Class-XII Biology PDF

BUREAU’S HIGHER SECONDARY Class-XII Prescribed by Council of Higher Secondary Education, Odisha, Bhubaneswar BOARD OF WRITERS Dr. Manoranjan Kar...

BUREAU’S HIGHER SECONDARY Class-XII Prescribed by Council of Higher Secondary Education, Odisha, Bhubaneswar BOARD OF WRITERS Dr. Manoranjan Kar Dr. Manas Ranjan Satapathy Formerly Professor in Botany, Lecturer in Botany, Utkal Univeristy, Vani Vihar, Bhubaneswar Dhenkanal (Autonomous) College, Dhenkanal Dr. Baman Chandra Acharya Dr. Subas Chandra Das Formerly Professor in Botany, Formerly Principal, Khalikote (Autonomous) College, Berhampur Sundergarh Women's College, Sundergarh Dr. Basanta Kumar Choudhury Dr. Pradeep Kumar Mohanty Formerly Reader in Botany, Presently, Editor, 'Bigyan Associate Professor in Zoology, Diganta', Odisha Bigyan Academy, Bhubaneswar Dhenkanal (Autonomous) College, Dhenkanal Dr. Ajay Kumar Acharya Dr. Jaya Krushna Panigrahi Reader in Botany, Swami Vivekananda Memorial Reader in Zoology, Sri Jayadev College of Education & (Autonomous) College, Jagatsinghpur Technology, Bhubaneswar Shri Uday Shankar Acharya Dr. Bairagi Charan Behera Associate Professor in Botany, Reader in Zoology, Samanta Chandra Sekhar (Autonomous) College, Puri Kendrapara (Autonomous) College, Kendrapara Dr. Hrushikesh Nayak Dr. Maheswar Behera Lecturer in Botany, Associate Professor in Zoology, Bhadrak (Autonomous) College, Bhadrak Samanta Chandra Sekhar (Autonomous) College, Puri BOARD OF REVIWERS Dr. Manoranjan Kar Dr. Basanta Kumar Choudhury Formerly Professor in Botany Formerly Reader in Botany, Presently, Editor, 'Bigyan Utkal Univeristy, Vani Vihar, Bhubaneswar Diganta', Odisha Bigyan Academy, Bhubaneswar Dr. Baman Chandra Acharya Dr. Subas Chandra Das Formerly Professor in Botany Formerly Principal, Khalikote (Autonomous) College, Berhampur Sundergarh Women's College, Sundergarh Dr. Pradeep Kumar Mohanty Associate Professor in Zoology Dhenkanal (Autonomous) College, Dhenkanal ODISHA STATE BUREAU OF TEXTBOOK PREPARATION AND PRODUCTION Pustak Bhavan, Bhubaneswar Published by : THE ODISHA STATE BUREAU OF TEXTBOOK PREPARATION AND PRODUCTION Pustak Bhavan, Bhubaneswar, Odisha, India First Edition : 2017 / 5000 Publication No. : 206 ISBN : 978-81-8005-400-6 © Reserved by the Odisha State Bureau of Textbook Preparation and Production, Bhubaneswar. No part of this publication may be reproduced in any form without the prior written permission of the publisher. Type Setting, Design & Printing : PRINT-TECH OFFSET PVT. LTD., Bhubaneswar Price : ` 205/- (Rupees Two Hundred Five Only)  FOREWORD In our continuous effort to evolve and to keep pace with this fast changing world, this Biology book for Class-XII students has been written. The book comprises of 5 units and there are 17 chapters wherein certain important aspects of Biological Science have been incorporated. As I find, the chapters include day to day affairs of the modern man. It will not only benefit our students to face the challenges of different types of examination but in the long run, help them to become complete citizens, also. The Council of Higher Secondary Education (CHSE), Odisha has implemented the Biology syllabus in line with National Council of Education, Research and Training, New Delhi from the session 2016-17, so as to integrate our students with national mainstream. This part, meant for Class-XII students are again, very important since our students have to appear at the Council Examination with the help of this textbook. This unique work has been done by the board of writers in Biology and were reviewed rigorously by the board of reviewers. Finally, the proofs have been meticulously read and recast by Dr. Basanta Kumar Choudhury and Dr. Pradip Kumar Mohanty. I sincerely thank all of them. This book could see light of the day because of their efforts only. My sincere thanks to Government of Odisha for its interest in the publication of this textbook. I also appreciate CHSE, Odisha for selecting us for the preparation and publication of the book. The Bureau will humbly appreciate the constructive comments and suggestions to improve the standard of the book. Sri Umakanta Tripathy Director Odisha State Bureau of Textbook Preparation and Production, Bhubaneswar  PREFACE Bioengineering expert, Catherine J. Paul, of Lund University, Sweden has the following observation on Biological Science, "a previously completely unknown ecosystem has revealed itself to us. Formerly, you could hardly see any bacteria at all and now, thanks to such as massive DNA sequencing and flow cytometry, we suddenly see 80 thousand bacteria per ml of drinking water". We know that the modern biological science which had a humble beginning at the turn of last century by rediscovery of Mendelism made a breakthrough with deciphering the structure of the wonder molecule, DNA. After that landmark event, the science of biology has made giant strides in many spheres last six or seven decades. We tamed the nature and exploited its natural resources beyond its replenishment level. We knew more and more about our anatomy, physiology etc. in order to live a sophisticated life. In the process, we got detached from nature. Our evolution and improvement in the quality of life had share of ill effects. A student of Biology should know all these so as live a life in harmony with nature. Class-XII Biology syllabus has been framed in such a way by National Council of Education, Research and Training (NCERT), New Delhi and corroborated by Council of Higher Secondary Education (CHSE), Odisha that all the discussed ingredients have been incorporated in it. The writers and reviewers in Biology here made an humble attempt to deal all the chapters so that it can be clearly understood. They hope it will cater to the needs of our students as well as teaching community. Keeping an eye on the requirements of our students certain chapters have been conveniently elaborated. It is now open to the students and teachers in Biology to go through all the chapters and provide their valuable constructive feedback. The boards never claim the write up is original one. It has been taken from certain primary and secondary sources. The writers and reviewers acknowledge their deep sense of gratitude to Odisha State Bureau of Textbook Preparation and Production, CHSE, Odisha and Department of School and Mass Education, Government of Odisha for relying on the same team to write the Class-XII Biology book. All the members of both the boards unequivocally thank Sri Biraj Bhushan Mohanty, Deputy Director of the Bureau for his unceasing efforts and inspiration. Lastly, they thank Sri Ashok Kumar Ojha, DTP Operator, M/s. Print-tech Offset Pvt. Ltd., Bhubaneswar, Odisha for undertaking the arduous task of typing and designing all text materials of the book. Srigundicha, 25.06.2017 Writers & Reviewers  BIOLOGY SYLLABUS CLASS - XII (Science) (Theory) I. REPRODUCTION (Periods - 22) (a) Reproduction in organisms : Reproduction, a characteristic feature of all organisms for continuation of species; Modes of reproduction - Asexual and sexual; Asexual reproduction; Modes- Binary fission, sporulation, budding, gemmule formation, fragmentation; vegetative propagation in plants. Sexual reproduction in flowering plants : Flower structure; Development of male and female gametophytes; Pollination-types, agencies and examples; Outbreeding devices; Pollen-Pistil interaction; Double fertilization; Post fertilization events- Development of endosperm and embryo, Development of seed and formation of fruit; Special modes- apomixis, parthenocarpy, polyembryony; Significance of seed and fruit formation. (b) Human Reproduction : Male and female reproductive systems; Microscopic anatomy of testis and ovary; Gametogenesis- spermatogenesis and oogenesis; Menstrual cycle; Fertilization, embryo development upto blastocyst formation, implantation; Pregnancy and placenta formation (Elementary idea); Parturition (Elementary idea); Lactation (Elemntary idea). Reproductive health : Need for reproductive health and prevention of sexually transmitted diseases (STD); Birth control- Need and Methods, Contraception and Medical Termination of Pregnancy (MTP); Amniocentesis; Infertility and assisted reproductive technologies - IVF, ZIFT, GIFT (Elementary idea for general awareness). II. GENETICS AND EVOLUTION (Periods - 20) (a) Heredity and Variation : Mendelian Inheritance; Deviations from Mendelism-Incomplete dominane, Co-dominance, Multiple alleles and Inheritance of blood groups, Pleiotropy; Elementary idea of polygenic inheritance; Chromosome theory of inheritance; Chromosomes and genes; Linkage and crossing over.  (b) Sex determination : In humans, birds, honey bee; Sex linked inheritance- Haemophilia, Colour blindness; Mendelian disorders in humans- Thalasemia; Chromosomal disorders in humans- Down’s syndrome, Turner’s and Klinefelter’s syndromes. (c) Molecular Basis of Inheritance : Search for genetic material and DNA as genetic material; Structure of DNA and RNA; DNA packaging; DNA replication; Central dogma; Transcription, Genetic code, Translation; Gene expression and regulation- Lac Operon; Genome and human genome project; DNA finger printing. (d) Evolution : Origin of life; Biological evolution and evidences for biological evolution (Paleontological, comparative anatomy, embryology and molecular evidence); Darwinism, Modern Synthetic theory of Evolution; Mechanism of evolution- Variation (Mutation and Recombination) and Natural Selection with examples, types of natural selection; Gene flow and genetic drift; Hardy-Weinberg’s principle; Adaptive Radiation; Human evolution (in brief). III. BIOLOGY AND HUMAN WELFARE (Periods - 08) (a) Health and Disease: Pathogens; parasites causing human diseases (Malaria, Filariasis, Ascariasis, Typhoid, Pneumonia, common cold, amoebiasis, ring worm); Basic concepts of immunology- vaccines; Cancer, HIV and AIDS; Adolescence, drug and alcohol abuse. (b) Improvement in food production: (i) Plant breeding, tissue culture, single cell protein, Biofortification; (ii) Apiculture and Animal husbandry. (c) Microbes in human welfare : In household food processing, industrial production, sewage treatment, energy generation and as biocontrol agents and biofertilizers. IV. BIOTECHNOLOGY AND ITS APPLICATIONS (Periods - 08) (a) Principles and process of Biotechnology : Genetic engineering (Recombinant DNA technology). (b) Application of Biotechnology in health and agriculture : Human insulin and vaccine production, gene therapy; Genetically modified organisms- Bt crops; Transgenic Animals; Biosafety issues- Biopiracy