Asexual reproduction Vegetative propagation
Category : NEET
Asexual Reproduction
Asexual reproduction.
The methods of reproduction which do not involve meiosis and fertilization are known as apomixis or asexual reproduction. Only mitotic divisions are involved in these methods, resulting into the formation of offsprings which are genetically similar to the parent plant.
Asexual reproduction is of following two types:
(1) Agamospermy: Agamospermy is a kind of plant apomixis in which the embryos and seeds are formed by asexual reproductive methods without involving meiotic gametogenesis and sexual fusion of gametes. It occurs widely in ferns and the flowering plants. There are three different types of agamospermy:
(i) Adventive embryony : Formation of embryo directly from the diploid sporophytic cells (nucellus or integument) of ovule is called adventive embryony. Such embryos are formed without involving meiosis and sexual fusion, e.g., Citrus, Opuntia, etc. In Citrus, a seed may possess upto 40 embryos (one normal and rest adventive).
(ii) Diplospory : In this case, the archesporium differentiates but megaspore mother cell directly gives rise to an unreduced (i.e., without meiosis) embryo sac. It may produce two types of embryos :
(a) Diploid parthenogenesis : Embryo develops from unfertilized diploid egg.
(b) Diploid apogamy : Embryo develops from any diploid cell of embryo sac except egg.
(iii) Apospory : It is the formation of complete embryo sac from the sporophytic cell without meiosis so that the gametophyte remains diploid. Apospory may be of two types :
(a) Somatic apospory : Embryo sac is formed from somatic cell.
(b) Generative apospory : Embryo sac is formed from archesporium without meiosis.
(2) Vegetative propagation: Regeneration or Formation of a new individual from any vegetative part of the body is called vegetative reproduction or vegetative propagation. The lower plants reproduce vegetatively through budding, fission, fragmentation, gemmae, resting buds, spores, etc. Among flowering plants, every part of the body such as roots, stem, leaves and buds take part in vegetative propagation. It is very common mode of reproduction and it may be natural vegetative propagation or artificial vegetative propagation.
(i) Natural methods of vegetative propagation : In natural vegetative propagation, a portion gets deattached from the body of mother plant and it grows into a new individual plant under suitable conditions. Different plant parts are variously modified for vegetative propagation. Some of these are given below.
(a) Vegetative propagation by stems: The modified stems like bulbs, runners, rhizomes, corms, tubers, offsets, etc., help the plant to multiply under favourable conditions.
(b) Vegetative propagation by roots: The roots of some woody plants produce shoots which grow into new plants; e.g., Murraya, Lebbeck tree (Albizzia), Sisham (Dalbergia sisso), etc. Modified tuberous roots of Sweet potato, Asparagus, Dahlia, Tapioca, Tinospora, etc. develop buds and each of which form a new plant.
(c) Vegetative propagation by leaves: The leaves generally do not help in vegetative propagation. However, in Bryophyllum pinnatum and B. daigremontianum, develop along the leaf margins which on deattachment produce independent plants. In elephant ear plant (Begonia) also, leaf buds are produced from petiole and veins throughout the surface of the leaf.
(d) Vegetative propagation by reproductive parts: Flowers are primarily associated with sexual reproduction. But in Globba, American aloe (Agave), Onion (Allium cepa), etc. special multicellular structures, called bulbils, occur on the inflorescence. These are the modifications of flowers. Bulbils grow into new plants when shed from the mother plant.
