sexual reproduction in flowering plants

• A flower is a modified shoot for the sexual reproduction of flowering plant Stamen, Microsporangium and Pollengrain
• Androecium

  Androecium is composed of Stamen. Stamen has two parts - the long and a slender stock called the filament and terminal bilobed structure called anther.
• Anther is bilobed, each lobe have two theca (dithecous) Anther is four-sided tertragonal structure consisting of four microsporangia located at the corners two in each lobe. When anther is young sporogenous tissue occupy the centre of each microsporangium. At maturity pollen sacs a are packed with the pollen grains
• Structure of microsporangia

  Circular in outline, surrounded by four wall layers – the epidermis, endothecium, middle layers and the tapetum. Outer three layers perform the function of protection and help in dehiscence of anther to release the pollen, the innermost layer is tapetum that nourishes the developing pollen grains.
• Figure : (a) Transverse section of a young anther; (b) Enlarged view of one microsporangyum showing wall layers; (c) A mature dehisced anther (check pg 20)
• Structure of pollen grains

  The pollengrain represent the male gametophyte. Spherical shape with 2 layered wall.

  1. Exine

  2. Intine
• Intine is made up of cellulose and pectin. Pollen grain has two cells; vegetative cell and generative cell. Vegetative cell is bigger, has abundant food reserves for developing male gametes.
• Hard outer layer is called exine and inner layer is intine. Exine made up of sporopollenin It can withstand high temperature and strong acids and alkali, no enzyme that degrades sporopollenin is so far known Pollengrains has prominent apertures called germpores where sporopllenin is absent . Pollen grains are well preserved a fossils because of sporopollinin.
• Economic importance of pollen grains Pollengrains are rich in nutrients, used as food supplements, in the form of tablets and syrup. It causes severe allergies and bronchial afflictions in some people. It lead to chronic respiratory disorders, asthma bronchitis eg. parthenium or carrot grass causes pollen allergy.

  Pollen bank : Pollen grains of a large number of species store for years in liquid nitrogen (-196oC: Cyropreservation). It can be used as Pollen Bank in plant breeding programme

  Pollen viability: The period for which pollen grains remain functional
• Pistil,Megasporangium (ovule), Megasporogenesis, Embryo sacs Gynoecium

  Unit of gynoecium is pistil or carpel.

  Carpel has three parts - Ovary, Style and Stigma.

  If gynoecium has a single pistil or carpal is called monocarpellary ovary.

  If gynoecium ha more than one pistil is called multicarpellary ovary.

  When more than one carpel and are fused together are called syncarpous ovary

  When carpelss are free are called apocarpous ovary.
• Structure of ovule or megasporangium

  Ovule is a small structure attached to placenta by a stalk called funicle. The body of ovule fuses with funicle in the region called hilum (hilum is the junction between ovule and funicle)
• Each ovule has one or two protective envelope called integuments.

  Integuments encircle the nucellus except at the tip where a small opening called the micropyle.
• Opposite to the micropylar end is the chalaza. Enclosed within integuments is a mass of cells called nucellus. Nyucellus cell have reserved food materials. In the nucellus there is a single embryo sac
• Structure of mature ovule ‘(imp)
• Megasporogenesis

  The process of formation of megaspore (by meiosis) from the diploid megaspore mother cell ( MMC) in micropylar region of nucellus. Out four megaspores one develop into functional megaspore and three degenerate.
• Embryo sac formation ( free nuclear embryo development ):

  The nucleus of functional megaspore divides mitotically into two nuclei that moves to opposite poles, then 4 nucleate and 8 nucleate ie., not followed immediately by cell wall formation. Later cellwalls are laid down and form embryosac. 6 of 8 nuclei forms to cells and remaining 2 nuclei move towards the centre of the embyosac and are called polar nuclei.
• A diagrammatic representation of the mature embryo sac(imp)
• Three cells at micropylar end is called egg apparatus . It consists of two synertgids and one egg cell. Synergids have special cellular thickening called filiform apparatus which guide the pollen tubes into the synergids three cells at chalazal end are called egg apparatus. The large central cell has two polar nuclei. Thus angiosperm embryosac is said to be 7 celled ans 8 nucleate .
• Pollination : Pollination is mechanism of transfer of pollen grains from the anthe to the stigma of a flower. Based on source of Pollen pollination three types :-

  1) Autogamy :

  2) Geitonogamy :

  3) Xenogamy:
• Autogamy : Transfer of pollen grains from the anther to the stigma of the same flower .

  In a flower which opens and exposes anther and stigma autogamy is rare.