and patents. V. ECOLOGY AND ENVIRONMENT (Periods 12) (a) Organisms and environemnt : Habitat and niche; Population and ecological adaptations; population interactions-mutualism, competition, predation, parasitism; Population attributes-growth, birth rate and death rate, age distribution. (b) Ecosystems : Patterns, components; productivity and decomposition; Energy flow; Pyramids of number, biomass, energy; Nutrient cycling (carbon and phosphorous); Ecological succession; Ecological Services- Carbon fixation; pollination, oxygen release. (c) Biodiversity and its conservation : Concept of Biodiversity; Patterns of Biodiversity; Importance of Biodiversity; Loss of Biodiversity, conservation; Hotspots, endangered organisms, extinction, Red Data Book: Biosphere reserves, National parks and Sanctuaries. (d) Environmental issues : Air pollution and its control; Water pollution and its control; agrochemicals and their effects; Solid waste management; Radioactive waste management; Greenhouse effect and global warming; Ozone depletion; Deforestation; Any three case studies as success stories addressing environmental issues. N.B. : Ia, II a, c; III b (i), c and v units are to be taught by Botany Faculty. I b; II b; III a, b(ii); IV units are to be taught by Zoology Faculty.) –––––––  QUESTION PATTERN AND DISTRIBUTION OF MARKS BIOLOGY (Theory) Class - XII (Science) SECTION - A (BOTANY) Time : 1.5 hours Full Marks : 35 Group A (Objective Type - Compulsory) Q1. Multiple choice / one word answer : 1 mark each x 5 = 5 marks Q2. Correct the sentences / Fill up the blanks : 1 mark each x 5 = 5 marks Group B (Short Answer Type) Q3. Answer within three sentences : 2.5 marks each x 3 = 7.5 marks Q4. Difference between (3 important differences) (1 bit to be answered out of 3 bits) : 3.5 marks = 3.5 marks Group C (Long Answer Type) Answer two questions out of four : 7 marks x 2 = 14 marks SECTION - B (ZOOLOGY) Time : 1.5 hours Full Marks : 35 Group - A (Objective Type-Compulsory) Q1. Multiple choice / one word answer : 1 mark each x 5 = 5 marks Q2. Correct the sentences / Fill up the blanks : 1 marks each x 5 = 5 marks Group - B (Short Answer Type) Q3. Answer within three sentences : 2.5 marks each x 3 = 7.5 marks (3 bits to be answered out of 6 bits) Q4. Difference between (3 important differences) (1 bit to be answered out of 3 bits) : 3.5 marks = 3.5 marks Group - C (Long Answer Type) Answer two questions out of four : 7 marks x 2 = 14 marks N.B : Long answer type questions are to be set only from the portions underlined in the syllabus. –––––––  CONTENT UNIT - I REPRODUCTION Pages Chapter-1 REPRODUCTION IN ORGANISMS 1 - 12 Chapter-2 SEXUAL REPRODUCTION IN FLOWERING PLANTS 13 - 68 Chapter-3 HUMAN REPRODUCTION 69 - 98 Chapter-4 REPRODUCTIVE HEALTH 99 - 123 UNIT - II GENETIC AND EVOLUTION Chapter-5 HEREDITY AND VARIATION 124 - 151 Chapter-6 SEX DETERMINATION 152 - 170 Chapter-7 MOLECULAR BASIS OF INHERITANCE 171 - 210 Chapter-8 ORGANIC EVOLUTION 211 - 249 UNIT - III BIOLOGY AND HUMAN WELFARE Chapter-9 HEALTH AND DISEASE 250 - 292 Chapter-10 IMPROVEMENT IN FOOD PRODUCTION 293 - 323 Chapter-11 MICROBES IN HUMAN WELFARE 324 - 335 UNIT - IV BIOTECHNOLOGY AND ITS APPLICATIONS Chapter-12 PRINCIPLES AND PROCESS OF BIOTECHNOLOGY 336 - 356 Chapter-13 APPLICATIONS OF BIOTECHNOLOGY 357- 376 UNIT - V ECOLOGY AND ENVIRONMENT Chapter-14 ORGANISMS AND ENVIRONMENT 377 - 403 Chapter-15 ECOSYSTEMS 404 - 425 Chapter-16 BIODIVERSITY AND ITS CONSERVATION 426 - 441 Chapter-17 ENVIRONMENTAL ISSUES 442 - 473 _______ UNIT - I : REPRODUCTION CHAPTER REPRODUCTION IN ORGANISMS 1 Till date not other planet in the solar system except the Earth is known to sustain life. Hundreds and thousands of living organisms abound every conceivable space in this planet and they form the biospere. The organisms in their size, shape, structure, function, habit and habitat etc. vary extensively. Yet they consistently exhibit a unique character, i.e. from a living being another living organism is produced. As a result of this, the constancy and continuity of the said species is maintained. This is called the phenomenon of reproduction. In other words, reproduction is said to be a bilogical process of perennation and continuation of a species from generation to generation. There is a great diversity in the living world in their method of reproduction. It is typical to the organism concerned. Factors like environment, physiology, habitat etc. of the organism influence simultaneously its mode of reproduction. Based on the type of reproductive unit, the reproduction process may be sexual or asexual. In the sexual reproduction, reproductive units called gametes which take part to produce zygote. The gametes when morphologically and physiologically similar are called isogametes. When differences occur between two fusing gametes, it may be called anisogametes or oogametes. The former one is the process of isogamy and the later, termed as heterogamy. In the cases, where such gametes are not developed in offspring production, it is called asexual reproduction. 1.1 ASEXUAL REPRODUCTION : In the asexual method of reproduction, the reproductive units are called spores which may be motile or nonmotile. Sometimes, the vegetative structures of the organisms may get segregated accidentally and grow into a new indivioluals. Later method may be called as vegetative propagation. Here, unlike to asexual or sexual method, no specialized reproductive units are formed for the production of progenies. Asexual reproduction is very common among lower plants, animals and particularly in the kingdoms of Monera, Protista and Fungi. In these cases offsprings are produced by the processes called fragmentation, fission, budding, gemmule formation, sporulation etc. 1.1.1 Fragmentation (Fig. 1.1) : This type of vegetative propagation occurs in most of the lower plants belonging to algae, fungi and bryophytes as well as many highly developed angiosperms. Among the animals 2 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) also such types of reproduction by fragmentation occur in lower animals like sponges, Hydra, etc. This is a process of accidental breakage of vegetative cells and development of the fragmented or broken parts into new organisms. The accidental breakage may occur due to wind, water current or injury caused by animals while feeding. In economically important horticultural or crop plants, human being uses this method of segregating plant parts for his own benefit. Fig.1.1 : Fragmentation in Spirogyra (an alga) 1.1.2 Fission : Bacteria, certain unicellular algae and such types of organisms propagate by the method of cell division. In this process, the nucleus and cytoplasm grow and divide into two euql parts. This is called binary fission. It is the predominating form of reproduction in bacteria (Fig.1.2) Fig.1.2 : Fission in Bacteria Other organism like Amoeba, Paramaecium and Euglena also show binary fession. In favourable conditions an Amoeba enlarges in size and withdraws its pseudopodia. It gets ready for cell division (Fig. 1.3). As a result of mitosis, two cells are Fig.1.3 : Fission in Amoeba produced which are capable of growing into independent organisms. Besides, binary fission, cells of certain algae like Stanieria divide into more than 2 cells and each part can develop to form new organisms (Fig. 1.4). This is called multiple fission. Under unfavourable conditions like drying of water reservoir or condensation of Fig.1.4 : Multiple fission in Stanieria (an alga) water into ice, Amoeba shows a different type Reproduction in Organisms y 3 Fig.1.5 : Multiple fission in Amoeba of fission. It withdraws its pseudopodia and encloses itself by 2 to 3 layered strong envelope. The process is called encystment. During the period, the metabolism of Amoeba is reduced to the minimum. But its nucleus divides repeatedly to form a large number of nuclei. Each nucleus remains surrounded by some amount of cytoplasm. When the suitable conditions sets in the cyst disintegrates and premature daughter Amoeba called Amoebula get released. This is an example of multiple fussion (Fig. 1.5). It is also called sporulation. Malaria causing Plasmodium also propagates by multiple fission. 1.1.3 Budding : The unicellular fungus, yeast (Saccharomyces) generally propagates by this method. Outgrowths develop at the peripheral region of the mother cell which are called buds. Into this, nucleus along with cytoplasm from the mother cell migrate. This causes daughter cells to enlarge in size. It remains attached to the mother cell by a narrow neck. Gradually, it loses contact from mother cell and grows into a new organism (Fig. 1.6). In the process of fission, the nuclear material gets segregated in equal amounts in the daughter cell, but this may or may not happen in the case of budding. Fig.1.6 : Yeast Budding Some other unicellular organisms and also muticellular ones like Sponge and Hydra reproduce by budding. These are of two types- (a) external buds, (b) internal buds. 4 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) (a) External budding : This occurs under favourable conditions when environmental conditions are suitable and enough nutirents are available. Initially, tiny swellings called buds develop in the median region of Hydra body. This gradually enlarges and coelenteron of Hydra migrates into it. At the Fig.1.7 : External Budding in Hydra. terminal region of bud mouth tentacles develop. This is the Hydra in its infant stage which when separated from mother cell develop into new Hydra (Fig. 1.7). (b) Internal budding : When conditions become unsuitable and there is scarcity of the nutrient supply, sponge forms internal buds called gemmules. In the process, special types of cells called archaeocytes get aggregated. A hard coat is formed around it and the Fig.1.8 : Internal Budding in Sponges. structure is now called gemmule (Fig. 1.8). It remains open outside by samll pore called micropyle. When suitable conditions again, sets in, the gemmules come out through micropyle and new sponges are formed. 