(ii) Artificial methods of vegetative propagation: Several methods of vegetative propagation are man-made and developed by plant growers and horticulturists for commercial production of crops. They are called artificial methods. In this method a portion is separated from the body of the plant and then it is grown independently. The potato tubers are organs of natural vegetative propagation but are also used artificially. This is useful commercially because the new individuals produced maintain the desirable characters of the parents. A population of these genetically identical plants obtained from an individual is called a clone. Some of the artificial methods are given below:
(a) Cuttings: The small piece of any plant organ (stem, root or leaf) use for propagation is called cutting. Leaf cuttings are used to propagate Sansevieria, Begonia, Bryophyllum, Glocinia and Kalanchoe. Root cuttings are used to propagate Citron and Tamarind. Stem cuttings are most commonly used for artificial propagation. When cuttings (about 20-30 cm. long pieces of stem) from such plants are put into the moist soil, they develop adventitious roots and buds at the base which develops into new plants. Sometimes roots are not easily produced in the cuttings and hence, they are treated with rooting hormone (IBA). Factors such as age of the parent plant, length and diameter of the cutting, season and the type of plants are taken into consideration for the propagation of particular species. Grapes, Sugarcane, Rose, Bougainvillea, Carnation, Coleus, Duranta, etc. are propagated by stem cuttings.
(b) Layering: In this method, roots are artificially induced on the stem branches while it is still attached to the parent plant for propagation. There are two common types of layering:
Air layering (Goatee): This is employed in plants with thick branches which cannot be easily bent. In this method, part of the stem is girdled (i.e., a ring of bark is removed) or slit at an upward angle. This part is covered with moist moss or cotton and enclosed in a polythene bag to prevent drying. The wrapped portion is called gootee. The roots appear after some time and at that stage the branch is cut and planted. It grows into a new individual. This method is used in vegetative propagation of Litchi, Pomegranate, Orange, Lemon, Guava, Bougainvillia, etc.
(c) Grafting: A new variety is produced by joining parts of two different plants is called grafting. The rooted shoot of one plant, called stock, is joined with a piece of shoot of another plant known as scion. The root stock is generally derived from a plant resistant to diseases and efficient in water and mineral absorption. The scion is a stem cutting from a superior quality plant. The grafting ends of both, stock and scion are cut obliquely and then placed over one another in such a way that the cambia of two come in close contact. The two pieces are firmly held together by tape, rubber tubing, etc. This results in fusion of cambia and formation of new vascular tissue. Grafting is generally done between the related varieties or species. This method has been practised for many economically useful plants, such as Rose, Mango, Apple, Pear, Guava, Citrus, Rubber etc. There are various methods of grafting like tongue or whip grafting, wedge grafting and crown grafting. Besides these a technique, called bud grafting, in which only a single bud along with a small portion of bark having intact cambium instead of a scion is employed for propagation.
(d) Propagation by plant tissue culture or Micropropagation: This method includes propagation of plants by culturing the cells, tissues and organs called tissue culture. Small pieces of plant organs or tissues are grown aseptically in a suitable nutrient medium. Initially it results in the formation of undifferentiated mass of cells called callus. Which later differentiates to produce a large number of plantlets. These plantlets are then transferred to separate pots or nursery beds to obtain a large number of plants. Tissue culture technique is useful in obtaining virus free plants, homozygous diploids and in commercial micropropagation of Orchids, Carnation, Gladiolus, Chrysanthemum and other Ornamental plants. This method is also employed for quick multiplication of plants.
Important Tips
Sexual Reproduction
Sexual Reproduction
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.
(1) 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).
(2) 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.
(i) 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.
(ii) 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 (perianth).
(iii) 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.
(iv) 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.
(3) Functions of a flower
(i) Flowers are modifications of shoot to perform the function of sexual reproduction. The fertile leaves become microsporophylls (stamen) and megasporophylls (carpels) which bear anthers and ovules respectively. The anthers produce pollen grains and the ovules possess eggs.
(ii) Flowers of most of the angiosperms are shaped variously to help diverse modes of pollination.
(iii) Flowers provide seat for germination of pollen, development of pollen tube, formation of gametes and fertilization.
(iv) The ovary part of the carpel gets transformed into fruit and the ovules are transformed into seeds after fertilization.
(v) Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and seeds.
(4) Inflorescence: The flowers are arranged in some definite manner on the plant in each species of the flowering plants. The mode of arrangement of flowers on a specialised branch on top of the plant which bears flowers is called inflorescence. The axis of the inflorescence is called peduncle.
