Reproduction in Organisms

“The very definition of the real becomes that of which it is possible to give an equivalent reproduction. The real is not only what can be reproduced, but that which is always already reproduced. The hyper real.” ~ Jean Baudrillard

In this article we are going to discuss about the topic of biology reproduction in organism so firstly you have to know what is reproduction? Types of reproduction in an organism and angiosperms types so, well let’s get started!

The term ‘reproduction consists of two words -(i) re = again and (ii) production= to generate or produce. This term, therefore means to produce again the same kind. The ‘again’ here indicates that a living organism can arise only from another living organism, but of same kind.Thus reproduction is a process in which a living organism produces agam a copy of its own kind (Species). Reproduction is the most important characteristic feature of all living organisms and its called Reproduction in Organisms

Like other organisms, flowering plants (angiosperms) also reproduce. Two methods of reproduction in these plants are asexual and sexual.

Asexual mode of reproduction

In asexual reproduction vegetative reproduction, no specialised structures are used. New plants develop from any vegetative part of the plant (i.e., plants are produced from somatic or diploid cells). The offsprings so produced are exactly similar to their parent.

lt is always uniparental, i.e., the offspring is produced by a single parent With or Without the involvement of gamete formation. As a result the daughter organisms are not only genetically identical to one another but are also the exact copies of their parent. Such type of genetically and morphologically similar individuals called clones.

Asexual reproduction is common in organisms like protozoans (e.g, Amoeba,Parame ecium, Euglena), Sponges (e.g. Sycon), coelenterates (e.g. Hydra), flat worms (e.g Planaria), annelids (e.g. Syllas), yeast, potato, ginger, penicillium, etc.Types

Sexual mode of reproduction

On the other hand, in sexual reproduction, plant produces specialised haploid structures called gametes. These are developed in sex organs. Gametes fuse to form Zygote, which on development gives rise to new plant. The off springs so produced are called hybrids and they differ in some of their characters from their parental plants.
Table Differences between Asexual reproduction and Sexual reproduction.

Sexual reproduction is a kind of reproduction, in which two parents (one male and other female) are involved in the formation of new individuals. These parents produce gametes (specialised haploid cells), which on fertilisation give rise to new individual. The individual is called hybrid. For sexual reproduction, angiosperms possess reproductive organs. These organs are the flowers

DIFFERENCE BETWEEN SEXUAL REPRODUCTION AND ASEXUAL REPRODUCTION

Sexual reproduction

1. Two parents male and female are involved in the formation of new individuals.
2. Sex organs take part in reproduction.
3. Vegetative parts have no role in sexual reproduction. Fusion of male and female gametes results in the development of new individual.
4. New individuals formed have genetic make up different form that of parents. Thus it produces variations.
5. It takes longer time
6. Specialised structures, called gametes are used.
7. Fertilisation essentially occurs.
8. The ploidy of the participating cells changes
9. Alternatively from diploidy to haploidy and again to diploidy.
10. The offsprings are not carbon copy of parental
11. plant. These are called ‘hybrids’

Asexual reproduction

1. New individuals are derived from a single parent.
2. Sex organs are not involved in asexual reproduction.
3. Any vegetative part of plant, such as stem, root or even few meristematic cells, are capable of giving rise to new individuals.
4. New individuals formed are genetically identical to the parent. So it does not produce variations.
5. it is relatively a quick method of multiplication.
6. No specialised structures are used.
7. There is no fertilisation.
8. The ploidy of the cells involved in
9. the reproduction remains the same, throughout the process.
10. The offsprings are the exact carbon copy of their parents. These can be called as natural clones.

VEGETATIVE REPRODUCTION

In this mode of reproduction, plant does not produce gametes or some specialised structure for reproduction. Simply, a vegetative part of the plant body (such as root,stem, leaves, twig etc.) gets detached from the main body and develops into a new plant, under favourable (suitable) conditions. The offsprings produced as a result of vegetative propagation are exactly similar to their parental plants and have the same genetic make up. They can therefore be called as ‘clones’.

This method of reproduction can be of two types

1. Natural Method
2. Artificial Method

(I) Natural Methods of Vegetative Reproduction

• In this type of reproduction the detachment of a plant part and its development
  into a new plant OCcurs naturally, without any involvement of mankind.
• Natural methods are widespread in angiosperms, especially in those plants, which show variOus types of modihcation of their body parts. Roots are modified into napiform, conical and fasiculated forms, while stems are modified to form rhizome, bulb, corm and tuber., Similarly, leaves are modified into various shapes and sizes.
• These nmodification play an important role in vegetative reproduction.