1.1.4. Sporulation : This is a very general process of reproduction in organisms like algae and fungi. Almost all algae, produce motile asexual reproductive units called zoospores (Fig. 1.9). Fig.1.9 : Zoospore formation This can float freely in water with its cilia or Reproduction in Organisms y 5 flagella. Example: (Ulothrix). The zoospores endogenously borne inside the structures known as sporangia (Fig. 1.10) and are liberated from it when become mature. Lower fungi like Saprolegnia reproduce by zoospores (Fig. 1.11, a) but higher fungi like Aspergillus, Penicillium possess nonmotile structures called conidia (Fig. 1.11, b) These are exogenously borne and can be easily disseminated by external agent like Fig.1.10 : Sporangium with sporangiophore air or water. Fig.1.11 (a, b) : Saprolegnia spores, (c) Penicillium conidia 1.2. VEGETATIVE PROGAGATION IN PLANTS : As described earlier no specialized reproductive units are produced in the process of vegetative propagation. When the plant is growing vegetatively, any part of it, say root, stem or leaf may get separated from this plant. If this separated part can grow into new individuals their it is called vegetative propagation. Several types of vegetative propagation are seen among the flowering plants depending on part involved in the process. This may occur under normal conditions or may be induced artificially. 1.2.1. By roots : Some tap or adventitious roots of sweet potato, Dahlia etc. become thick, swollen due to storage of food (Fig-1.12). The adventitious buds are borne on such structures. The buds produce leafy shoots, called slips. When such roots with adventitious buds are planted in the soil, they produce new plants and thus, vegetative propagation occurs. 6 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) 1.2.2 By stems : The stems are efficient means of vegetative propagation. This may be of the following two types. (a) Subaerial stems : Subaerial stems may develop as lateral branches from the mother plant. This may break up from the parent plant and then, grow into new plants. (Fig.1.13) Example- Runners (Oxalis), sucker - (banana, Chrysanthemum), stolon (Jasmine), offset (Eichhornia) A B C Fig.1.12 : A. Root Tuber of Sweet potato, B. Fasciculated root of Dahlia, C. Nodulose roots of mango ginger A B Runner Fig.1.13 : Vegetative propagation by stem modifications : A. Runner, B. Stolon. Reproduction in Organisms y 7 C D Fig.1.13 : Vegetative reproduction by stem modifications : C. Sucker, D. Offset (b) Underground Stems : In certain plants (Fig. 1.14) the underground stems become modified for storage of food during the active phase of the growth. Examples- Rhizome (Ginger), tuber (Potato), bulb (Onion) and corm (colocasia) 1.2.3 By leaves : The fleshy succulent leaves of Bryophyllum (Fig.1.15) bear adventitious buds in their notches located in the margins. When the leaves fall on moist soil, these buds develop into small plants completing the process of vegetative propagation. 1.3 ARTIFICIAL METHODS : Farmers, gardeners, horticulturists have taken advantage of this type of propagation in plants. They have manipulated the process for their own benefit. 13.1. Cuttings : This is a very common method. Here a piece of stem up to a suitable length is taken from the parent plant. This stem piece is called the cutting. It should have few nodes and internodes. The cutting is planted in moist soil with suitable nutrients. After sometime, roots emerge from the nodes of the basal portions of the cuttings and the upper buds give rise to the shoot. The plants of China rose, sugar cane, Bougainvillea etc are commonly grown by this method. 1.3.2. Grafting : In this process, a detached part of one plant (i.e. twig or bud) is inserted into the stem or root system of another plant (Fig 1.16). The former is called scion (short piece of detached shoot containing several dormant buds) and the latter stock (lower portion of the plant which is fixed to the soil by its root system). The grafted portion is covered by grafting wax to avoid 8 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) infection. The scion becomes part of the plant into which it is grafted. The new plant developed bears flowers and fruits, characteristic of scion. Mango, rose, orange, guava etc. are generally propagated by graftings. 1.3.3 Layering : Here, roots are artificially induced to grow on the branches before they are detached from the parent plant. There are three types of layering : (i) Serpentine layering : Branches at the lower portions of the stem are put in the soil at many places to form new plants from them. A B Fig.1.14 : Underground stem modifications : A. Bulb, B. Corm for vegetative propagation. Adventitious buds Fig.1.15 : Reproduction by leaf Fig.1.16 : Grafting by stem cutting (A-C) different stages. Reproduction in Organisms y 9 (ii) Mound layering : Soft lower branches are selected and a ring of epidermal layer is removed. This part is then pegged in the soil with the apical portion remaining outside. After an interval of time, adventitions roots develop. Then the ringed portion is cut off to allow new plant grow independently. (iii) Air layering : Here, a ring of bark is removed from the aerial branches. It is then covered by grafting clay (water, clay, cow dung) with a little amount of root inducing promoter. The entire portion is wrapped with polythene bandage. At a particular time interval, roots are developed and when separated it can grow into a new plant. Example - Pomegranate, orange, lemon etc. Significance : This is a quick method of reproduction where survival rate of the progenies is very high. Endangered or threatened plant species can be saved by such propagations. Plants reproducing vegetatively take a short time to mature. Potato for example, takes three months to mature. Plants with desirable qualities may be developed by this method. The vegetative method of reproduction among angiosperms has a lot of agricultural and horticultural applications. 1.4 MICROPROPAGATION : This process is similar to rooting of plant cuttings and is, in a way, another method of vegetative propagation of plants. However, it differs from the conventional procedure since it is carried out in aseptic condition and requires an artificial nutrient medium. A small plant cutting or explant (usually axillary bud) is sterilized and inoculated into culture vessel containing semi-solid nutrient medium. The inoculated culture vessel is incubated at room temperature. In a short span of time, a large number of shoots develop from the axillary buds through a process called axillary bud proliferation. Each growing point is then subcultured to give rise to shoot. This phenomenon is called adventitious shoot formation. Each shoot is stimulated by auxin to develop roots (Fig. 1.17 A,B). The new plantlet or propagule is then transferred to field. This method is generally practised for ornamental, fruit and crop plants. This is useful because (i) the healthy propagules can only be obtained (pathogen free) (ii) rapid rates of multiplication can be ensured (iii) Development of plant materials with desired traits and their maintenance in a small space can be done. Examples : Substantial benefits can be expected to occur in the production of crops such as tea, coffee, oil palm, date palm, coconut, fruit yielding plants like papaya, banana, Citrus and apples. Significant progress has also been achieved in developing protocols of micropropagation of tree species. Mass propagation, in vitro, of teak, Eucalyptus spp., sandal wood, different species of bamboo and many other trees has been successfully done. Similarly, considerable progress has been made in commercial harvesting of medicinal plants such as Dioscorea deltoidea, D. floribunda, Atropa belladona, Solanum Spp., Rauwolfia serpentina etc. by microprogation techniques. 10 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) A B Fig.1.17 : A. Micropropagation of meristematic plant shoot (axillary bud) cutting through plant tissue culture technique; B. Development of shoot and root, one following the other, through plant tissue culture technique (organogenesis). Some species of potato, Cassava, sugarcane and banana are severely and chronically affected by viruses. Yields of these crops can be increased significantly by planting disease free stocks. Potato is of most important and widely grown food crop in the world. But it is susceptible to many viral pathogens, some of which may be present without perceptible symptoms. The pathogens cause gradual decrease in vigour and yield of potatoes. Eradication of viruses can be achieved by employing micropropagation techniques where healthy meristematic buds are cultured. More than 500 plants can be obtained in about three to four months starting from a single meristem. By manipulating the medium composition, light intensity and temperature, plantlets can be induced to produce microtubers. These disease free microtubers can be grown under controlled conditions in soil to form minitubers. The minitubers can be planted directly in the field to raise a disease free crop. Apart from the application of micropropagation techniques for generating true-to- type planting material from elite genotypes, micropropagation holds special significance in situations where rapid bulking of extremely limited stock material is required. The desired genetic gains achieved through plant breeding can be multiplied several fold on an economic and rapid time scale. ______ Reproduction in Organisms y 11 SAMPLE QUESTIONS GROUP - A (Objective-type Questions) 1. Fill in the balnks with correct answers from the choices given bracket : (i) Nonmotile asexulal reproductive units are called ________. (Zoospores, Buds, gametes, conidia) (ii) In ________, a living organisms divide equationally. (fragmentation, fission, budding, sporulation) (iii) Yeast, generally, reproduces by ________. (fission, budding, sporulation, gametangia) (iv) Dahlia propagates by ________. (roots, stem, leaf, seed) (v) The process by which one plant part is inserted into another to grow a new individual plant is called ________. (layering, cutting, grafting, micropropagation) 2. Anwser in one word only : (i) What is called the motile asexual reproduction units? (ii) In which asexual method do yeasts generally divide? (iii) What can be called to sexual reproductive units? (iv) What is the general asexual method of reproduction in Amoeba? (v) In the process of grafting, what is called to detached part? (vi) In which process can large number of adventitious buds be formed? 3. Correct the statements without changing underlined words only : (i) In mound layering, branches at lower portion of the stem are put in the soil at many places. (ii) Dahlia reproduces vegetatively by stems. (iii) Aspergillus reproduces asexually by zoospores. (iv) Internal buds in sponges are called gemma cups. (v) In binary fission, many cells can be produced from one cell. 4. Fill in the balnks : (i) The process of perennation of species takes place by ________. (ii) Zoospores are borne inside ________. 