Depending upon the arrangement of flowers, inflorescence is classified as follows :
(5) Relative position of floral organs on thalamus: Depending upon the form of thalamus and the position of floral whorls with respect to the ovary, the flowers are of the following three types :
(i) Hypogyny: In this case the thalamus is convex-like and ovary occupies the highest position on it. The outer three whorls, viz. sepals, petals and stamens and inserted one above the other but below the ovary. Since the ovary lies above the other parts, it is described as superior and the rest of the floral whorls as inferior. A flower having hypogyny is called hypogynous. e.g. China rose, Brinjal, Mustard, etc.
(ii) Perigyny : In some cases, the receptacle or the thalamus forms a swallow or deep cup-shaped structure around the ovary. The pistil is attached at the centre of the concave thalamus. The sepals, petals and stamens are attached at the margins of the thalamus, the flowers are said to be perigynous and ovary is superior. Different type of flowers show different degrees of perigyny. e.g. Rose, Pea, Bean, Prunus, etc.
(iii) Epigyny : In this condition the margin of thalamus grows further upward completely enclosing the ovary and getting fused with it and bear the sepals, petals and stamens above the ovary. The ovary in such cases is said to be inferior and the rest of the floral members superior. e.g. Apple, Sunflower, Cucumber, Guava, etc.
(6) Placentation: The ovary contains one or more ovules, which later become seeds. The ovule bearing regions of the carpel is called placenta. The arrangement of placentae and ovules within the ovary is called placentation. The placenta is the cushion-like structure to which the ovules are attached inside the cavity of the placenta, placentation is of the following types :
(i) Marginal : In this type of placentation, the ovary is simple, unilocular and the ovules are arranged along the margin of the unilocular ovary. The placenta develops along the ventral suture of the ovary. e.g. Pea, Gram, Goldmohur, etc.
(ii) Axile : It is found in a compound ovary which is two or more chambered, usually as many as the number of carpels e.g. Petunia and Asphodelus. The placentae bearing the ovules develop from the central column or axis which is formed by the fusion of margins of carpels. In certain cases the number of chambers (loculi) increases due to the false septum formation. e.g. Datura, Tomato, etc.
(iii) Free central : In this free central placentation, the gynoecium is polycarpellary and syncarpous. The ovary in early stages is multilocular, but soon the septa break down leaving it as a unilocular structure. e.g. Dianthus, Slience, Primula, etc.
(iv) Parietal : In parietal placentation, the ovary is usually one-chambered but in some cases it becomes bilocular due to the formation of false septum, e.g. Brassica compestris (Sarson). The placentae bearing the ovules develop on the inner wall of the ovary at places where the margins of two adjoining carpels meet. The number of placentae corresponds to the number of fused carpels. e.g. Poppy, Mustard, Cactus, etc.
(v) Basal : In this type of placentation, ovary is bicarpellary, syncarpous and unilocular and a single ovule is borne at the base of ovary. e.g. Marigold, Sunflower, etc.
(vi) Superficial : The ovary is multicarpellary, syncarpous, and large number of loculi without specific order e.g. Waterlily (Nymphea).
(7) Symmetry of the flower: The number, shape, size and arrangement of floral parts (i.e. calyx, corolla, androecium and gynoecium) in a flower determines its symmetry. On the basis of symmetry, flowers can be of following three types :
(i) Actinomorphic or Regular flowers : Such flowers can be divided by vertical plane into two equal and similar halves. e.g. Mustard, Hibiscus, Brinjal, etc.
(ii) Zygomorphic or Irregular flowers : These flowers can be divided into two equal halves only along one median longitudinal plane. e.g. Pea, Iberis, Ocimum, etc.
(iii) Asymmetrical flowers : These flowers cannot be divided into two equal halves along any vertical plane. e.g. Canna, Maranta.
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