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Chapter 16 Reproduction in Plants Objectives Students should be able to: • Define asexual reproduction as the process resulting in the production of genetically identical offspring from one parent • Define sexual reproduction as the process involving the fusion of nuclei to form a zygote and the production of genetically dissimilar offspring • Identify and draw, using a hand lens if necessary, the sepals, petals, stamens and carpels of one, locally available, named, insect-pollinated, dicotyledonous flower, and examine the pollen grains under a microscope • State the functions of the sepals, petals, anthers and carpels • Use a hand lens to identify and describe the anthers and stigmas of one, locally available, named, wind-pollinated flower, and examine the pollen grains under a microscope • Outline the process of pollination and distinguish between self-pollination and cross-pollination • Compare, using fresh specimens, an insect pollinated and a wind-pollinated flower • Describe the growth of the pollen tube and its entry into the ovule followed by fertilisation (production of endosperm and details of development are NOT required) 16.1 What is Reproduction? • No organism lives forever • To ensure continuity of species, organisms must produce new organisms like themselves Production of new organisms is called reproduction Two types of reproduction 1. Asexual reproduction 2. Sexual reproduction Asexual Reproduction • Asexual Reproduction is process resulting in the production of genetically identical offspring from one parent, without the fusion of gametes • Gametes are reproductive cells containing half the chromosomes as the normal body cell Asexual Reproduction Identical offspring produced – Mitosis occurs. – A cell divides to give 2 identical daughter cells – Each daughter cells has the same no. of chromosomes as parent cell. – These daughter cells have identical genes as the parent cell. They are also called clones. Parent with 2n chromosomes 2n Cell divides 2n Two daughter cells formed, each with 2n chromosomes (mitosis) 2n • No. of chromosomes in cells varies with the species Sexual Reproduction • Is the process involving the fusion of two gametes to form a zygote, resulting in the production of genetically dissimilar offspring. • Gametes are produced in a process known as meiosis • In meiosis, – A cell divides to form 4 daughter cells called gametes – Each gamete has half the number of chromosomes as the paent cell. – When gametes fuse during fertilisation, the normal number of chromosomes is restored. Parent cell with 2n chromosomes 2n nucleus Cell divides (meiosis) n n n n Four gametes formed, each with n chromosomes How are gametes involved in sexual reproduction? • The process of fusion of gametes is called fertilisation • The nuclei of the two gametes (male and female) fuse to form a zygote. • Zygote contains hereditary information from two different gametes, hence offspring produced is genetically different from both parents 16.2 Sexual reproduction in Flowering Plants • New plants produced from seeds • Seeds are found in fruits • Each fruit is produced by a flower • Hence, flowers bear the reproductive organs of flowering plants General Parts of a Flower • Pedicel – Is a flower stalk • Receptacle – Enlarged end of the flower stalk on which the other parts of the flower are borne • Sepals** – Modified leaves – Function : enclose and protect the other parts of the flower in the bud stage – Sepals together forms calyx • Petals** – Modified leaves and forms the most obvious (conspicuous) part of the flower – insect-pollinated flowers, petals are • Brightly coloured to attract insects for pollination • Provide Landing platform for insects – Petals together form the corolla carpel Parts of a Flower stigma carpel style anther ovary ovule stamen filament Sepal • Sepals are modified leaves which enclose and protect the other parts of the flower in the bud stage. • All the sepals together make up the calyx. • The sepals usually form the outermost layer of floral leaves. • However, some flowers, for example, Hibiscus, have another layer of Copyright © 2006-2011 Marshall Cavendish International (Singapore) Ltd. May 2017 floral leaves outside the sepals, whichPte.make up the epicalyx of22 the flower.19 Parts of a Flower stigma carpel style anther ovary ovule stamen filament Sepal Receptacle The receptacle is the enlarged end of the flower stalk which bears the other parts of the flower. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 20 Parts of a Flower stigma carpel style anther stamen ovary ovule filament Sepal Receptacle Pedicel The pedicel is the flower stalk. Some flowers have no pedicels and are attached to the plant directly at the base. They are called sessile flowers. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 21 Parts of a Flower stigma carpel style anther stamen ovary ovule Sepal Receptacle filament Petal Petals are modified leaves forming the most obvious (conspicuous) part of the flower. All the petals together make up the corolla. In insectpollinated Pedicelflowers, petals: • are brightly coloured to attract insects for pollination; and • provide a platform for insects to land. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 22 Parts of a Flower stigma Petal carpel style ovary anther ovule stamen filament Sepal Receptacle Pedicel Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 23 • Stamens** (Androecium) – Male part of the flower – Consists of a filament and an anther – All stamens together make up the androecium • Filament – Stalk that holds the anther – Anther contains a vascular bundle (xylem and phloem) • Anther** – Consists of two lobes – Each lobe contains two pollen sacs – Pollen sacs contain pollen grains Parts of a Stamen Side view of a stamen Stamen • The male part of the flower. • It consists of an anther and a filament. • All the stamens together make up the androecium (plural: androecia). Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 25 Parts of a Stamen vascular bundle Side view of a stamen Stamen with upper half of anther cut off Stamen Filament The filament is the stalk that holds the anther in a suitable position to disperse the pollen. The anther also contains a vascular bundle. The vascular bundle contains: • phloem which translocates sucrose and amino acids to the developing anther. Sucrose provides the energy for cell division. Amino acids are used for synthesis of proteins and new protoplasm. 22 May 2017 • xylem which transports water and mineral salts to the anther. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 26 Parts of a Stamen pollen sacs vascular bundle pollen grains anther lobes Side view of a stamen Stamen Filament Stamen with upper half of anther cut off Anther The anther consists of two lobes. Each lobe contains two pollen sacs. In the pollen sacs are the pollen grains. The anther produces pollen grains. When the anther matures, it splits open to release the pollen grains. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 27 Parts of a Stamen pollen sacs vascular bundle pollen tube nucleus pollen grains anther lobes Side view of a stamen Stamen Filament generative nucleus Stamen with upper half of anther cut off Anther Mature pollen grain Pollen grains • Pollen grains contain the male gametes, which are produced by meiosis. • The nuclei in pollen grains contain the haploid number of chromosomes. • Each pollen grain has two nuclei, the generative nucleus and the pollen tube nucleus or vegetative nucleus. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 28 • Carpel** (pistil or gynoecium) – Female part of the flower – Consists of: • an ovary, • a style above the ovary • and one or more stigmas • Stigma** – Receives pollen grains – Mature stigma secretes sugary fluid that stimulate the pollengrains to germinate Parts of a Carpel Carpel stigma • The female part of the flower. • It consists of: style - an ovary; - a style above the ovary; and ovary Side view of a carpel - one or more stigmas. • A flower may have one or more carpels. • All the carpels together make up the pistil or gynoecium (plural: gynoecia). Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 30 Parts of a Carpel Stigma • The stigma is a swollen structure at the end of the style. style Carpel ovary • It receives the pollen grains. • The mature stigma secretes a sugary fluid that stimulates the pollen grains to germinate. Side view of a carpel Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 31 Parts of a Carpel Style Stigma • The style is a stalk that connects the stigma to the ovary. • It holds the stigma in a suitable position to trap pollen grains. Carpel ovary Side view of a carpel Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 32 Parts of a Carpel stigma style ovary wall definitive nucleus ovule ovum funicle L.S. of a carpel placenta Ovary • The ovary contains one or more ovules. • Within each mature ovule is a female gamete or egg cell called the ovum (plural: ova) and the definitive nucleus. The ovum is produced by meiosis. It is therefore haploid. • The ovules are attached to a region in the ovary called the placenta. The ovule is attached to the placenta by a stalk called the funicle. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 33 Pollination • Pollen grains must be transferred from the anthers to the stigmas for the fusion of gametes • Can be done by insects or wind • Flowers pollinated by insects possess characteristics distinct from wind-pollinated flowers What is Pollination? • The transfer of pollen grains from the anther to the stigma • If the pollen grains are transferred to a flower in another plant of the same kind, it is called crosspollination Self-pollination • Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower or of a different flower of the same plant Advantages 1. Only one parent plant is required 2. Offspring inherits its genes from the parent plant. Hence good qualities passed down to the offspring 3. High probability of pollination 4. Less pollen grain and energy wasted in self-pollination Disadvantages 1. Less varieties are produced. Hence species is less adapted to environmental changes 2. Continued self-pollination may lead to offspring becoming weaker, smaller and less resistant to diseases Cross-pollination • Cross-pollination is the transfer of pollen grains from one plant to the stigma of a flower in another plant of the same species Advantages of crosspollination 1. Offspring may have inherited good qualities from both parents 2. More varieties produced. This increases the chance of species surviving environmental changes 3. More viable seeds are produced Disadvantages of crosspollination 1. Both parents required 2. Depends on external factors such as insects or wind for pollination 3. Lower probability of crosspollination 4. More energy and pollen grains wasted Self Pollination vs Cross Pollination • Self Pollination results in the production of – offspring that are weaker and – less adaptable to changes in he environment • Cross Pollination may produce – Offspring with valuable qualities – Abundant and more viable seeds – More varieties of offspring The Structure and Pollination of an Insect-Pollinated Flower Structure of Clitoria flower • Belongs to the legume (pea) family External features of Clitoria The Clitoria flower is pollinated by insects. The flower is complete, bisexual and bilaterally symmetrical. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 46 External features of Clitoria pedicel calyx 1 • The calyx consists of five green sepals. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 47 External features of Clitoria pedicel epicalyx calyx 2 • The base of the calyx is enclosed by the epicalyx. The epicalyx consists of two green leaves. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 48 External features of Clitoria pedicel epicalyx calyx corolla • The corolla consists of five brightly-coloured petals 3 of different shapes and sizes. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 49 External features of Clitoria pedicel epicalyx calyx corolla standard petal 4 standard petal • The large, deep blue standard petal has lines on it that guide insects to where the nectar is located. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 50 External features of Clitoria pedicel epicalyx calyx corolla standard petal nectar guide standard petal 4 • The large, deep blue standard petal has lines on it that guide insects to where the nectar is located. • Thus, these lines are called nectar guides. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 51 External features of Clitoria pedicel epicalyx calyx wing petal wing petals corolla nectar guide standard petal standard petal 5 • Two deep blue wing petals are enclosed by the standard petal. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 52 External features of Clitoria pedicel keel petals epicalyx wing petal calyx wing petals corolla nectar guide standard petal standard petal 6 • Two small, yellowish-green keel petals are enclosed by the wing petals. The keel petals enclose the reproductive organs. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 53 Stamens and Pistil of Clitoria Androecium • The androecium consists of ten stamens with long filaments. • The filaments of nine of the stamens are fused along most of their length to form a hollow tube or ‘trough’. This trough encloses the pistil. • Nectar, secreted by the flower, collects at the bottom of the stamen trough. Thus, the nectar can be reached only by an insect with a long tubular mouth or proboscis, for example, a bee or a butterfly. stamen trough Stigma and stamens of Clitoria Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. stigma 22 May 2017 54 Stamens and Pistil of Clitoria Androecium • The androecium consists of ten stamens with long filaments. • The filaments of nine of the stamens are fused along most of their length to form a hollow tube or ‘trough’. This trough encloses the pistil. • Nectar, secreted by the flower, collects at the bottom of the stamen trough. Thus, the nectar can be reached only by an insect with a long tubular mouth or proboscis, for example, a bee or a butterfly. stamen trough Stigma and stamens of Clitoria The tenth stamen is free. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. stigma 22 May 2017 55 Stamens and Pistil of Clitoria Pistil The pistil consists of a single carpel. Section of the carpel of Clitoria Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 22 May 2017 56