12 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) (iii) Under unfavourable conditions when a number of tiny Amoeba are produced by multiple fission, it is called ________. (iv) Internal buds in Hydra are called ________. (v) In Bryophyllum, adeventitious buds are borned on ________ GROUP - B (Short Answer-type Questions) 1. Write notes on the following with at least 2 valid points : (i) Asexual reproduction (ii) Micropropagation (iii) Cutting (iv) Layering (v) Fission (vi) Budding (vii) Fragmentation (viii) Micropropagation 2. Differentiate between the following with at least 3 valid points : (i) Zoospores and condia. (ii) Asexual reproduction and Sexual reproduction (iii) Grafting and Layering (iv) Budding and Fission. (v) Internal budding and Enternal budding. (vi) Fragmentation and budding. GROUP - C (Long Answer-type Questions) 1. Give and account of vegetative reproduction in angiosperms 2. Describe the process of micropropagation and its advantages. 3. Describe asexual reproduction process in lower animals. 4. Describe the asexual reproduction process in lower plants. qqq  SEXUAL REPRODUCTION IN CHAPTER FLOWERING PLANTS 2 2.1 INTRODUCTION : Sexual reproduction is a natural phenomenon among the angiosperms. The flowers bear sex organs (stamens and carpels) or reproductive organs which finally produce haploid male and female gametes. These haploid male and female gametes fuse to produce a diploid zygote that develops into a new diploid plant body, the sporophyte. The flowering plants like other living organisms have two distinct phases in their life cycle, such as; a diploid sporophytic phase and a haploid gametophytic phase. Both the phases alternate with each other. The flowering plants, as we see them from outside are the sporophytes which bear reproductive structures in flowers. The flower performs very important role in the life cycle of Angiosperms as various steps of sexual reproduction occur in such structures. 2.2 STEPS IN SEXUAL REPRODUCTION : 2.2.1 Development that leads to formation of haploid male gametes : 1. Formation of microsporangia in the anther of stamen. 2. Formation of haploid microspores called microsporogenesis which involves meiosis in the microspore mother cells developed within the anther. The haploid microspore or pollen grain is the first cell of the male gametophyte. 3. Pollination - The microspores once released from the anther are dispersed in various ways to reach the stigma of the carpel. 4. Development of male gametophyte in the pollengrain that produces two male gametes (sperms). 2.2.2 Development that leads to formation of haploid female gamete : 1. Development of ovule (megasporangium) and female gametophyte within it. 2. Formation of megaspores (megasporogensis) within the female gametophyte (Embryo sac). The haploid megaspore is the first cell of the female gametophyte. 3. Development of female gamete into embryo sac with formation of egg apparatus, secondary or definite nuclei and antipodal cells. 14 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Fig. 2.1 : Life-Cycle of an Angiospermic plant Sexual Reproduction in Flowering Plants y 15 2.2.3 Fertilization : The fusion of the two haploid gametes, i.e. one male gamete and the female gamete results in the formation of diploid zygote. By this way the sporophytic phase is restored. By this, the angiosperms exhibit the phenomenon of double fertilization to form the zygotes. The second male gamete fuses with definite nuclei and forms endosperm. This is called triple fusion. 2.2.4 Development of embryo (Embryogeny) and Endosperm : Zygote develops into an embryo. Formation of endosperm is due to triple fusion (secondary nucleus - 2 nuclei + one male nucleous). Endosperm provides nutrition to the developing embryo. 2.2.5 Development of seed and fruit The ovule devlops into the seed and the ovary develops in to the fuit. Therefore while studying various embryological processes (steps of sexual reproduction) it is essential to acquire familiarity with the organisation of the flower. 2.3 THE STRUCTURE OF THE FLOWER (Fig. 2.2) : The flower is the branch of the shoot (stem) specially modified for sexual reproduction. Plant morphologists regard flower as a shoot of determinate growth with highly condensed or supressed internodes in between nodes and the leaves. It is also specialized variously to suit the functions of different floral organs. During floral initiation, the shoot apex gets transformed into a floral apex of the flower which is formed during onset of reprodctive phase of growth of the plant. The flower may occur singly or as a part of an inflorescence. During floral initiation, the apex of the flowering axis develops floral whorls in acropetal succession. The floral axis bearing floral organs is known as the receptacle. Since there is no significant elongagation of the internodes on the receptacle, the various floral whorls differentiate close to each other in a flower. Typically, a flower has four sets of appendages or floral parts, arranged in whorls called floral whorls such as; Calyx (outermost whorl), Corolla, Androecium and Gynoecium (innermost whorl). The calyx and corolla whorls also called accessory whorls consist of sterile appendages called sepals and petals respectively. The sepals and petals differ in form, size and other characters. But in some families, particularly monocotyledons, they are alike and collectively called perianth. The calyx, corolla (or perianth) do not take part in sexual reproduction, thus represent the non-essential parts of the flower. But they are protective in function and attract insects etc. during pollination. The androecium and gynoecium both are called essential whorls which bear the sexual reproductive organs in angiosperms. The individual members (organs) constituting androecium whorl are called the stamens or the male sex organs. Similarly, the carpels constituting the gynoecium whorl are the female sex organs. Flowers exhibit a great variation in size, colour, shape and insertion of various floral whorls. The flower of Duckweed, (Wolffia microscopica) has a size of 0.1 mm across and is the 16 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) smallest flower among the angiosperms. The flower of certain species of Rafflesia (a root parasite found in forests of Malaysia) is the largest known flower measuring a diameter upto one meter. The colour of flower is mostly due to the colour of petals and ranges from dead white through ivory, yellow, orange, red, violet and blue. The flowers exhibit nearly the whole range of visible spectrum of colours. A flower is regarded as complete if it has all the floral whorls. If any one whorl is absent it is called an incomplete flower. An incomplete flower is said to the perfect (bisexual or hermaphrodite) when both the androecium (bearing male sex organs) and gynoecium (bearing female sex organs) are present. A flower is said to be imperfect (or unisexual) when either of the two sexes are missing. Fig. 2.2 : Dianthus caryophyllus. A. Flowering twig. B. Longitudinal-half of a flower to show its various parts. C, D. Anthers with a part of the filament; C. dorsal view, and D. ventral view. E. Carpels; the ovaries of both the carpels are fused completely whereas the styles and stigmas are free. F, G. Longitudinal (F) and transverse (G) sections of the ovary. The ovules are borne on a central axis, the placenta. Sexual Reproduction in Flowering Plants y 17 In unisexual flowers, if carpels are absent, it is called a male flower or Staminate flower and when stamens are absent the flower is female or pistillate flower. Sometimes, both the essential whorls are absent in a flower which is known as neuter. A plant may have both male and female flowers borne separately (e.g. Maize, Cucurbita). It is called monoecious. When male and female flowers are present in different plants, such plants are called dioecious (e.g. Papaya, Mulberry, Date palm, Coccinia). When both perfect and imperfect flowers are borne on the same plant, they are called polygamous (e.g. Mango, Marigold). In spite of limitless variations seen among the flowers, their basic organization (structure) is uniform. 2.3.1 The Structure of a Perfect Flower : The stalk of the flower through which it is connected to the main plant is called a pedicel. The leaf-like structure(s) bearing a flower in it’s axil is called a bract. Similar foliar (leaf-like) structure present at the summit or anywhere of the pedicel below the receptacle of the flower is called a bracteole. The swollen part of the flowering (floral) axis at which all floral parts such as; Calyx, Corolla, Androecium and Gynoecium are attached is called the receptacle. (Figs. 2.1, 2.2) 2.3.1.1 Calyx : This is the outermost whorl of flower i.e. the lowermost whorl on the receptacle. The individual members (apendages) of calyx is called a sepal. The sepals may be united (gamosepalous) or free (polysepalous) and are generally green in colour or inconsipicuously coloured. Sepals are usually leaf-like (foliaceous) in texture and primarily serve to protect the flowering bud. They usually fall off soon after the flower opens or sometimes are persistent (e.g. Brinjal). When sepals look like a petal (petaloid) they attract the insects for pollination (e.g. Clematis). Green sepals perform photosynthetic function. Sepals sometimes form a spur which stores nectar. They may help in seed dispersal such as in fruits of some members of Asteraceae. 2.3.1.2 Corolla : Inner to the calyx, lies the corolla whorl of the flower. The corolla whorl arises in the receptacle from a node just above that of calyx. The individual members (appendages) of corolla are called petals. The flowers owe their charm mostly to the bright and gorgeous colours and attractive forms of corolla. Corolla with free petals is called polypetalous and that with united or fused petals called gamopetalous. This character has been used in the system of classification of flowering plants by Bentham and Hooker (1862-1833). All dicotyledonous flowering plants in this system are further classfied into three groups, such as; (i) Gamopetalae (plants with gamopetalous Corolla), (ii) Polypetalae (with Polypetalous Corolla) and (iii) monochlamydeae (plants lacking a corolla) 18 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) When these sterile appendages of a flower are not clearly distinguishable into Calyx and Corolla as in Monochlamydeae (as above) they are collectively called a perianth and the individual members of it are knwon as tepals. The corolla or the perianth protect the young reproductive structures in bud condition and also help in pollination by attracting insects through their attractive colours and curious forms. 