Modes of Reproduction

MODES OF REPRODUCTION
    Plants reproduce by various methods. These methods can be divided into two types: (i) asexual reproduction and (ii) sexual reproduction.
    In asexual reproduction, the new individual is produced by a single parent. Seeds are not produced and there is no union of gametes (sex cells). The new individual is identical to the parent.
   In sexual reproduction, fusion of two types of gametes takes place. Two parents take part in the formation of gametes and new individuars are produced from these. The new individual is not identical to either of the parents. It has features of both the                 parents.

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Sexual Reproduction

SEXUAL REPRODUCTION
(i)    In this kind of reproduction two opposite sexes  i.e. male and female are required. 
(ii)    The flowers are the reproductive parts of a plant. The stamens are the male reproductive part and the carpel or pistil is the female reproductive part.
(iii)    The flowers which contain either only the pistil or only the stamens are called unisexual flowers. 
(iv)    The flowers which contain both stamens and pistil are called bisexual flowers. 
(v)    Corn, papaya and cucumber produce unisexual flowers, whereas mustard, rose and petunia have bisexual flowers. Both the male and the female unisexual flowers may be present in the same plant or in different plants.

(vi)    Anther contains pollen grains which produce male gametes. 
(vii)    A pistil consists of stigma, style and ovary. The ovary contains one or more ovules. The female gamete or the egg is formed in an ovule. In sexual reproduction, a male and a female gamete fuse to form a zygote.

1. Pollination
    The transfer of pollen from the anther to the stigma of a flower is called pollination.
    Types of pollination:
    (a) Self pollination
    (b) Cross-pollination
(a)    If the pollen lands on the stigma of the same flower it is called self-pollination. 
(b)    When the pollen of a flower lands on the stigma of another flower of the same plant, or that of a different plant of the same kind, it is called cross-pollination. In plants pollination is followed by fertilization.

Figure: Pollination in Flower

2. Fertilisation
    The process of fusion of male and female gametes (to form a zygote) is called fertilisation. The zygote develops into an embryo.

Figure: Fertilization (Zygote formation)

 

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Fruits and Seed Formation

3. Fruits and Seed Formation
(i)    After fertilisation, the ovary grows into a fruit and other parts of the flower fall off. 
(ii)    The fruit is the ripened ovary. 
(iii)    The seeds develop from the ovules. 
(iv)    The seed contains an embryo enclosed in a protective seed coat. Seed contain cotyledon, which store food materials.Some fruits are fleshy and juicy such as mango, apple and orange. Some fruits are hard like almonds and walnuts.

Figure: (a) Section of an apple   (b) Almond

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Seed Dispersal.

4.    Seed Dispersal
(i)    Seeds are dispersed to different places by means of some special structure through wind, water, animals etc. 
(ii)    Dispersal of seeds is very essential, if all seeds fall at the same place and grow than it will cause competition between germinating seeds for space, nutrients, water, sunlight, minerals etc. and seed would not grow into healthy plants.

 

 

 

 

 

 

(iii)    Thus, seed dispersal prevents overcrowding and competition between the plant and its own seedlings for sunlight, water and minerals. 
(iv)    It also enables the plants to invade new habitats for wider distribution. 
(v)    Winged seeds such as those of drumstick and maple. (a) and (b)], light seeds of grasses or hairy seeds of aak (Madar) and hairy fruit of sunflower. (a), (b)], get blown off with the wind to far away places. 
(vi)    Some seeds are dispersed by water. These fruits or seeds usually develop floating ability in the form of spongy or fibrous outer coat as in coconut. 
(vii)    Some seeds are dispersed by animals, especially spiny seeds with hooks which get attached to the bodies of animals and are carried to distant places. Examples are Xanthium and Urena. 
(viii)    Some seeds are dispersed when the fruits burst with sudden jerks. The seeds are scattered far from the parent plant. This happens in the case of castor and balsam.

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Sexual Reproduction part 2

Asexual reproduction
•    In asexual reproduction new plants are obtained without production of seeds or spores. A single parent can produce offsprings, which are genetically and morphologically similar to their parent and called as clone. It occurs by different methods :

1. Vegetative propagation
(i)    It is a type of asexual reproduction in which new plants are produced from roots, stems and leaves. For eg. by stem in potato and ginger, by roots in sweet potato, raddish and by leaves in bryophyllum. Since reproduction is through the                         vegetative parts of the plant, it is known as vegetative propagation. 
(ii)    Plants produced by vegetative propagation take less time to grow and bear flowers and fruits earlier than those produced from seeds. The new plants are exact copies of the parent plant, as they are produced from a single parent.

(iii)    Cut branch of rose or champa with node called as cutting. When these cuttings are buried in the soil than from node new leaves arises and from buried part roots arises.

Figure: Stem-Cutting of Rose

(iv)    Over the stem, multicellular out growths appear called buds. Buds, which develops into flower called floral buds. Some buds are present in axil of leaves, which develop into shoots. These buds are called vegetative buds. A bud consists of a short            stem around which immature overlapping leaves are folded. Such vegetative buds can give rise to new plants.

(v)    Over a fresh potato, some scars are found having buds. These scars are called “eyes”. A piece of potato having eye when buried in soil can produce a new plant.

Figure: Potato Plant Sprouting from an ‘eye’

Similarly ginger and turmeric can be grown.

Figure: Ginger with new plants sprouting from it 

(vi)    Bryophyllum (Sprout leaf plant) has fleshy leaves with serrated margin. Over margins buds are present, when leaf falls on a moist soil, each bud can give rise to a new plant.

Figure: Leaf of Bryophyllum with buds in the margin

(vii)    The roots of some plants can also produce new plants. For e.g. sweet potato, Dahlia, raddish, carrot etc. The pairs of cacti plant, when get detached from the main plant body can produce in plants.


