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Sexual Reproduction in Flowering Plants INTRODUCTION Plants are a boon to the whole mankind. They provide us with food, fruits, medicine fiber, textile for humans. Thus, these plants need to reproduce continuously so that the plants can be maintained on the earth and the balance of the ecosystem is maintained. Plants reproduce asexually and sexually. The propagules through which the plants reproduce are the leaves, roots, stem while the reproductive organ for sexual reproduction is the flower. FLOWER-THE REPRODUCTIVE ORGAN OF A PLANT Flowers have been of great importance to humans. It has aesthetic, ornamental, social, cultural and religious values. Flowers are structurally divided into the following different whorls: © Calyx-group of sepals, usually green in colour. © Corolla-group of petals, brightly coloured except green. s | Anther<—+_#\4—\—_ 2 Stamen| _. Filament¢————_-— Receptacle < Sepal Pedicel PARTS OF A FLOWER Stigma Fig. LONGITUDINAL SECTION OF A FLOWER Pistil EP ape Ppp pa cm TPs PRd ee prea 8 = | a a = 3 i £ = 3 3B 2 =] 3 2 a & @ =| x & Androecium-male reproductive part of a isting of the stamen having filament and anther. © Gynoecium-ferale reproductive part of a flower known as pistil or carpel consisting of stigrn: tyle and ovary. fé are many structural and hormonal flower con anges that take place in the formation of the flower. These changes lead to the modification of the floral primordium and thus the arrangement of the different whorls of the flower Sexual reproduction in flowering plants has been divided into the following events: Pre—Fertilisation: Structures and Events O Fert © Post-Fertilisation: Structures and Events ation Event PRE-FERTILISATION: STRUCTURES AND EVENTS STRUCTURE OF STAMEN Stamen consists of two parts Filament—Sterile part which connects the anther to the thalamus or pe Anther-Fertile part, wit! pollen grains. Anther is usually bilohed, each lobe has two s of the flower, which are formed the theca and thus known as dithecous. Each theca has two microsporangia (four in total) and tl a typical angiosperm anther is tetrasporangiat Each microsporangium develops and become the pollen sac that produces a large nu microspores known as pollen grains. ANTHER DEVELOPMENT Anther is parenchyrnatous in natu Hypodermal archesporial cells are differentiated, one in each angle. Each arehesporial call undergoes a peri 1 on to form outer primary parietal cell and inner prima sporogenous cell, The primary parietal cell undergoes divisions to form 3-5 layered anther wall. (Gray Matter Alert!!! Exceptions: Moringa and Wolfia-Each anther lobe has one microsporangium Arceuthobium-One microsporangium per anther. » Pollen gra Pollen sac Anther Filament FIG, THREE-DIMENSIONAL CUT SECTION OF AN ANTHER Definition fd Microsporangium: Structure formed in the anther of the flowering plants where the development and maturation of microspores into pollen grains take place. a 2 is a a. iT = Ls 2 ik £ c o P= o 3 3 6 a a i a = Pi Tapetum is the innermost hment to deve tapetum is of two types © Amooboid or periplasmodial or invasive: * cells break up releasing the t into the pollen chambe # All such protoplasts now fuse to form m. Micre plasr pe me get surrounded by periplasm Secrotory or glandular or parietal * Tape ils main throughout the sment of microspores and finally they degenerate. Sporogenous Tissue yenous tis cell divides te ess knowr form m L{PMC) ever mic in follow’ t and produce the 2. Microsporogene: Different events through which haploid microspores are formed in the microsporangium. Definition Archesporial call (2m) Sporogenous tissue (2n) | Microspor mother cell (2n) Psion (Giet . Micraspore (n) Tetrahedral — Isobilateral Decussate e2@6 8% FIG, ARRANGEMENT OF MICROSPORES Linear T-shaped Development of the Microspore or Microsporogenesis After meiosis, four haploid microspores a produced which are enclosed by the callose w During the process of development, the enzyme Callase degrades the wall between the microspores and the microspores arate to form the pollen grains. The pollen grain further d unequal cells. The larger cell is the vegetative cell that forms the pollen tube. The smaller cell forms the generative cell. The generative cell lies