2.3.1.3 Androecium : Androecium whorl is located inner to the corolla and its individual members (appendages) are called stamens. These are the male reproductive organs. A stamen typically consists by a long stalk-like sterile filament to which usually a fertile bilobed anther is attached at its distal end. Each of the anther lobes typically has two microsporangia or pollen sacs which contain microspores or pollen grains. The tissue that joins filament with the anther lobes is called connective. So each anther typically has two another lobes and four microsporangia. (Fig. 2.2 C, D & 2.3). 2.3.1.4 Gynoecium : Inner to the Androecium whorl lies the gynoecium which is the female reproductive apparatus (organs) in a flower. The individual members constituting the gynoecium whorl are called carpels. As a rule, Carpels are borne laterally on the receptacle. A typical Carpel comprises of a basal swollen ovary with a terminal stigma held on stalk like structure called style. A flower having a single carpal is called simple or monocarpellary (e.g. fabaceae). When more number of carpels constitute the gynoecium whorl it is termed as compound or multicepellary. The multicarpellary gynoecium may be apocarpous with totally free carpels (e.g. Clematis) or syncarpous with united (fused) carpels forming a compound gynoecium. Syncarpous condition is seen in most of the angiosperms, (such as; Mustard, Hibiscus, Datura etc.). Based on number carpels formed in the gynoecium a flower may be monocarpellary (single carpel), bicarpellary (with two carpels e.g. Allium), tetracarpellary (with four carpels, e.g. Datura, Berberis), Pentacarpellary with five carpels, (e.g. Melia, Hibiscus) and multicarpellary with more than five carpels (e.g. Papaver). The ovary contains ovules which are enclosed within the ovary wall and hence the name angiosperms. The portion of the carpellary tissue to which ovules are attached is called the placenta (Fig. 2.2). The mode of distribution of ovlues inside the ovary is called placentation. The elaborate description of the structure of the flower is in Volume-I of this book (Bureau’s Higher Secondary (+2) Biology, Vol-I, 2016, Pages 163-181). The terminologies relating to a flower have great value in understanding the development of embryological processes in angiosperms. Sexual Reproduction in Flowering Plants y 19 Main reproductive structures associated with a noraml flower are given below : Flower - A reproductive structure Stamen - Male sex organ Carpel - Female sex organ Anther - Develops microsporangia (pollen sacs) Ovule of ovary - Develops megasporangium Pollen grain - Develops into male gametophyte Embryosac within ovule - Female gametophyte Sperm - Male gamete Egg - Female gamete 2.4 DEVELOPMENT OF MALE GAMETOPHYTE : To study the development of male gametophyte, one must start with the stamen and followed by structure of anther, microsporangia, microsporogenesis and microspores or pollen grains. The microspore develops into a male gametophyte. 2.4.1 The stamen : As stated earlier , the stamen is the male reproductive organ and consists of the lower sterile, long, narrow stalk-like filament and upper fertile part, the broader knob-like anther (Fig. 2.3). The anther and filament are Fig. 2.3 : Stamen. A. Ventral view, B. Drosal view, connected by a connective. C. T.S. of Anther (Enlarged) 2.4.2 Structure of Anther : The anther shows great variety in form but externally it is typically two lobed, called anther lobes. Each lobe contains two longitudinally running chambers or pollen sacs. Each pollen sac represents a microsporangium which contains several haploid microspores or pllen grains. Therefore an anther generally contains four microsporangia. (Figs. 2.3, 2.4) 2.4.3 Formation of Microsporangia (Pollen sacs) : Young anther is a homogenous mass or meristematic cells surrounded by an epidermis (Fig. 2.5). Groups of hypodermal cells in each of the four corners of it, become distinguished from the surrounding cells by their larger size, dense cytoplasm and prominent nuclei. The 20 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Fig. 2.4 : T.S. anther, showing stomium and pollen grain Fig. 2.5 : A-F. Development of microsporangium : A-E Successive stages of the development of microsporangium; F. A mature pollen sac in a transerve section. Sexual Reproduction in Flowering Plants y 21 conspicuous cells which are arranged in plate-like or crescent-shaped vertical rows form the archesporium of anther. The rows of archesporial cells may vary from one to few in different species. The archesporial cells enlarge radially and divide periclinally to form outer primary parietal cells and inner primary sporogenous cells (Fig. 2.5, 2.6). The primary parietal cells undergo repeated periclinal and anticlinal divisions giving rise to 3-5 concentric layers of cells which eventually form the wall layer of the anther, (anther wall). The primary sporogenous cells may directly function as microspore mother cells (MMC) or undergo several mitotic divisions and finally each of them function as MMC (Fig. 2.5 E). Fig. 2.6 : Transverse section of a mature anther. Thus, at four corners of the anther, four microsporangia are formed. Each microsporangium consists of 3-5 layered anther wall which surrounds the core of microspore mother cells (Fig. 2.5). The anther wall is made up of (i) one layered outermost epidermis, (ii) single layer of endothecium (sub-epidermal layer), (iii) middle layers and (iv) tapetum. (Fig. 2.6). The cells of endothecium are radially elongated, may be U-shaped or ring shaped and they attain the maximum growth when pollen grains mature. They help in dehiscence of anthers.The middle layers lie inner to endothecium. Cells of middle layers are ephemeral and degenerate completely before microspore mother cells undergo meiosis. They store food materials in some taxa. The innermost of wall layers having larger and centripetally extended cells is the tapetum which surrounds the microspore mother cells (Fig. 2.7). Tapetal cells may be multinucleate and provide nutrition to the sporogenous cells and ultimately to the developing microspores. Thus the developing microspores consume products of middle layers and tapetum. 22 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Fig. 2.7 : Dimorphic tapetum in Alectra thomsonii 2.4.4 Microsporogenesis : The sporogenous cells as stated above function as microspore mother cells, which are polygonal in shape and closely packed. As the anther enlarges the pollen sacs (microsporangia) become spacious i.e. get loosely arranged. A few microspore mother cells become non-functional and are finally absorbed by the developing microspores. Each viable or functional microspore mother cell undergoes meiotic cell division and forms four haploid microspores. This process of formation of microspores from the microspore mother cell through meiosis is known as microsporogenesis. (Fig. 2.8) After microsporognesis, the mature anther dehisces by means of slits. Once the slit (opening) is made the microspores come out of the anther. 2.4.5 The Microspore or Pollen grain : Typically, the microspore or pollen grain is a haploid and unicellular body with a single nucleus. The mature microspore or pollen grain may be oval, ellipsoidal, triangular, lobed or even crescent shaped. The microspore has a well defined two-layered wall, consisting of outer thick exine and inner thin intine. The intine surrounds the cytoplasm. The outer exine may have spines, ridges or furrows which may vary in different species. So the exine is either sculptured or smooth. The exine is chiefly composed of sporopollenin, a substance considered to be the oxidative polymer of carotenoids or carotenoid esters. Sporopollenin is a tough substance providing resistance to physical, chemical and biological decomposition and checks natural decay of pollen grains. In insect pollinated pollen grains, the exine is covered by a yellowish viscous and sticky substance called pollenkitt which emits smell. The definite function of pollenkitt is not known but it is believed that it helps in attracting insects and protects the pollen from ultraviolet radiation. Sexual Reproduction in Flowering Plants y 23 Fig. 2.8 : A-E. Simultaneous division of cytoplasm in microspore mother cell. The intine is composed of pectin and cellulose. Intine is usually thicker near the germ pores and at these points also contain enzymatic proteins. Cytoplasm below the intine contains dictyosomes, mitochondria, endoplasmic reticulum, rich starch contents and unsaturated oils. Pollen grains are densely cytoplasmic as long as they are in tetrad condition. Later on when they become free from tetrad condition, their cytoplasm become considerably enlarged and highly vacuolated. At certain places, the exine remains thin. These areas are called germ pores through (one germ pore) which the intine protrudes outside and forms the pollen tube (Fig. 2.10) Fig. 2.9 : Structure of pollen grain. 24 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Study of pollen grains is known as palynology. The pollen grain on further development forms a male gametophyte. 2.4.6 Formation of Male Gametophyte : 1. Pre-pollination development : The development of male gameophyte is more or less uniform in angiosperms (flowering plants). It may start in pollen grains while still within the microsporangium or pollen sac (precocious germination). Before the cell division, the nucleus of microspore migrates from the centre to periphery and many vacuoles appear in between the nucleus and the wall of the microspore. The microspore undergoes only two mitotic divisions. The first mitotic division leads to the formation of a bigger vegetative cell (also called tube cell) and a smaller generative cell (Fig. 2.9). There is no distinct cell wall between these two cells. Both the cells are bound by the cell membrane only. A temporary callose wall is laid down between vegetative and generative cells (2.10). The callose wall (plug) spreads between generative cell and intine to finally pinch the generative Fig. 2.10 : Successive stages of development of Male Gametophyte and callose plugs in developing pollen tube Sexual Reproduction in Flowering Plants y 25 cell off. The callose wall then dissolves and the generative cell (bound by membrane only) lies freely in the cytoplasm. The cytoplasm of the generative cell is almost hyaline and does not contain much of stored food. The texture of generative cell is relatively uniform. It may be elliptical, lenticular or even spindle shaped (Figs. 2.9, 2.10). The elongated form of generative cell however facilitates it’s passage through pollen tube. The larger vegetative cell contains various stored food such as; fat, starch and some protein granules. Now the microspore contains two cells such as vegetative cell and generative cell (Fig. 2.10B). It is usually at its two celled stage, the microspores are liberated from the microsporangia of the anther. Upto this stage, the development of male gametophyte is said to be under pre-pollination stage. Fig. 2.11 : A-B. Gynoecium: A. External view, B. Transverse section of ovary. 