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Sexual Reproduction in Flowering Plants | 13 SËXÜÅL RËPRØDÜÇTÏØÑ ÏÑ FLØWËRÏÑG PLÅÑTS CHAPTER 02
14 | A Handbook of Biology All flowering plants (angiosperms) show sexual reproduction. Flowers are the sites of sexual reproduction. PRË-FËRTÏLÏSÅTÏØÑ: STRÜÇTÜRËS & ËVËÑTS Several hormonal and structural changes result in differentiation and development of the floral primordium. Inflorescences bear the floral buds and then the flowers. ÅÑDRØËÇÏÜM (STÅMËÑ) It consists of a whorl of stamens representing the male reproductive organ. Their number and length are variable in different species. Å STÅMËÑ HÅS 2 PÅRTS: Filament: Long and slender stalk. Its proximal end is attached to the thalamus or the petal of the flower. (NEET-I 2016) Anther: Terminal and typically bilobed. Each lobe has 2 thecae (dithecous), often a longitudinal groove runs lengthwise separating the theca. A B TRÅÑSVËRSË SËÇTÏØÑ ØF ÅÑTHËR: The anther is a tetragonal structure consisting of four chambers called microsporangia located at the corners. Each lobe consists of two microsporangia. Hence, the dithecous anthers are tetrasporangiate. The microsporangia develop to pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains. 1 2 3 STRÜÇTÜRË ØF Å MÏÇRØSPØRÅÑGÏÜM A typical microsporangium is nearly circular in outline. It is surrounded by four wall layers– the epidermis, endothecium, middle layers & tapetum. Line of dehiscence
Sexual Reproduction in Flowering Plants | 15 The outer 3 layers give protection and help in dehiscence of anther to release the pollen. The tapetum (innermost layer) nourishes the developing pollen grains. (NEET 2013) Cells of the tapetum contain dense cytoplasm and generally have more than one nucleus. When the anther is young, a group of compactly arranged homogenous cells (sporogenous tissue) occupies the centre of each microsporangium. MÏÇRØSPØRØGËÑËSÏS Epidermis Tapetum Sporogenous tissue Endothecium Epidermis Connective Microspore mother cells Middle layers Middle layers Endothecium Tapetum T.S. of young anther Microsporangium showing wall layers A mature dehisced anther 1 2 As the anther develops, each cell of sporogenous tissue undergo meiotic divisions to form microspore tetrads (microspores are arranged in a cluster of four cells). Each one is a potential pollen (microspore mother cell). The formation of microspores from a pollen mother cell through meiosis called microsporogenesis.
16 | A Handbook of Biology 3 4 As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. Each microsporangium contains thousands of pollen grains. They are released with the dehiscence of anther. PØLLËÑ GRÅÏÑ (MÅLË GÅMËTØPHÝTË) Generally spherical and measure up to generally 25-50 mm in diameter. Cytoplasm is surrounded by a plasma membrane. A pollen grain cell wall is made up of exine and intine. Exine: The hard outer layer. Made up of sporopollenin (highly resistant organic material). It can withstand high temperature and strong acids and alkali. (Karnataka NEET 2013) Enzymes cannot degrade sporopollenin. - Exine has apertures called germ pores where sporopollenin is absent - Pollen grains are preserved as fossils due to the presence of sporopollenin. Exine exhibits patterns and designs. Intine: The inner wall. It is a thin and continuous layer made up of cellulose and pectin. MÅTÜRËD PØLLËÑ GRÅÏÑ ÇØÑTÅÏÑS 2 ÇËLLS: 1. Vegetative Cell: It is bigger, has abundant food reserve and a large irregularly shaped nucleus. 2. Generative cell: It is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus. In over of 60% angiosperms, pollen grains are shed at the 2 - celled stage In others, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3- celled stage). n n 2n n n Pollen tube cell (Vegetative cell) Pollen wall Meiosis Mitosis Microscope mother cell Tetrad of microspores 2-celled stage of pollen grain Generative cell Mitosis of generative cell 3-celled stage of pollen grain Gametes