in the cytoplasm of the vegetative cell. The generative cell is elongated that helps it to move through the pollen tube The vegetative cell has most of the cytoplasm and is needed to provide nutrition to the pollen Brain. This pollen grain having the generative and vegetative cell is known as the two-celled stage. In some plants like the pollen grain is shed at two-celled stage. In other plant species like the divides by mitosis to form two mi sperms When the pollen grain has the vegetative cell and the two sperm cells, then, is known as three-celled stage. les and forms two enerative cell gametes or Previous Year's Question @ n an angiosperm, how many microspore mother cells are required to produce 100 pollen grains? (1) 75 (2) 100 (3) 25 (4) 50 Previous Year's Question '?) Which one is wrong? (1) Vegetative cell is larger than generative cell. (2) Intine is made of cellulose and pectin. (3) Pollen grains of some plants remain viable for months. (4) Double fertilisation is absent where pollen is shed in 2-celled stage Asymmetric spindle Generative cell FIG. STAGES OF A MICROSPORE MATURING INTO A POLLEN GRAIN $exual Reproduction in Flowering Plants. ANTHER DEHISCENCE The dehiscence of the anther occurs due to 18 the known as the the endothecium layer. The point endothecium layer breaks omium. Pollen grains at sed frorn the dehisced anther, FIG, A MATURE DEHISCED ANTHER APPLIED ASPECTS OF PALYNOLOGY The study of pollens has sped us to study the allergy caused by the p 1s, Airborne pollens cause allerg' ike hayfever, asthma, watery eyes running nose. It can be caus like Amaranthus, Parthen came into India as a contaminant with imported wheat. Pollens are used for making tonics and energy tablets. These tonics and tablets are used by athletes performance sd by some plants n, Parthenium was have certain useful properties and thus crea and race horses to respectively Pollens are present in the honey which provide it » amino acid, vitamins and minerals. 1 grains of some plants are viable for a few ks like In leguminous plants, the viability of the pollen ined for six months, To increase the he pollens are maintair at lower temperature i.e., 196°C in pollen banks. This is known as cryopreservation These stored poile b can be used later for plant ing to improve certain characteristics of the Previous Year's Question 2) Which one of the following statements is not true? (1) Pollen grains of many species cause severe allergies. (2) Stored pollen in liquid nitrogen can be used in the crop breeding programmes. (3) Tapetum helps in the dehiscence of anther (4) Exine of pollen grain is made up of Sporopollenin Gray Matter Alerti! Pollenkitt: it is formed by tapetal cells. It is pr nt as a coating over the pollen grain, It is made up of carotenoids or flavonoids, glycoprotein lipids and monosaccharides. Carotenoids and flavonoids give yellow or orange colour to the pollens, a?) Pollen tablets are available in the Previous Year Que: market for (1) in vitro fertilization (2) breeding programmes (3) supplementing food (4) ex situ conservation i & a ‘at & i hes my = a S BS ny e-5 a} & = & at a = & 3exual Reproduction in Flowering Plants » used if the plants face extinction ally endang cium a © Stigma-Landing surface for the pollens ) Style-P; ) Ovary-Embryo sac with the female » for the pollen tube to the can be unicarpellary or Unicarpellary -Having Multicarpellary- |t further divided into apocarpous or Syncarpous ¢ Apocarpous- Multicarpellary condition in which carpels are not fused together t ¢ Syncarpous- Multicarpellary condi h the ¢ Example: Mic in wt sls are fused toget Example: Pag Structure of the Ovule The ovule is attached to with the help of a stalk k An ovule con af the nui nta in the ary y s funicle. @ protected by one or two coats called integuments. 