2. Post-pollination development : When the microspores or pollen grains fall on the stigma of the pistil, post-pollination changes occur. The pollen grains absorb water and nutrients available on the stigmatic surface. The intine of the pollen grain protrudes out through one of the germ pores and a pollen tube is formed. The pollen tube pierces the stigmatic surface and moves down through the style of the pistil (Figs. 2.11, 2.27). Now, in the generative cell, the nucleus divides mitotically to form two male nuclei which 26 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) become surrounded by a thin cytoplasmic sheath and appear as distinct non- motile male gametes. Since there is no cell wall in the male gametes, they may be called naked. The nucleus of the generative cell, migrates to pollen tube (Fig. 2.10D). Formation of male gametes may also occur prior to formation of pllen tube. This three-celled male gametophyte remains viable for a short time. As the pollen tube elongates, the distal part bcomes highly vacuolated and becomes separated from the anterior part containing the three nuclei, by formation of callose plug (Fig. 2.10E). The male gametophyte in flowering plants is a highly reduced structure. 2.5 DEVELOPMENT OF THE FEMALE GAMETOPHYTE : To study the development of the female gametophyte one must be thoroughly acquainted with structure of the ovary and ovule. Fig. 2.12 : Ovule : Structure (Anatropous Ovule). Sexual Reproduction in Flowering Plants y 27 2.5.1 The Ovary and Ovule : The gynoecium consists of one to many carpels (Fig. 2.11). A typical pistil (carpel) consists of a basal portion called ovary, a stalk (or style) and the terminal receptive disc (the stigma) (Fig. 2.11A). The ovary may contain a single or may ovules arranged in specific placentation. Similarly if more than one carpel is present in the syncarpous ovary, the ovary may contain a number of locules (chamber), usually corresponding to number of carpels. (Fig. 2.11B). The ovule after fertilization develops into a seed, ovary finally develops into a fruit. The ovule is the megasporangium. The ovlue is attached to the placenta (Fig. 2.11). The placenta is a ridge of tissue (a parenchymatous mass) in the inner wall of the ovary to which the ovules are attached. The mode of arrangement of ovules along the placenta in the cavity of the ovary is known as placentation (axile, parietal, free central etc.) 2.5.2 Structure of Ovule : Each ovule in a flowering (angiospormic) plant has the following structure. The ovule is attached to the placenta by a slender stalk called funicle or funiculus (Fig. 2.12). This point of attachment of the body of the ovule to its stalk (funiculus) is known as hilum. In an inverted (anatropous) ovule. The part of funiculus remains attached beyond the hilum alongside of the Fig. 2.13 : A-F. Various types of ovules : A. Orthotropous, B. Anatropous, C. Campylotropous, D. Hemi-anatropous, E. Amphitropous, F. Circinotropous. 28 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) body of the ovule forming a sort of ridge called raphe. The main body (swollen portion) of the ovule consists of a mass of thin walled parenchymatous cells forming a central body, called nucellus, The nucellus is surrounded and protected by one or two multicellular coats (or sheaths) called integuments. The ovules with one integument are called unitegmic (e.g compositae) and with two integuments are known as bitegmic. Majority of anigosperms have bitegmic ovules. The small opening in the integumentary sheath at apex region of ovule where the tip of nucellus remains exposed is called a micropyle. The basal part of the ovule where the nucellus, integuments and funiculus merge is called the chalaza (Fig. 2.12). Depending upon the relative position of micropyle and chalaza at maturity of ovules, different types of ovules have been reported in angiosperms, such as (i) orthotropous (upright or erect ovule), (ii) anatropous (inverted ovule), (iii) campylotropous, (iv) hemianatropous, (v) amphitropous and (vi) circinotropous (Fig. 2.13). 2.5.3 Development of the ovule : The ovule primordium arises on the placenta as a hemispherical projection or a parenchymatous mound (Fig. 2.14A). Periclinal division followed by anticlinal divisions in the very young protruberence (projection) results in enlargment of the same. There is an early differentiation of the archesporial cell which becomes conspicious due to their larger size and dense cell contents (Fig. 2.14B). Differentiation of archesporial cells (archesporium) is followed by initiation of the inner and outer integuments (Fig. 2.14 C, D). Integuments arise as a complete ring, right below the nucellus and grows upwards to cover the whole nucellus except at the micropylar opening at the tip region. The central part of the ovule inner to the integument is a parenchymatous mass of cells which becomes differentiated in due course of time to a mature nucellus containing a female gametophyte or embryo sac (Fig. 2.12). Thus the mature ovule consists of outermost shealths, the integuments and Fig. 2.14 : A-D. Successive stages in the the nucellus which encloses an embryo development of ovule. sac (the female gametophyte) (Fig. 2.12). Sexual Reproduction in Flowering Plants y 29 2.5.4 Megasporogenesis : The mucellus towards the micropylar end, differentiates a hypodermal cell into an archesporial initial. This archesporial initial divides periclinally to form outer primary parietal cell and inner primary sporogenous cell. The primary sporogenous cell forms the megaspore mother cell (Fig. 2.14 B-D) which undergoes meiotic cell division to form four haploid megaspores. (Fig. 2.15 A). The formation of megaspores from the megaspore mother cells is known as megasporogenesis. Out of four megaspores in a linear tetrad, usually the upper three megaspore degenerate and the lower most megaspore (the chalzal one) enlarges to become functional megaspore (Figs. 2.15, 2.16 A). Thus the megaspore mother cell is the last cell of the female sporophytic (2n) or diploid generation. Similarly the haploid (n), functional megaspore represents the first cell of the female gametophytic generation. This functional megaspore organises to form a female gametophyte or embryo sac. (Fig. 2.16) Fig. 2.15 : A-C. Megaspore tetrads : A. Linear tetrad, B. T-shaped tetrad, C. Tetrahedral tetrad 2.5.5 Organization of the female gametophyte (Embryo Sac) : The functional megaspore grows in size and many small vacuoles appear in its cytoplasm. The vacuoles, later on, join together to form a large vacuole. The nucleus of the megaspore undergoes three mitotic divisions to form eight nuclei. Generally four nuclei are seen at micropylar end (pole) and the other four, seen at the chalzal region (pole) of the enlarged megaspore. 30 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) This enlarged megaspore organises itself to form an embryo sac. It has been observed that the separation of nuclei and presense of four nuclei at each pole is due to enlargement of the central vauole which pushes the nuclei towards the opposite poles of the embryo sac (Fig. 2.16 C-D). Fig. 2.16 : A-E. Successive stages of the embryo sac development. Sexual Reproduction in Flowering Plants y 31 The most common type of embryo sac is eight nucleate embryosac developing from a single megaspore. It is found in about 81% of flowering plants. Since this embryo sac develops from a single megaspore it is called monosporic type of embryo sac or (Polygonum type), which is described below. Out of four nuclei, one nucleus from each group at a pole migrates to the centre of embryo sac. These two nuclei which have migrated to center are called polar nuclei. Two polar nuclei fuse to form a definitive nucleus. The three nuclei left at the chalazal pole are surrounded by walls and are called antipodals or antipodal cells. Again, out of three nuclei located at micropylar pole (or micropylar end) are organised to form the egg apparatus consisting of one egg (or oosphere) and two synergids (Fig. 2.17). The egg is the female gamete. The egg hangs between the synergids. The embryo sac is the female gamete bearing part or the female gametophyte (Figs. 2.16 E, 2.17). The egg (the female gamete) on fertilization with a male gamete forms a zygote which develop into an embryo. The synergids help the passage of pollen tube bearing two male gametes. Pollen tube passes through the synergids to effect fertilization. One male gamete fertilizes the female Fig. 2.17 : Mature Embryo sac. gamete (egg) called syngamy and the other male gamete fuses with the two polar nuclei (secondary nucleus) in the center of embryosac, forming the primary endosperm cell. Primary endosperm cell or the endosperm mother cell grows into an endosperm which provides nutrition to the growing embryo (Figs. 2.16, 2.17). The antipodal cells sooner or or later get disorganised. 2.6 POLLINATION : When pollen grains are shed from the anther they are disseminated by means of various agencies. Some pollen grains may be by one means or the another, finally reach the stigma of a pistil, either of the same or another flower of same plant or another. Therefore the process of transfer of pollen grains from anthers to the stigma is called pollination. The process of pollination ends when the pollen grain has reached the stigma. 2.6.1 Types of pollination : The two broad categories of pollination are self pollination and cross pollination. The transfer of pollen grains from an anther to the stigma of the same flower or to a flower on the same plant is known as self pollination or autogamy. On the contrary, the transfer of pollen grains from anther of the flower to the stigma of a flower of another plant is called cross pollination or allogamy. 