Tiêu đề SEXUAL REPRODUCTION IN FLOWERING PLANTS.pdf ✅

 Digital www.allendigital.in [ 23 ] 1. Introduction: - • Floriculture – Science of cultivation, breeding and marketing of flowers. • Most of the important angiospermic characters are found in Capsella (shepherd's purse) and it is easily available so that it is considered as a "Typical Angiosperm". • It is an annual plant and grows as a weed during the winter season in the field. • The main plant body of the angiosperms is a sporophyte (diploid) and it is differentiated into root, stem and leaves. • Angiosperms are is a heterosporous plant, it means two different types of spores are formed in the life cycle which are classified into two categories in which male spores are called microspores and female spores are called megaspores. • Flowers are object of aesthetic, ornamental, social, religious and cultural values. • Flower is a fascinating organ of angiosperms. • Flowers are seat of sexual reproduction in angiosperms. • To a biologist, flowers are morphological and embryological marvels and site of sexual reproduction. Flower is a modified shoot :- • According to Goethe, Flower is a modified shoot. Flower has a stalk called pedicel. Free end of the pedicel is flattened or dome shaped which is called thalamus. The thalamus is a type of modified stem, on which nodes and internodes are present. • Nodes are present very close to each other because internodes are small, highly reduced in the thalamus. • The whorls present in the flower are the modifications of leaves and these are arranged in four circles on the thalamus. • The four nodes are present on the thalamus, in which first modified leaves (calyx) are attached on the first lowermost node. • The corolla born on the second node, androecium is present on the third node and gynoecium on the fourth node in uppermost position. Sexual Reproduction in 02 Flowering Plants Pedicel Sepal Petal Ovary Style Stigma Anther Filament Stamen Carpel Thalamus
NEET : Biology [ 24 ] www.allendigital.in  Digital Sexual Reproduction: - • Sexual reproduction involves the formation and fusion of gametes. It is a complex and slower process as compared to asexual reproduction. • Events of sexual reproduction may be categorised into pre-fertilisation, fertilisation and post - fertilisation events. 2. Pre-Fertilization Structures and events:- • Pre-fertilization events include gametogenesis and gamete transfer. • Much before the actual flower is seen on a plant, the decision that the plant is going to flower has taken place. Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium. Inflorescences are formed which bear the floral buds and then the flowers. In the flower the male and female reproductive structures, the androecium and the gynoecium differentiate and develop. (A) Male Reproductive Whorl – Androecium :- • Male reproductive organ is called androecium consists of a whorl of stamens. • The number and length of stamens are variable in flowers of different species. • Stamen is equivalent to microsporophyll. • A typical stamen is differentiated into two parts -a long, thin stalk called the filament and the terminal generally bilobed structure called the anther. • The proximal end of filament is attached to the thalamus or the petal. • Anther and filament or two anther lobes are attached together with help of a region, called connective. Connective contains vascular tissues. • A typical angiosperm anther is bilobed with each lobe having two theca i.e. they are dithecous. Often a longitudinal groove runs lengthwise separating the theca. • The anther is consisting of four microsporangia located at the corners, two in each lobe. • The microsporangia develop further and become pollen sacs, they extend longitudinally all through the length of an anther and are packed with pollen grains. Connective Microspore Mother cells Stomium Tapetum Middle layer Endothecium T.S. of anther, showing four microsporangia and anther walls Anther Distal end Pollen grains Line of dehiscence Filament (stalk) (b) (a) (a) A typical stamen; (b) Three-dimensional cut section of an anther Proximal end Pollen sacs
