Class 12 Subject: Biology Chapter-2 SEXUAL REPRODUCTION IN FLOWERING PLANTS Sexual reproduction is the process of fusion of male and female gamete resulting in the production of a diploid zygote which ultimately develops into a new organism. All flowering plants show sexual reproduction. FLOWER: Flowers are the site of sexual reproduction in flowering plants. A typical angiospermic flower has following parts arranged in four whorls. They are Calyx, Corolla, Androecium and Gynoecium. 1. Calyx: It is the outer most whorl of the flower. It is composed of leaf like green sepals. 2. Corolla: It is the second whorl of flower and consists of number of petals. 3. Androecium: It is the third whorl of flower consisting of stamens. Stamen is the male reproductive organ of flower. Each stamen is made up of filament and anther. 4. Gynoecium: This is the last and fourth whorl of the flower consisting of pistil or carpel. Pistil is the female reproductive organ of the flower. Each pistil is composed of ovary, style and stigma. Fig : structure of a flower STAMEN : A typical stamen consists of two parts -- i. The long and slender stalk called the filament ii. The terminal bilobed structure called the anther. ANTHER: It is a bilobed and dithecous structure. In transvers section, it is a tetragonal structure consisting of four microsporangia located at the corners , two in each lobe. Microsporangia develops further into pollen sacs. Pollen sacs contain pollen grains. STRUCTURE OF MICROSPORANGIUM Microsporangium is circular in outline. It is surrounded by four wall layers—the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers are protective in function and help in dehiscence of anther to release the pollen. The inner most wall layer i.e. tapetum provides nourishment to the developing pollen grains. A sporogenous tissue occupies the centre of each microsporangium. Fig : T.S of an anther Fig: Stamen MICROSPOROGENESIS : The process of formation of haploid microspores from a pollen mother cell(MMC) through meiosis is called microsporogenesis. Microspores are arranged in a cluster of four cells—the microspores tetrad. As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. POLLEN GRAIN: The pollen grains represent the male gametophytes. Each pollen grains has a two-layered wall. The outer hard layer called the exine is made up of sporopollenin which is one of the most resistant organic material that enables them to resist high temperature and strong acid and alkali. No enzyme is yet known to degrade sporopollenin. The region on exine where sporopollenin is absent are called the germ pores. It helps in the formation of pollen tube. The inner layer is thin called as intine. It is composed of cellulose and pectin. A mature pollen grain contains two cells ,the vegetative cell and the generative cell. The vegetative cell is bigger, has abundant food reserve and a large irregular shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. In about 60% of angiosperms, pollen grains are shed at this 2- celled stage. In the remaining species, the generative cell divides mitotically to give rise to two male gametes before pollen grains are shed (3 celled stage). PISTIL / GYNOECIUM The gynoecium represents the female reproductive part of flower. A flower may be monocarpellary (having one pistil) or multicarpellary (having more than one pistil). Pistils may be syncarpous (fused together) or apocarpous (free). Each pistil has three parts- the stigma, style, and ovary. The stigma serves as a landing platform for pollen grains. The style is the elongated slender part beneath the stigma. The basal bulged part of the pistil is the ovary. Placenta is located inside the ovarian cavity. Megasporongia commonly called ovules arise from the placenta. THE MEGASPORANGIUM (OVULE) The main parts of megesporangium (ovule) are – i. Funicle—stalk that attached ovule to placenta ii. Hilum—Junction between ovule and funicle. iii. Integuments—one or two protective envelops around the ovules. iv. Micropyle—a small opening at the tip of integuments. v. Chalaza—basal part of ovule. vi. Nucellus—mass of cells enclosed within the integuments having abundant reserve food material. vii. Embryo sac or femal gametophyte—it is located inside the nucellus MEGASPOROGENESIS - The process of formation ofhaploid megaspores from megaspore mother cell (MMC) through meiosis is called megasporogenesis. The megaspore mother cell divides meiotically to form four haploid megaspores. One of the megaspore is functional while the other three degenerate in majority of the angiosperms. Only the functional megaspore develops into female gametophyte or embryo sac. This method of embryo sac formation from a single megaspore is termed as monosporic development. Development of Embryo sac or female gametophyte: Nucleus of the functional megaspore undergoes mitotic division to form two nuclei which move to the opposite poles forming, the 2-nucleate embryo sac. To more sequential mitotic divisions result in the formation of 4-nucleate and