32 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Pollination may also be divided into three types basing on the source of the pollens. They are (i) Autogamy (ii) Geitonogamy (iii) Xenogamy. (i) Autogamy - Here the source of the pollen to fill on stigma is the same flower. (ii) Geitonogamy - When transfer of pollen grains takes place between two flowers borne by the same plant it is called geitonogamy. It is functionally a cross pollination but genetically a self pollination. (iii) Xenogamy - In this process, the transference of pollen grains between two different (separated) plants of the same species. Here pollination causes transference of genetically different types of pollen grains of a plant to the stigma of another plant. 2.6.2 Self Pollination : Self pollination (as defined above) can take place in bisexual flowers in which both male and female sex organs mature at the same time. Self pollination can also occur in unisexual flowers (male and female) of the same plant (monoecious condition) when their sex organs mature at the same time. The process of self pollination can be classified into the flowering two types. 1. Autogamy : Autogamy means pollination of a flower by its own pollens. So it is the transfer of pollen grains from anther of a flower to the stigma of the same flower. Naturally autogamy is possible in bisexual plants only (e.g. Tea, Wheat, Rice etc.). 2. Geitonogamy : This type of pollination is the transfer of pollen grains from the anther of a flower to the stigma of another flower borne on the same plant. In otherwords, this is a type of self pollination that occurs between two different flowers present on the same plant. Here, only one plant is involved. The flowers may be bisexual or unisexual borne by the same plant. 2.6.2.1 Contrivances (Adaptations) of self pollination : There are certain adaptations or devices in the flowers to effect the self pollination. 1. Homogamy : It takes place in bisexual flowers in which both the anther and stigma mature at the same time (e.g. Mirabilis, Potato, Wheat, Rice etc). 2. Dichogamy : In many bisexual flowers, when the anthers and stigma mature at different times, it is known as dichogamy. Normally, it favours cross pollination. But, if cross pollination fails, the stigmas move back and touch the anthers to activate self pollination (e.g. Sun flower). In the flowers of Ixora, and Vinca, the sessile anther may lie at the mouth of narrow corolla tube, that may brush against stigma of the same flower. This results in self pollination. Sexual Reproduction in Flowering Plants y 33 3. Cleistogamy : The bisexual flowers which never open are called cleistogamous or closed flowers. In this case, pollen grains have to be pollinated on the stigma of the same flower, so that self pollination is obligatory. Such a flowers are very small, not coloured and do not emit any smell. Cleistogamy is seen in the underground flowers of Commelina benghalensis (Fig. 2.18). It is also seen in the case of Impatiens, Oxalis, Portulaca etc. On the other hand, flowers which open and expose their reproductive organs to pollinating b agents are called chasmogamous. Commelina benghalensis has both cleistogamous and chasmogamous flowers. 2.6.2.2 Advantages of self pollination : 1. Fertilization and production of the progeny are Fig. 2.18 : Chasmogamous and always certain by this method. cleistogamous flowers in Commenlina benghalensis. 2. It maintains purity of race and superiority of a- normal chamogamous flower; the variety. b- underground cleistogamous flowers. 3. Here there is less wastage of pollens. 2.6.2.3 Disdvantages : 1. It leads to loss of viability and vigour of the plant in the long run. 2. If lethal genes become homozygous, the effect may be disastrous. 2.6.3 Cross pollination : Cross pollination is also called allogamy. It is the transference of pollen grains from anther of one flower to stigma of flower on another plant of the same or allied species. Cross pollination within a species (different variety) is called xenogamy. Since it occurs between two different strains of the plant, it yields hybrids. It is effected by external agents which carry the pollens of one flower to the stigma of another flower. These flowers are borne by two seperate plants of the same or allied species. It occurs in both dioecious and monoecious species. But dioecious species are necessarily cross pollinated. 2.6.3.1 Contrivances (adaptations) of cross pollination : There are various adaptations for the process of cross pollination. In unisexual and bisexual flowers, certain devices are present for effective cross pollination and avoiding self pollination. Some of these devices of cross pollination are as follows. 34 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) 1. Dicliny or unisexuality - Here, the flowers are unisexual, borne either in a monoecious plant (bearing male and female flowers in the same plant) or in a dioecious plant (male and female flowers are borne in two separate plants). In monoecious plants although cross pollination takes place by several agents, geitonogamy may occur. Some of the such monoecious plants are Cucurbita, Ricinus, Zea etc. In dioecious plants, cross pollination is the rule. The examples are Piper, Cannabis, Morus etc. 2. Self sterility - It is the condition when a flower cannot be pollinated by the pollen grains of the same flower or from any flower of the same plant. It is found that stigma of some orchids wither away if the pollen grains from the same flower are deposited on it. Many species of Solanaceae (Solanum, Nicotiana) and tea plant are self sterile and are cross pollinated. 3. Dichogamy - It is found in bisexual flowers where stamens and carpels mature at different times, hence, the self pollination is prevented naturally. There are two conditions for dichogamy. When the gynoecium matures earlier than the anthers, the stigma receives the pollen grains from another flower. This condition is known as protogyny. Common examples of protogynous flowers are Anona, Polyalthia, Magnolia, Michelia etc. The other condition of dichogamy is protandry where the anther matures earlier than the stigma. Here self pollination is naturally impossible. Hence, the pollen grains are carried over to the respective stigma of another flower in which gynoecium is matured. It occurs in Hibiscus, Gossypium, Helianthus, Tagetes, Coriandrum etc. Under above conditions, geitonogamy or xenogamy can only occur. 4. Herkogamy - In some bisexual flowers, there are certain adaptations of floral parts like anthers and style which act as barriers to self pollination. Here autogamy is mechanically impossible (Fig. 2.19), and thereby, favouring cross pollination. In many cruciferous and caryophyllaceous plants, the style is much longer and the stigma is exerted far Fig. 2.19 : Trimorphism in flower of Lythrum salicaria. Three types of flowers show pistils and stamens of three different heights. Pollination usually takes place between organs of the same height. Sexual Reproduction in Flowering Plants y 35 beyond the stamens, preventing pollens to reach the stigma. In flowers of Gloriosa, the anthers are extrose-(facing outwards) and dehisce at a distance, Thus discouraging self pollination. The peculiar arrangement of stamens and pistils in Salvia achieve cross pollination only by insects. (Fig. 2.20) 5. Heteromorphism - Plants may have two (dimorphic), three (trimorphic) or different forms of flowers, based on the position of anthers and stigmas at differnt levels. Such heteromorphous flowers may have heterostyly (styles of different length) or heteroanthy (different types of anthers). One form has short stamens and long style while the other has long stamens and short style (e.g. Primula, Lythrum) (Fig 2.19). In this case, one with short style will be cross pollinated by pollens from lower anthers and vice versa by insects having the capacity to enter in to particular depth of the flower. Dimorphism is observed in Jasminum, Linum etc. Some species of Oxalis, Linum, Lathyrus exhibit trimorphism, which show three types of flowers at three different positions of anthers and stigmas. It results in cross pollination only (Fig. 2.19). 2.6.3.2 Agents and types of Cross pollination : Cross pollination is brought about by external agents, as it involves two separate plants of the same or closely allied species. These agents can be categorized (Table-2.1) as biotic agents (insects, birds, bats, snail etc.) and abiotic agents (wind, water). Fig. 2.20 : A-D. Structure of Ficus carica inflorescence : A. Longitudinal section of inflorescence. B. Staminate flowers, C. Long styled pistillate flower, D. Abortive (short-styled) pistillate flower. 36 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Table - 2.1 Pollinating agents for cross pollination Category Agent Type of cross pollination Biotic Agents Insect Entomophily Animals Zoophyly (Birds (Ornithophilly Bats Cheiropteriphily Snails) Malacophily) Abiotic Agents Air Anemophily Water Hydrophily 1. Entomophily Insect pollinated plants are entomophilous. In these cases, the flower attracts the insects in a variety of ways and the sticky pollens easily adhere to the body or body parts of the insects. Similarly, the stigma is also sticky to receive the pollen grains. The flowers develop the following adaptations to attract the insects. (i) Conspicuous and coloured flowers Here the petals of corolla are large sized, irregular and beautifully shaped to attract insects. Bracts, sepals or even stamens become coloured in some plants, (e.g. Mussaenda, Bougainvillea, Musa). (ii) Nectar - Nectary glands secrete the nectar which attract the bees. Nectar provides nutrition to these insects, (e.g. Oranges). (iii) Scent - Flowers that open during nights emit good scent which attract many nocturnal insects, (e.g. Nyctanthes, Cestrum). Flowers with offensive smell and nauseating to human beings attract swarm of carrion flies (e.g. mature inflorescence of Amorphophallus, Rafflesia and some aroids) (iv) Edible sap - There are certain plants which do not have nectaries to attract insects. Edible sap secreted by such plants attracts insects. (e.g. Some Orchids). (v) Edible pollens - Wax on the pollen is utilized to build the honeycomb and pollen may be required to nourish the young insects, (e.g. Papaver, Rosa, Clematis). (vi) Special mechanisms (a) In the case of Bignonia, if it has not been pollinated, the stigma gets exposed again by opening the flap closed by insect visit. Sexual Reproduction in Flowering Plants y 37 Fig. 2.21 : A-D. Insect pollination in Salvia: A Structure of flower, B. Diagrammatic presentation of changes taking place in the position of anther lobes during the entry of insect in the flower, C. Entry of insect in the flower (note the dusting of pollen grains on the back of the insect), D. Insect entering another flower and the pollens on its backdate beingjcollected by stigma. (b) In