4 small opening is left at one end, known as micropyle. the pore s absorber through which water e@ ovule where funiculus is chalazal end. Note: Ovule in which integurnen rit is called Ategmic ovule, e. Santalum. + — | Apocarpous: Multicarpellary condition in which the carpels Definition are not fused together. Syncarpous: Multicarpellary condition in which the carpels are fused together. syn ovary « Fig. FIG MULTICARPELLARY, =MULTICARPELLARY, APOCARPOUS SYNCARPOUS GyNoecium PISTILOFPAPAvER OF MICHELIA Chalaza Integuments ex Embryo Nucellus « Micropyle Funiele € FIG. ANATROPOUS OVULE Sexual Reproduction in Flowering Plants cells at the chalazal end known as the antipodal cells. They degenerate before or soon after the fertilisation Three cells are formed at the micropylar end. Two of them are known as synergids. In between the synergids is present the egg or male gamete and together the egg and the synergids are known as egg apparatus. The epe apparatus ched to the walls of the embryo sac at the micropylar end. The synergids at their micropylar end have projections known as the filiform apparatus. One of the synergids degenerates before the entry of pollen tube into the embryo sac and the other one degenerates after the embryo sac has received the pollen tube. Synergid secretes chemicals which attract the pollen tube towards the embryo sac. The degenerating synergid forms the seat for pollen tube discharge. The central cell consists of two nuclei. The cells of the antipodals and the egg apparatus are haploid while the central cell is diploid. The most common type of embryo sac (usually found in 41% of the plants) is the Polygonum type. it develops from the chalazal megaspore while the three megaspores towards the micropylar end degenerates. Thus it is known to show monosporic development as it develops from a single functional megaspore. Gray Matter Alert!!! Oenothera type Embryo Sac: This type of an embryo sac is derived from the micropylar megaspore of the tetrad. It consists of the eff apparatus and a single central nucleus. Antipodal cel are absent, Nea apparatus Previous Year's Question '?) Functional megaspore in an angiosperm develops into an (1) endosperm (2) embryo sac (3) embryo (4) ovule Synergids: Central cell <— Polar nuclei Antipadals <4 FIG, DEVELOPMENT OF EMBRYO SAC FROM FUNCTIONAL MEGASPORE Chalazal end Antipodals Central cell Polar nuclel Ege > Synergids ——> Filiform Mictopylarend apparatus 10. POLLINATION Definition é3 * The process of transfer of pollen gra from the anther of a flower to the stigma of another flower ation * They are of the following type Self-Pollination: Transfer of pollen grains fromm the anther to the stigma of the same flower or flower on the same plant. ( POLLINATION 9 is known as * Self-pollination is further divided into two types: © Autogamy Geitonogamy Autogamy Geitonogamy Same flower Same plant different flower Autogamy * The transfer of poll the stigma of the same flow * Example: Orchid grains from the anther to Geitonogamy * The transfer of pollen grains from the her to the stigma of a different flower on the same plant. * Example: Maize Cross-pollination or Xenogamy * itis the transfer of pollen grains from the of a flower to the st of another flower on ry another plant of the same or different species. FIG, SELF POLLINATION | Sexual Reproduction in Flowering Plants Chasmogamous flowers-These are the =< flowers which have well-exposed reproducti Definition parts They can undergo both self o Cross-Pollination: Transfer of cross-pollination pollen grains from the anther to the stigmaof another flower on Agents of pollination different plants of the same or * There are two main agent different species. © Abiotic Biotic * Abiotic-The a its like air and water are abiotic agents that help in pollination Sed Anemophily Anemophily (aN oF ‘ f nation of pl Hydrophily acteris The following are the chi ‘watter) and stigma of plants pollinated by air * Flowers are small and are usually borne in Abiotic pondant catkins that sway to release the pollens ( in the air. Exar Mulberry and Hazel Pollinatng ans * Pollen grains are i oth and dry so that ¢ 1ces by the a el F Biotic maturity the file the anthers beyond the floral parts * The stigma of these flo 8 feathery and large. Corrente * OQvules duced Mego! * If the f are ‘, then the cement of —— the flower help in pollination. Example: In maize, tae the male flowers known { while the fem T (ange mre aot? rally at the lower ends le Hydrophily . of © by water is ar hydrophily. The following are the characteris of pollens and stigma of a plant pollinated by ater lation is Ccom@© . t grown on the surface are (2) lilies d by wind or by certain (4) orchids 4 & a ‘at [3 a rs © is S 6 oO = 5 g a oo 3 o = 8B 