Some examples of natural method of Vegetative Reproduction
A. Vegetative Reproduction by Roots
B. Vegetative Reproduction by Underground stems
C. Vegetative Reproduction by Creeping stems
D. Vegetative Reproduction by Arerial Shoots
E. Vegetative Reproduction by Leaves
F. Bulbils

(II) Artificial Methods Of Vegetative Reproduction

In some plants, where vegetative reproduction by natural means is difficult to Occur, special techniques can be used. We know that plant cells are more totipotent than animal cells. So they can be forced to reproduce vegetatively. Thus all the techniques or methods which are carried out by human beings to produce plants vegetatively are called artificial methods. These include stem cuttings, root cuttings, layering, grafting and gootee, etc.

A. Cutting
B. Layering
C. Grafting
D. Gootee

To understand reproduction in plants, you must read the following basic Concepts very carefully.

Types of Asexual Reproduction:-

The various ways of asexual reproduction are as follows or the organisms that reproduce asexually :-

(i) Binary fission
In this type of asexual reproduction the parent organism divides into two equal halves, each half forming an independent daughter organism. It involves mitosis. The resultant offsprings are genetically identical to each other and to the parent.

Examples: Yeast, Euglena, Amoeba, Paramecium, Planaria, etc.ois

(ii) Multiple Fission
In this process, the parent body divides repeatedly to form many daughter organisms.
Examples: Amoeba, Plasmodium.

(ii) Plasmotomy
In this process, the multinucleate individuals divides into many multinucleate off springs without the division of nuclei.
Examples: Pelomyxa and Opalina.

(iv) Budding
In this process, a daughter organism is formed from a small projection, the bud arising on the parent body. It is of two types:
(a) External Budding
(b) Internal Budding

(a) External Budding: In this type of budding, an out growth or bud grows externally on the surface of the body. The bud may split away from the parent and take up an independent existence.

Examples: Yeast, Hydra, Sycon, Salpa.

(b) Internal Budding: In this type of budding the buds are formed within the parents body. They are called gemmules.
Examples: Fresh water sponges (Spongilla).

(v) Fragmentation

In this process, the parent body breaks into pieces and then each piece develops into a whole organism.
Examples : Algae, Sponges, Coelenterates, etc.

(vi) Sporution
Algae and fungi reproduce through special asexual structures called spores. The Common type of spores are motile, These are flagellated, motile and microscopic
structures. Another type of spores are conidia, these are formed exogenously on a branch called conidiophore. On germination these spores give rise to new organisms.
Zoospores are found in algae like Chlamydomonas where as conidia are found in fungi like Penicillium.

BASIC CONCEPT OF SEXUAL REPRODUCTION IN ANGIOSPERMS

in all the categories of higher plants, two distinct phases are found in the life cycle. These are
(i) Sporophytic phase and
(ii) Gametophytic phase

Both these phases regularly alternate with each other. In lower plants like Bryophytes, gametophytic phase is a dominant phase. But in higher plants like pteridophytes, gymnosperms and angiosperms, the dominant and chief phase is sporophytic phase In angiosperms gametophytic phase is highly reduced and microscopic.

Sporophytic Phase

• It is the diploid phase (always).
• It is developed from diploid zygote. Therefore, zygote represents the first cell of sporophytic phase.
• Sporophytic phase is called so, because it gives rise to spores.Spores are haploid cells (but these are not the gametes, so students should note this fact).
• A sporophyte (since it is diploid) produces haploid spores with the help of meiosis (also called reductional division).
• In higher plants, like angiosperms, spores are of two types. Male plants produce small sized spores, called microspores (or pollen grains) whereas the female plant produces larger spores, called megaspores (macrospores) (student should note that pollen grains or microspores are not the male gametes or sperms).
• In angiosperms, the entire standing plant body is sporophytic (diploid).
• Gametophytic tissues are highly reduced and develop inside the small part of flower.
• Microspores are developed inside microsporangia (anthers), while megaspores develop inside megasporangium (ovule), within ovaries

Gametophytic Phase

• It is the haploid phase of the plant.
• It 1S a very reduced phase in higher plants like angiosperms, but is very important.
• It develops from spores (which are haploid structures). Therefore, spores represent the first cells of gametophytic phase.
• Since there are two types of spores – microspores (pollen grains or male spores) and megaspore (temale spores), the gametophytes are also of two types – Male gametophyte (it develops from no microspore) and female gametophyte (it develops from megaspore).
• Gametophytes on development give rise to gametes.
• Male gametophyte produces male gametes (sperms), while female gametophyte produces female gametes (egg cell or ovum).
• In angiosperms, the mature male gametophyte consists of only three cells (one
  tube cell and two sperms or male gametes).
• Its development starts when the microspores are within the anther but gets completed only when it is transferred to the stigma of the flower (through
  pollination).
• The mature female gametophyte in angiosperms is mostly 7 celled (but have 8 nuclei). It is also called as embryo sac. It contains, an egg cell, 2 synergid cells,3 antipodal cells and one large embryo sac cell.
• The gametes from two types of gametophytes unite (fertilise) to produce a diploid cell,
  called zygote, which sets seeds and again start the sporophytic phase.