dense, capitulum type of infloerscence, the inconspicuous, small individual flowers become attractive when grouped together so that these are visited by insects, (e.g. Helianthus, Tridax, Tagetes). (c) In hypanthodium inflorescence, the insects enter through the ostiole and bring about pollination. (Fig. 2.20) Here the infloresence is a closed one and open to outside by a pore called ostiole. The receptacle is hollow with three types of flowers. Near the ostiole the flowers are male. The middle portion flowers are female with long styles. The lowest at base of cavity are sterile female flowers. The insect enters through the ostiole passes through anthers of male flowers and reaches upto the female flowers on the middle part of infbrescence. So the pollen grains are carried by the insect and passes onto the stigma of the female flowers (Fig. 2.20). (d) A peculiar type of adaptation is seen in the case of Salvia (Fig. 2.21) where there is occurrence of a bilabiate corolla with two epipetalous stamens. The bilobed anther of each stamen is widely separated by the elongated curved connective 38 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) which swings freely on the filament. Out of the bilobed anther, the upper lobe is fertile and the lower one is sterile. The lower sterile lobe of anthers receive the insect which enters the bilipped corolla tube. During this event, the connective swings down, so that the upper fertile lobe comes down, and strikes the back of the insect and dusts it with pollen grains. The flower is protandrous. So, when the stigma matures it bends down and touches the back of the insect covered with pollen grains and bring about Fig. 2.22 : A humming bird collecting nector pollination. This type of adaptation is form Bignonia capreolata flowers, thereby called ballistics. pollinating them. (After Bates). 2. Zoophily : Animals act as useful agents of pollination. Birds, squirrels, bats, snails and other animals take part in pollination. Based on the type of animals involved, the zoophily is classified into different types. (a) Ornithophily - Several birds such as tiny humming birds, the honey thrushes feed on the nectar of the flowers and pollinate. Birds like crows also help in pollination. (e.g. Bignonia) (Fig. 2.22), Bombax, Erythrina and Callistemon also show this type of pollination. (b) Cheiropteriphily - In this type of pollination, the flowers are pollinated by bats. They visit the flowers to collect nectar during which pollination occurs, (eg. Anthocephalus, Bombax etc.). (c) Malacophily - Snails and slugs help in this type of pollination. Chrysanthemum, Lemna are some of the examples. 3. Anemophily : In this case, plants are pollinated by wind. The flowers are inconspicuous and small. They are never coloured and showy. They neither emit any odour nor secrete any nectar to attract the insects. However, the pollens are produced in enormous numbers. For example, a Cannabis flower produces approxmately 5 lakh pollen grains. They are light and dry. Such pollens are easily carried away by wind and transferred to the stigmas. The occurrence of Sexual Reproduction in Flowering Plants y 39 branched bushy stigma and comparatively larger protruding stigma in grasses, bamboos, cereals, millets, sugarcane and other such plants help wind pollination. In Zea mays (Fig. 2.23), the male flowers are borne in a terminal panicle of spikelets. A few female spadices are borne, each in a axil of leaf surrounded by spathes. The style consists of long and silky threads. These are seen to hang in tufts from the spadix. When the anthers burst a cloud of pollen grains, these float in air, close round the plant. Some of these floating pollen grains are received by protruding stigmas which bring about pollination (Fig. 2.23). 4. Hydrophily : This type of pollination takes place in aquatic plants. Water is the medium for transfer of pollen grains. Particularly, the submerged plants are adapted for this type of pollination (e.g. Naja, Vallisneria, Hydrilla etc.) There are two possibilities, Fig. 2.23 : Maize-an anemophilous plant. either pollination takes place completely under water or it takes place on the water surface. In the former case, it is known as hypohydrogamous (e.g. Najas) while in the latter, it is known as epihydrogamous. (e.g. Vallisneria, Hydrilla). In Vallisneria, the plant grows in mud (Fig. 2.24). It is dioecious and leaves are strap-shaped. The male plant bears flowers in small spadix surrounded by spathe. These are short stalked and borne low down amongst radical leaves. The female flowers are borne singly on long wiry stalks enabling the flowers to float on water when mature. The individual male flowers get detached and float freely in large numbers on the water surface. These flowers open on the water surface exposing two stamens vertically. The male flowers cluster around the female flower. As anthers burst and the sticky pollen grains get Fig. 2.24 : Vallisneria spiralis showing attached to the stigma. free-floating male flower. 40 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) 2.6.3.3 Advantages of cross pollination : (1) This always results in much healthier offsprings. (2) The offsprings produced in this method are better adapted ones. (3) As a result of the cross pollination, hybrids are produced. (4) More abundant and viable seeds are produced which store greater quantities of food material. (5) The process eliminates defective characters and is helpful in production of new varieties. 2.6.3.4 Disadvantages of Cross Pollination : (1) This is wasteful process, as large number of pollen grains get damaged in the course of pollination. (2) The chance of fertilization is limited here since it can be effected only if the pollen reaches the matured stigma. 2.7 OUTBREEDING DEVICES : Pollination (as described in previous pages) is the process of transferring pollen grains from anther to stigma of the pistil. Pollination can be either cross pollination or self pollination. Majority of flowering plants are bisexual or hermaphrodite which commonly promote self pollination. But self pollination always is not desirable. Successive series of self pollination affects the progeny negatively and causes inbreeding depression. This results in formation of homozygous genes. Therefore nature has provided certain adaptations or devices to promote cross pollination in order to produce healthy progeny. This is known as outbreeding. Outbreading is a phenomenon where individuals within a species will tend to breed with others who are neither close relatives nor distant genetic relations but a middle ground of both. The devices or factors which encourage outbreeding are briefly presented below (More descriptions made in previous pages) 2.7.1 Unisexual flowers (Dicliny) : Nature has created unisexual flowers which contain only one sex either male or female. So cross pollination is the only choice. Therefore formation of unisexual flowers or dicliny is one of the outbreeding devices. 2.7.2 Non-synchronisation (Dichogamy) : Timing is important for successful pollination. Release of pollen grains from anther and receptibility of stigma should happen simultaneously. Sometimes pollen grains in a hermaphrodite flower mature and get released before stigma is open which leads to loss of pollen vitality or vice-versa. This phenomenon i.e. dicliny includes two processes such as protandry (anthers Sexual Reproduction in Flowering Plants y 41 mature earlier than carpels) and protogyny (carpels mature much earlier than it’s anthers). So dichogamy is an outbreeding device in which cross pollination is the only method to develop seeds. 2.7.3 Heterostyly and Herkogamy : In heterostyly due to great disparity in length of style and stigma, effective self pollination is not possible. In herkogamy, the homogenous flowers adapt certain devices in which only cross pollination is possible. 2.7.4 Self Incompatibility : Incompatibility is the inability of certain gametes even from genetically similar plant species to fuse with each other. Here, even though pollination takes place, it cannot proceed to fertilization due to failure of the pollen tube growth. This is also known as intraspecific incompatibility, self sterility or self incompatibility, which has been reported in nearly 66 families of flowering plants. Self incompability within a flower (or plant) may be due to prevention of some physiological and morphological mechanism. It involves very complex mechanisms associated with interactions of pollen and stigmatic tissues. Self incompatibility helps to prevent self pollination. If the incompatibility is due to genotype of the sporophyte (e.g. stigamtic tissue), it is termed as sporophytic incompatibility. On the other hand if incompability is due to the genotype of the pollen, it is termed as gametophytic incompability. Self incompability usually develops with maturation of stigma. It may also be due to preventing of pollen germination, retardation of pollen tube growth, deorientation of pollen tube or even failure of nuclear fusion. Incompatibility is controlled by genes with multiple alleles. In self incompatible fruit trees, it is necessary to plant two cross compatible varieties to ensure good results. Self incompatibility may be used in hybrid seed production. 2.8 POLLEN-PISTIL INTERACTIONS : A special character of sexual reproduction in flowering plants is the interaction of pollen grain (the male gametophyte) with massive sporophytic tissue of pistil particularly stigma and style, before discharging the male gametes near the egg (female gamete) inside the embryo sac. All pollinations do not lead to a successful fertilization of male and female gametes. For successful fertilization, the stigma of the pistil has to recognise the pollen of the same species. Once a compatible pollen is recognised and accepted by the stigma, then the various subsequent events for fertilization will proceed. In this recognition process (pollen-stigma), the incompatible pollens are rejected by the stigma. 42 y Bureau’s Higher Secondary, BIOLOGY (Class - XII) Pollen-pistil interaction is a long papilla term (prolonged) interaction of pollen grains and pistil resulting in a successful fertilization. The chain of events of pollen- pistil interaction proceed as follows : l Landing of pollen on the stigma which recognises its compatible pollen. l Germination of pollen and formation of pollen tube where the pollen releases its contents (Fig. 2.25) l Pollen tube growth through style of the pistil towards ovary and then into ovule (Fig. 2.27). l Entry of pollen tube containing male gametes F