12th Class Biology Sexual Reproduction in Flowering Plant / फूलों के पौधों में यौन प्रजनन 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. 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 stem. These are the modifications of axillary buds. 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. 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. 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 :

• Mound layering : In this technique a lower branch of stem is bent and covered in such a way that the tip of the branch remains above the ground. After a few days, the covered part of the stem produces adventitious roots. At this stage the branch is cut off from the parent plant and it grows into a new plant. This method is commonly employed for propagating Strawberry, Jasmine, Grape vine, Raspberry, etc.
• Air layering (Gootee) : This is employed in plants with thick branches which can not 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.