In this way a regular alternation between sporophytic and gametophytic stage is clearly apparent in angiosperms

MECHANISM OF REPRODUCTION

Reproductive Organs
Flower is the reproductive organ of angiospermic plants. It is found only in angiosperms. lt contains male and female reproductive organs, which are responsible for the producion of gametes (sex cells). In general, a flower consists of four whorls, calyx, corolla, androecium and gynoecium. Calyx and corolla are called accessory whorls, while other two are essential whorls or reproductive whorls. Androecium is called as male whorl, as it contains stamens, the male sex organs. Gynoecium is called as female whorl as it contains pistils (or carpels), the female sex organs.

Different whorls have been summaried below –

The Calyx

• The calyx is the outermost whorl of a flower.
• It consists of sepals that are usually green.
• The sepals may be free from each other (polysepalous) or fused (gamosepalous)
• The calyx encloses and protects the inner whorls in the bud stage.
• Since the sepals contain chlorophyll, they can also synthesize food.
  

The Corolla

• corolla is found on the innerside of the calyx and is the most Conspicuous part.
• It is usually white or brightly coloured.
• Its are called petals which are much larger than sepals. The petals may be Separate from each other (polypetalous) or become partly or completely fused
  (gamopetalous). If the petals are completely fused they form a corolla tube. The undifferentiated calyx and corolla together are called the perianth. The units of the perianth are called tepals.
• The brightly-coloured corolla attracts agents of pollination such as insects and birds

Androecium

• The androecium is the male whorl of the flower and is found inner to the corolla
• Each stamen is made up of a slender flexible filament and swollen anther.
• The anther is the male reproductive organ of the flower,
• The anthers (microsporangia) produce microspores (pollen grains) which produce male gametophyte.
• The filament bears and supports the Anther in the most suitable position for pollen transfer to take place.

Gynoecium

• The gynoecium is the fourth and the innermost whorl of the flower. It represents the female whorl of the flower.
• The units of gynoecium are called carpel or pistil or megasporophyll.
• Each pistill (carpel) consists ofa basal swollen portion, the ovary, from which a
  long and slender style arises. The stigma is found at the tip of the style.
• The Sigma 1s the most suitable part for receiving pollens during Pollination,The ovary internally contains the ovules or microsporangium. The Sticky stigma at the tip of the style can trap the pollen grains.

Important Concepts Of Organisms in reproduction for NEET

• Gynoecium and androecium are called essential whorls. A flower must contain at least one of the two whorls.
• When only one whorl is present, the flower is called unisexual. When only AndroeCium is present, flower is called staminate, and when only gynoecium is
  present the flower is called pistillate.
• When both essential whorls are present in the same flower the flower, 1s Called as bisexual or hermaphrodite.
• The calyx and the corolla are referred to as the non-essential whorls, since they are not responsible for the formation of gametes and seeds. a remi
• When all the four whorls are present in the flower, it is called complete ilower.
• If any of the non-essential whorls is not present, the flower is called incomplete.
• When both essential whorls are absent, the flower is called neuter.
• A plant containing flowers of only one sex is called dioecious. When unisexual flowers of both sexes are found on the same plant, the plant is called monoecious.
• When many types of flowers (male, female and neuter) are present on the same plant, the plant is called polygamous.
• When each whorl of a flower has equal number of units, then such a flower is called isomerous. In opposite condition, the flower is called heteromerous.

STEPS IN SEXUAL LIFE CYCLE OF A TYPICAL ANGIOSPERM

Following steps (sequences) occur in sexual reproduction in a typical angiospermic plant (a synopsis is given below)-

A. Developments that lead to the formation of male gametes (sperms)

1. Development of anther (microsporangium)
2. Formation of microspore (pollen grain) microsporogenesis
3. Pollination
4. Development in microspore to form mature male gametophyte and formation of sperms (Microgametogenesis)

B. Developments that lead to the formatioon of female gametes (egg cell)

1. Development of ovule (megasporangium)
2. Formation of megaspore (megasporogenesis)
3. Development in megasore-formation of female gametophyte (embryosac) and female gamete (Megagametogenesis).

C. Fertilisation

D. Development of embryo and seed formation

“Art, as I see it, is any human activity which doesn’t grow out of either of our species’ two basic instincts: survival and reproduction.” “Love is only a dirty trick played on us to achieve continuation of the species.” “Reproduction is more pleasurable than death.”

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