Sexual reproduction in flowering plants Class 12 ppt
Sexual Reproduction :
It involves fusion of two compatible gametes or sex cells. All organisms reach to the maturity in their life before they can reproduce
. In plants, the end of juvenile or
vegetative phase marks the begining of the reproductive phase and can be seen easily in the higher plants at the time of flowering. The flower is specialized reproductive structure of a plant in which sexual reproduction takes place. The function of flower is to produce haploid gametes and to ensure that fertilization
will take place. Typical flower consists of four different whorls viz. calyx, corolla androecium and gynoecium.
Sexual reproduction involves two major events viz. meiosis and fusion of gametes to form diploid zygote and the production of genetically dissimilar offsprings. Variations are useful from the point of view of the survival and the evolution of species, over the time.
Sexual reproduction is characterised by fusion of the male and female gametes (fertilization), the formation of zygote and embryogenesis. Sequential events that occur in sexual reproduction are grouped into three distinct stages viz, Pre-fertilization, Fertilization
The male reproductive whorl of flower is called androecium. Individual member of androecium, is called stamen. Stamen consists of filament, connective and anther.
Structure of Anther : An immature stage of anther is represented by group of parenchymatous tissue surrounded single layered epidermis. Anther is generally dithecous (having two lobes) and tetrasporongiate. Each monothecous anther contains two pollen sacs. In dithecous anther four pollen sacs are present. Therefore, it is tetrasporongiate. The heterogenesity (differenciation) arises when some hypodermal cells get transformed into archesporial cells.
Reproduction is the production of young ones like parents. Reproduction is an essential process as it leads to continuation of species as well as to maintain the continuity of life. Each organism has its own particular method of reproduction.
i. Asexual reproduction
1.1 Asexual Reproduction :
Asexual reproduction does not involve fusion of two compatible gametes or sex cells. It is the process resulting in the production of genetically identical progeny from a single
organism and inherits the genes of the parent. Such morphologically and genetically identical individuals are called clones. Organisms
choose to reproduce asexually by different modes or ways:
i. Fragmentation
Reproduction is the production of young
Multicellular organisms can break into fragments due to one or the other reasons. e.g. Spirogyra. These fragments grow into new individuals.
ii. Budding :
It is the most common method of asexual reproduction in unicellular Yeast. Usually it takes place during favourable conditions by producing one or more outgrowths (buds). These buds on seperation develop into new individual.
iii. Spore formation In Chlamydomona asexual reproduction occurs by flagellated,
motile zoospores which can grow independently into new individuals.
Vegetative Reproduction : Plants reproduce asexually through their vegetative parts. Hence, the new plants formed are genetically identical to their parents. There are also few methods which would not occur naturally in the plants. Agriculture and horticulture exploit vegetative reproduction in order to multiply fresh stocks of plants. Artificial methods are used to propagate desired varieties according to human requirements. The various methods are as follows :
a. Cutting :
The small piece of any vegetative part of a plant having one or more buds is used for propagation viz. Stem cutting - e.g. Rose, Bougainvillea; leaf cutting - e.g. Sansvieria; root cutting e.g. Blackberry.
b. Grafting :
Here parts of two plants are joined in such a way that they grow as one plant. In this method, part of the stem containing more than one bud (Scion) is joined onto a rooted plant called stock, is called grafting. Whereas budding is also called bud grafting in which only one bud is joined on the stock, e.g. Apple, Pear, Rose, etc.
Pollen grain marks the begining of male gametophyte. It undergoes first mitotic division to produce bigger, naked vegetative cell and small, thin walled generative cell. The vegetative cell is rich in food and. having irregular shaped nucleus. The generative cell floats in the cytoplasm of vegetative cell.
The second mitotic division is concerned with generative cell only and gives rise to two
non-motile male gametes. The mitotic division of generative cell takes place either in pollen grain or in the pollen tube. The pollen grains are shed from the anther, at this two- celled
stage in most of the angiosperms. Female reproductive whorl of flower is gynoecium (Pistil). Individual member of gynoecium is called carpel (megasporophyll). A flower with many, free carpels is called apocarpous (e.g. Michelia). A syncarpous flower is one that has many carpels fused together (e.g. Brinjal). Typical carpel has three parts viz, ovary, style and stigma. The number of ovules in the ovary varies e.g. paddy, wheat and mango are uniovulate whereas tomato and lady’s finger are multiovulate.
T. S. of Anther :
The archesporial cell divides into an inner sporogenous cell and outer primary parietal cell. Sporogenous cell forms sporogenous tissue.
Each cell of sporogenous tissue is capable of giving rise to a microspore tetrad. Parietal cell undergoes divisions to form anther wall layers. The wall of mature anther consists
of four layers. Epidermis is the outermost protective layer made up of tabular (flattened) cells. Endothecium is sub-epidermal layer made up of radially elongated cells with fibrous thickenings. Inner to endothecium is middle layer made up of thin walled cells (1-2 layered), which may disintegrate in mature anther. Tapetum is the inner most nutritive layer of anther wall. It immediately encloses the sporogenous tissue (microspore mother
Structure of Anatropous ovule :
Each ovule develops inside the ovary and is attached to the placenta by a small stalk called funiculus. The place of attachment of funiculus with the main body of ovule, is called hilum.
In angiosperms, the most common type of ovule is anatropous in which micropyle is directed downwards and is present adjacent to the funiculus (funicle). The ovule consists of central parenchymatous tissue, the nucellus
which is surrounded usually by two protective coverings called integuments viz. Outer and an inner integument.
A narrow opening at the apex of the ovule is called micropyle. Chalaza is the base of ovule directly opposite to micropyle. Embryo sac (female gametophyte) is oval multicellular structure embedded in the nucellus.
It is the process of formation of haploid megaspores from diploid megaspore mother cell (MMC). Megaspore mother cell becomes distinguished in the nucellus, more or less in the centre but towards micropylar end of ovule.