Sexual Reproduction in Flowering Plants  Digital www.allendigital.in [ 25 ] • A typical anther has four microsporangia i.e. tetrasporangiate. • A typical anther is bilobed, dithecous and tetrasporangiate. • In Capsella, which is member of the cruciferae or brassicaceae, anthers are dithecous and tetrasporangiate type. • But in malvaceae, the anthers are monothecous and bisporangiate. Structure of Anther :- • In the transverse section of anther, it is seen almost tetragonal (4 sided) Microsporangium is generally surrounded by four different wall layers. (i) Epidermis :- It is the outermost layer of anther. It is single celled thick layer. It forms the outermost protective layer. (ii) Endothecium (Fibrous layer) :- This layer is present below the epidermis. It is single celled thick layer. During the maturation of anther, various changes take place in different walls of cells of endothecium. The outer wall of these cells remains thin, but inner walls and radial walls become thick due to thickening of –cellulose fibers. Callose bands are also present along the radial walls. At some places callose bands and fibrous thickenings are absent. These places are called stomium. The dehiscence of anther takes place only from these places. Endothecium becomes hygroscopic in nature due to presence of fibrous thickening. Endothecium helps in dehiscence of anther. (iii) Middle layer: - Middle layer consist of parenchymatous cells. This layer is one to three celled thick structure. Food is stored by parenchymatous cells in this layer. Middle layer is ephemeral (Short lived) in nature and absent in a mature anther. • The outer three wall layers perform the function of protection and help in dehiscence of anther. (iv) Tapetum: - It is the innermost wall layer which acts as nutritive layer. This is single layered thick. The cells of tapetum possess dense cytoplasm and generally have more than one nucleus. The cells of the tapetum are initially diploid but they become polyploid and multinucleate due to endomitosis and nuclear division respectively. • Tapetum absorbs food from the middle layers and provides nutrition to the microspore mother cells or developing microspores. The tapetum disappears in the mature anther. Epidermis Endothecium Middle layers Microspore mother cells Tapetum Connective Epidermis Endothecium Sporoganous tissue Tapetum Middle layers (A) T.S. of young anther (B) Enlarged view of one microsporangium
NEET : Biology [ 26 ] www.allendigital.in  Digital • Before degeneration of cells of tapetum, they form special granules called proubisch bodies in cytoplasm. Here they are surrounded by sporopollenin. Now they are called ubisch bodies or orbicules. At last tapetum degenerates and ubisch bodies are released into pollen sacs. • Ubisch bodies (sporopollenin) participate in the formation of outer covering (exine) of pollen grains. Functions of Tapetum :- (1) Tapetum provides nutrition to the Microspore mother cells (MMC) or Pollen mother cells (PMC) and Developing pollens. It nourishes the developing pollen grains. (2) Secretion of enzyme and hormone. (3) Secretion of sporopollenin. (4) Formation of ubisch bodies. (5) Secretion of pollen Kitt substances. (i) Microsporogenesis :- • Each microspore mother cell or cell of sporogenous tissue divides to form four haploid microspore or pollen grain by meiotic division or reduction division. The process of formation of microspores from pollen mother cell through meiosis is called microsporogenesis. • At the initial stage all four microspores are attached together with the help of callose layer. The microspore are arranged in a cluster of four cells - the microspore tetrad. After some time, this callose layer is dissolved by callase enzyme, which is secreted by tapetum. • During this period spherical bodies are formed inside the tapetal cells before their disintegration. These spherical bodies are known as Ubisch bodies or orbicules. Ubisch body is mainly made up of a complex substance sporopollenin. It is a polymer of carotenoids. • After the formation of ubisch body, the tapetum layer degenerates. Ubisch bodies participate in the formation of exine of the microspores inside the pollen sacs. Now thick-walled microspores are called pollen grains. • Inside each microsporangium thousands of microspores or pollen grains are formed that are released at the time of anther dehiscence. • All the above types of tetrads are found in Aristolochia elegans. • Most common type of tetrad is tetrahedral. Tetrahedral Deccusate T-Shaped (Dicots) Isobilateral Linear (Monocots) POLLEN TETRANDS OF ANGIOSPERMS A POLLEN GRAIN TETRAD