later the 8-nucleate stages of the embryo sacs. The cell wall is laid down leading to the formation of embryo sac. Structure of an Embryo sac: Embryo sac consist of— i. Egg apparatus – present at the micropylar end. It consists of two synergids and one egg cell. ii. Antipodals – Three cells present at the chalazal end. iii. Central cell—It has two polar nuclie. Thus a typical angiosperm embryo sac at maturity is 8-nucleate 7-celled structure. POLLINATION The transfer of pollen grains from anther to stigma of a pistil is called pollination. Kinds of pollination: Autogamy : Transfer of pollen grains from anther to stigma of the same flower. e.g viola,oxalis Geitonogamy: Transfer of pollen grains from anther to the stigma of another flower of the same plant. e.g cucurbits Xenogamy:Transfer of pollen grains from anther to the stigma of another flower of the different plant. e.g. papaya. Agents of pollination: Abiotic agents- a) wind- Anemophily b) water- Hydrophily Biotic agents- a) Insects- Entomophily b) Birds- Ornithophily c) Bats- Chiropterophily d) Mammals- Zoophily Adaptation in flowers in pollination Wind pollination- Pollen grains: light, non-sticky winged Anther- well exposed Stigma- large and feathery Eg: corn cob, grasses Water pollination- Pollen grains are long ribbon like Pollen grains are protected by mucilaginous covering Produces large number of pollen grains Insect pollination- Flowers –large colourful, fragnant, rich in nectar Pollen grains and stigma are sticky OUTBREEDING DEVICES Plants have many mechanism and devices that promote cross pollination. Dichogamy- In this mechanism the stigma and the anther mature at different time. Heterostyly- Stigma and anther are placed at different levels. Self-sterility or self-incompatibility – This is a genetic mechanism that prevent self pollen from fertilizing the ovules by inhibiting pollen germination. Unisexuality- (Dioecism) –The plant produces either male or female flower. Artificial Hybridisation Techniques Emasculation- Removal of anther from the bisexual flower before the anther is mature is known as Emasculation. Bagging- The emasculated flower is then covered with a bag to prevent unwanted pollination. This process is called Bagging. POLLEN PISTIL INTERACTION - All events from pollen deposition on the stigma until pollen tubes enter the ovule are together called as pollen pistil interaction. Recognition of compatible pollen Germination of pollen grain and pollen tube growth DOUBLE FERTILISATION: The pollen tube releases two male gamete into the embryo sac. One of the male gamete fuses with egg to form diploid zygote. This is called syngamy. The second male gamete fuses with two polar nuclei to produce triploid endosperm nucleus(PEN). As this involves the fusion of three haploid nuclei it is termed as triple fusion. Since two types of fusion takes place in an embryo sac the phenomenon is called double fertilization. PEN develops into endosperm and zygote develops into embry POST FERTILISATION EVENTS: 1. Endosperm development 2. Embryo development 3. Maturation of ovule into seed 4. Maturation of ovary into fruit ENDOSPERM The primary endosperm cell(PEC) divides repeatedly and forms triploid endosperm tissue. The endosperm tissue is filled with reserve food material and are used for the nutrition of developing embryo. Two types of endosperm development: 1 Free nuclear type 2 Cellular type Non-albuminous seed-Endosperm is completely utilized before maturation of seed. E.g pea, groundnut Albuminous seed – A portion of endosperm remains in mature seed. E.g wheat, maize EMBRYO DEVELOPMENT Zygote divides by mitosis an gives rise to the proembryo and subsequently to the globular ,heart shaped mature embryo. A typical dicotyledonous embyo consists of two cotyledon and an embryonal axis between them. The part of embronal axis above the level of cotyledon is the epicotyl which becomes plumule(shoot). The part ofembryonal axis below the level of cotyledon is the hypocotyl which becomes radicle(root). Monocot embryo consists of only one cotyledon which is termed as scutellum. Embryonal axis has the radicle on its lower end, it is covered by a sheath called coleorhiza. Plumule is covered by coleoptile. Dicot Embryo SEED Seed is a fertilized ovule. Seed consists of : 1. Seed coat: composed of two layer a)Testa- thick outer layer b)Tegmen- thin inner layer. 2. Hilum: scar on seed coat. 3. Micropyle: A small pore on seed coat through which oxygen and water exchange occurs during seed germination. 4. Cotyledon: It stores food which provides nourishment to the developing embryo. 5. Radicle: embryonic root 6. Plumule: embryonic shoot. Monocot seed FRUIT Fertilised ovary is called fruit. Wall of ovary forms fleshy or dry fruit wall called pericarp. Types of fruit: 1) True fruit: only ovary contributes in fruit formation. 2) False fruit: thalamus also contributes in fruit formation.e.g. apple, strawberry,cashew. Parthenocarpic Fruit: Fruit which develops without fertilization is called parthenocarpic fruit.