexual contact are called Sexually Transmitted Diseases or STDs. Examples are AIDS, syphilis, gonorrhea.


1. Asexual reproduction: The process of producing offsprings which involves a single parent without the formation of gametes is called asexual reproduction.	8. Primary sex organs: They are the gonads i.e., testes and ovaries which produce gametes and secrete sex hormones.
2. Spore: A spore is a single-celled or multi-celled reproductive structure which gets separated from its parent and under the favourable conditions gives rise to a new individual.	9. Gametes: The special cells involved in sexual reproduction to produce the offsprings are called gametes or sex cells.
3. Seed: A seed is the reproductive unit of a plant from which a new plant grows.	10. Puberty: The age at which sex hormones are produced, reproductive organs become matured and have the capacity to give rise to new individuals and there is development of secondary sexual characters in both males and females.
4. Vegetative propagation: It is mainly seen in plants and is an asexual mode of reproduction where a new plant grows from different parts of plant like roots, stem, leaves etc., rather than from a seed.	11. Fertilisation: The process of fusion of male and female gametes to produce the zygote is called fertilisation.
5. Tissue culture: The production of new plants from a small piece of plant tissues or cells removed from the growing tips of a plant in a suitable growth medium is called tissue culture.	12. Gestation period: It is the time from fertilisation till the birth of the new born.
6. Pollination: The process of transfer of pollen grains from anthers of stamens to the stigma of carpel within the same flower or different flower of same plant or to any other flowers of different plants but of same species is called pollination.	13. Parturition: The delivery of full term baby from the uterus of mother after the end of gestation period is called parturition.
7. Double fertilisation: The process by which a male gamete fuses with an egg to form zygote and the second male gamete unites with two polar nuclei to form endosperm is called double fertilisation.

How Do Organisms Reproduce Class 10 Notes Biology Science
Chapter 8