Advantages of Vegetative fropagation
    •    Plants produced by this method take less time to grow and bear flowers and fruit faster than those produced from seeds.
    •    Seedless plants can be obtained.
    •    Plants produced are exact copies of the parent plant. No variations are present.

2. Budding

(i)    Yeast is a single-celled microscopic organism.
(ii)    In yeast small bulb-like projection coming out from the yeast cell is called a bud. 
(iii)    The bud gradually grows and gets detached from the parent cell and forms a new yeast cell. 
(iv)    The new yeast cell grows, matures and produces more yeast cells. Sometimes, chain of bud arises from single yeast cell. 
(v)    If this process continues, a large number of yeast cells are produced in a short time. Yeast can be easily grown in sugar solution. Yeast is commercially used in bakery etc.

3. Fragmentation

(i)    Slimy green patches in ponds, or in other stagnant water bodies can be easily seen. 
(ii)    These are the algae i.e. spirogyra or pond silk. 

(iii)    When water and nutrients are available algae grow and multiply rapidly by fragmentation. 
(iv)    An alga breaks up into two or more fragments. 
(v)    These fragments or pieces grow into new individuals. This process continues and they cover a large area in a short period of time. e.g. Spirogyra.

4. Spore formation

(i)    The fungi on a bread piece grow and form spores which are present in the air. When spores are released they keep floating in the air. As they are very light they can cover long distances.
(ii)    The spores are asexual reproductive bodies. 
(iii)    Each spore is covered by a hard protective coat to withstand  unfavourable conditions such as high temperature and low humidity. So, they can survive for a longer time. 
(iv)    Under favourable conditions, a spore germinates and develops into a new individual. Plants such as mosses and ferns also reproduce by means of spores.

Activity : To observe spore formation in bread mould. 

•    Take a slice of bread and slightly moisten it.
•    Leave it in air for 3 days.
•    Observe after 3 days with a magnifying glass. You will notice fine thread-like structures (hyphae) and small spherical structures (sporangia) on long stalks are seen in Fig:
•    Now, take a spherical structure on a slide in a drop of water and put a cover slip on it.
    On observed under the microscope.
    You will see very small rounded bodies called the spores.

Artificial Methods of Vegetative Propagation
    Vegetative propagation carried out by human beings is called artificial propagation. There are several methods of artificial propagation. Two of the methods are stem cutting and grafting. 
•   Stem cutting is generally used in plants like rose, champa, sugar cane and bougainvillea. A stem cutting is a short piece of a branch of a plant having a node. This cutting when placed in the soil under suitable      conditions develops roots and leaves. Finally the complete plant develops.

•    Layering is done when the other propagation methods are unsuccessful and only a few plants are needed. It is a method in which roots develop on a shoot or branch that is still attached to the parent plant.           The stem is cut off from the parent plant only after it has rooted. Layering is generally used in plants like honeysuckle, rhododendron and forsythia.
•    Grafting is a very common method of artificial vegetative propagation in fruit plants like mango. New varieties can be developed by this method. In this method, the root portion is taken from one plant. This is        called the stock. The stem portion, with several buds, is taken from another plant called the scion. 
    The scion is taken from a plant which has the desired features that are intended to be introduced in the root portion. The ends of the stock and the scion are obliquely cut and firmly tied together. In this manner,      a new plant variety is developed.

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Modes of Reproduction

Chapter 12: Reproduction in Plants

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from either parent. Asexual reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unless mutations occur. Asexual reproduction does not involve the production and fusion of male and female gametes. Asexual reproduction may occur through budding, fragmentation, spore formation, regeneration and vegetative propagation.

Modes of reproduction

Depending on the number of parents involved, there are different modes of reproduction. Every living organism reproduces by either of the two modes i.e. sexual reproduction or asexual reproduction. The different modes of reproduction in animals and plants are explained below.

Sexual reproduction

It is the mode of reproduction in which gamete cells from two organisms, one male and one female, combine to form a singular zygote. This zygote shares half of its genetic information with the father and the other half with the mother.

Asexual Reproduction

It is the mode of reproduction which involves only one organism. The offspring that is produced is genetically identical to the mother and almost always has the same number of chromosomes. Hence, they are called clones. Moreover, they are exact copies of their parent cell.

Sexual Reproduction

Sexual reproduction

Humans reproduce sexually by the fusion of sperm with the egg (ovum) to produce a zygote called fertilization. Fertilization is the crucial stage of sexual reproduction. Without fertilization, sexual reproduction will not be complete.

Sexual Reproduction in Plants

Plants that reproduce sexually have sex parts which we know as flowers. The process of reproduction involves the fusion of male gametes, namely, pollen with the female gamete, also known as ovules. This fusion results in the formation of a zygote and an endosperm nucleus, which grow into seeds and fruits respectively.

Sexual Reproduction in Animals

Among the different modes of reproduction in animals, sexual reproduction is the most common one. Most animals reproduce sexually, though in a variety of different ways.

Sexual reproduction in animals involves the union of a haploid sperm and a haploid egg to form a diploid zygote, which shares its DNA with both the parent cells.

Fruits and Seed Formation

Fruits and seeds formation

Seeds and fruits are formed by fertilization. In angiosperms, two structures are formed as a result of double fertilization – a diploid zygote and a triploid primary endosperm cell. The zygote develops into an embryo, whereas, the endosperm cell gives rise to endosperm. It provides nourishment to the growing embryo.

Seed Dispersal

Seed dispersal

Seed dispersal is the mechanism by which plant seeds are transported to new sites for germination and the establishment of new individuals. Animals commonly mediate this process, and consequently, the ultimate fate of seeds depends on their effectiveness as seed dispersers.

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