  Autogamy requires

  i) Synchrony : Pollen release and stigma respectively in same time

  ii) anthers and stigma should lie close to each other. In some plants eg. Viola, oxalis, commelina there are two type of flowers. a) Chasmogamous flowers : Flowers which are similar to flowers of other species with exposed anthers and stigma. b) Cleistogamous flowers : Flowers which do not open at all. Here another and stigma lie close to each other.
• Geitonogamy : Transfer of pollen grains from the anther to the stigma of another flower of the same plant. This type of pollination is genetically similar to autogamy
• Xenogamy : Transfer of pollen grains from anther to the stigma of a different plant. During pollination it brings genetically different type of pollen grains to the stigma.
• Agents of pollination :

  1. Wind pollinating flowers :
  2. Water pollinating flowers :
  3. Insect pollination (Entamophily)
• Wind pollinating flowers : Pollen grains are light and non sticky, well exposed stamens, so that pollens are easily dispersed. Large feathery sigma, single over in each ovary, numerous flowers packed into inflorescence.Example:- coconut,rice,wheat,grasses,maize,datepalam.
• Water pollinating flowers : a) In water hyacinth and water lily flowers above the level of water and are pollinated by insects or wind. b) In Vallisneria and Hydrilla the female flowers reach the surface of water by long stalk and pollen grains are released on to the surface of water and they are carried passively by water currents. c) In seagrasses ( eg. Zostera) female flowers remain submerged in water and the pollen grains are released inside the water.
• Characters of water pollinating flowers

  1. Pollen grains are long ribbon shaped and are carried passively inside the Water
  2. Protected from wetting by a mucilaginous covering
  3. Flowers are unisexual and nectorless
  4. Stigma is sticky and unwettable
• Insect pollination (Entamophily)

  1. Flowers are large and colourful
  2. Produce rich nectar.
  3. Commonly produce fragrance
  4. Pollen grains are usually sticky and spiny
• Pollen release and stigma receptivity are not synchronized, meaning pollen is released before the stigma becomes receptive or the stigma becomes receptive before the release of pollens.
• Anther and stigma are placed at different positions, preventing pollen from coming in contact with the stigma of the same flower.
• Self incompatibility is a genetic mechanism that prevents self pollen of the same flower from fertilizing the egg by inhibiting pollen tube growth in the pistil.
• In castor and maize, male and female flowers are present on the same plant, preventing autogamy but not geitonogamy.
• In papaya, male and female flowers are present on different plants, preventing autogamy and geitonogamy.
• filiform apparatus

  The synergid cells in embryo sac have special cellular thickenings at micropylar tip is called filiform apparatus. It play an important role in guiding the Pollen tubes into the synergids.
• Artificial hybridization : It is the process of crossing different species or genera having desirable characters to combine and to produce commercially superior varieties. It is important that only desired pollen grains are used and stigma is protected from contamination ( from unwanted pollens ). It has two techniquet – Emasculation and Bagging

  1) Emasculation : The process of removal of anther from the flower buds of female parent before anther dehisces. Emasculation prevent self pollination

  2) Bagginng : Emasculated flower is covered with butter paper bags.
• Double fertilization :

  Syngamy : One of the male gamete moves to egg cell and fuse with its nuclear is called syngamy. It results in diploid zygote.
• Triple fusion : The other male gamete move towards the two polar muclei in central cell and fuse to produce triploid primary endosperm nucleus (PEN) is called triple fusion. This two type of fusion syngamy and triple fusion takes place in embryo sac is termed as double fertilization. It is an event unique to flowering plants. The central cell after triple fusion become primary endorsperm cell (PEC) and develop into endosperm. Zygote developos into embryo.
• Endosperm development : PEN undergoes successive nuclear division to give rise to free nuclei is called free nuclear endosperm. Later cell wall formulation occurs and endosperm becomes cellular. The coconut water from tender coconut is nothing but free nuclear endosperm, made up of thousands of nuclei and surrounding white kernel is the cellular endorsperm
• Embryo development ( embryogeny) : Embryo developes at micropylar end of the embryo sac. Endosperm provide nutrition to the developing embryo. Though early stage of embryo development are similar in monocots and dicots their seeds differ greatly.
• Four stages of embryo development : Pro embryo, Globulr embryo, Heart shaped embryo and Mature embryo.
• Strucure of Dicot embryo A Dicot embryo consists of an emgryonal axis and two cotyledons, the portion of embryonal axis above cotyledons is the epicotyl and portion below the level of cotyledon is hypocotyl. Epicotyl ende in plumule or stem tip and hypocotyl end in the radical or root tip. Root tip is covered with root cap.