SEXUAL REPRODUCTION IN FLOWERING PLANTS

                                                                                                                                                                                                                                                                 Flowers:

Flowers are modified shoots, with a condensed axis known as thalamus and bearing appendages called floral organs. They are meant essentially for sexual reproduction.

A typical flower is hermaphrodite, i.e. bearing fertile organs of both male and female. The stalk that bears the flower is condensed, so that the internodes become shorter and the nodes come to lie close to each other. The tip of the stalk or receptacle is swollen; and bears four different organs; from inside out: Sepals (calyx), Petals (corolla), Stamens (Androecium) and Carpels (gynoecium). These floral whorls are modified leaves.

The stamens (Microsporophyll) and carpels (megasporophylls) are essential parts of flower. They are directly involved in the process of reproduction. The remaining two floral whorls sepals and petals are non essential and aid in the process of reproduction.

Figure : Parts of a flower

The table describes the main parts of a flower and their functions:

Structure	Function
Sepals	Protect the unopened flower
Petals	May be brightly coloured to attract insects
Stamens	The male parts of the flower (each consists of an anther held up on a filament)
Anthers	Produce male sex cells (pollen grains)
Stigma	The top of the female part of the flower which collects pollen grains
Ovary	Produces the female sex cells (contained in the ovules)
Nectary	Produce a sugary solution called nectar, which attracts insects

TERMS RELATED TO FLOWER

·         Bracts: Bract is modified, usually reduced leaf like structure present beneath flower or inflorescence. Flower with bracts is called bracteate flower and without bracts are known as ebracteate. Bracts protect the flower in bud condition.                  

·         Based on attachment:

 Pedicel: Stalk of an individual flower

Peduncle: stalk of an inflorescence

Sessile: If pedicel absent

·         Complete/Incomplete Flower:

Complete: A flower with 4 whorls of floral parts; pistil, stamens, sepals and petals

Incomplete: A flower lacking one or more of the four kinds of floral whorls sepals, petals, stamens or pistils.

·         Floral Symmetry:

Actinomorphic/ Regular: flowers in which each whorl s similar in shape, and the flower can be divided in to two halves along more than one median longitudinal plane

Zygomorphic/ Irregular: flower in which members of some or all of the floral whorls are unequal. The can be divided longitudinally in to two equal halves in only one vertical plane.

Figure : On the basis of symmetry; a. actinomorphic; b. zygomorphic

·         On the basis of reproductive organs:

Hermaphrodite or Bisexual: Both stamens (male reproductive organ) and pistil (female reproductive organ) present

Unisexual: flower with reproductive organ of only one sex. They can be of two types based on types of reproductive organs present:

·         Staminate

·         Pistillate

·         Based on the number of floral parts

Dimerous/ Trimerous/ Tetramerous/ Pentamerous

·         Based on Position of Floral organs on Thalamus:

·         Hypogynous: A flower in which ovary is superior and all floral organs are below the level of ovary

·         Perigynous: Ovary at same level as remaining floral organs.

·         Epigynous: Ovary inferior to remaining floral organs.

Figure 

Smallest Flower: Wolffia microscopic

Largest Flower: Rafflesia

National Flower of India: Lotus (Nelumbium)

Pre Fertilization Changes:

The decision to flower occurs much before the actual flowers are seen.

Flowers originate from shoot meristem, which normally generate leaves and shoots. Several hormonal and structural changes are started that lead to differentiation and further development of floral primordium.

The meristem stops vegetative growth when flowering begins and either produces a single flower or a group of flowers. Inflorescences are formed first which then bear floral buds that from flowers. The flowering process involves two steps:

·         Induction or the change in form and function of the vegetative meristem to develop a flower

·         Evocation: Development of floral meristem

The floral meristem, like shoot meristem is divided in to two layers; Tunica and corpus.

In the flower, male (androecium) and female (gynoecium) reproductive structures develop.

STAMEN, MICROSPORANGIUM & POLLEN GRAIN

Stamen in a typical flower comprises of two parts:

·         Filament: long and slender stalk

·         Anther: upper broad, bilobed structure

The lower end of filament is attached to the thalamus or the petal of the flower.

The number and length of stamens is different in different species.

Figure 2.2

Anther Structure:

·         Tetragonal

·         Bilobed or made of two anther lobes that are connected by a strip of sterile part called connective.

·         Dithecous or each lobe is made of two theca.

Each theca contains 1 microsporangia. Thus, each anther lobe is dithecous with 2 microsporangia in each. The total number of microsporangia in each anther is 4.

The microsporangia develop in to pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains.

Structure of Microsporangium:

A transverse section of a microsporangium appears almost circular. It is surrounded by 4 wall layers:

·         Epidermis

·         Endothecium

·         Middle layers

·         Tapetum

The outer three layers are protective and help in dehiscence of anther to release pollen grain.

The fourth layer, tapetum provided nourishment to the developing pollen grain. The cells of tapetum are generally multinucleate or may have large polyploidy nucleus.

Development of Microsporangium (Microsporogenesis)

The process of formation of microspore from microspore mother cell through meiosis

It involves the following steps:

·          A young anther has groups of homogenous cell. They are compactly arranged in the centre of each microsporangium and are known as sporogenous tissues.

·         Each of the cells of sporogenous tissue is capable of forming microspores, and hence is also known as potential pollen or micropsore mother cell (PMC)

·         As anther develops, cells of sporogenous tissue develop in to loosely arranged microspore mother cells or Pollen mother cells (PMC).

·         The PMCs undergo meiotic division to form microspore tetrads.

·         The microspore, as they are formed are arranged in a cluster of four cells – microspore tetrad.

·         As the anthers mature and dehydtrate,the microspores dissociate from each other and develop into pollen grains.

Thus inside each microsporangium several thousand microspores or pollen grains are formed.

Pollen Grain

The study of pollen grains is known as Palynology

A pollen grain is a microscopic body that contains the male reproductive cell of a plant. It is crucial in a plant's fertilization process.

·         Pollen grains develop from microspore and represent the male gametophyte

·         Typically haploid, unicellular with single nucleus

·         Usually spherical, measuring 25-50 micrometers in diameter

Shape Colour and Size: Pollen grains of various species can vary quite a lot in size (from about 10 to nearly 100 micrometer; exceptions are the thread-shaped pollen grains of some eelgrass). They also vary in shape: round, oval, disc or bean-shaped and sometimes filamentous. The natural color is mostly white, cream, yellow or orange. The texture of the cell wall shows also great variations, from smooth to spiky.

Structure:

Prominent two layered wall

·         Exine: Outer, hard layer. Made of sporopollenin which is one of the most resistant organic materials known. It is resistant to strong acids, alkali and high temperatures. No enzyme that degrades sporopollenin is so far known. Pollen grains are well preserved as fossils due to sporopollenin.

Exine has prominent apertures where sporopollenin is absent and the exine is thin. These are known as germ pore when they are circular and germ furrows when they are elongated.

·         Intine: Inner wall of pollen grain. It is a thin and continuous layer made of cellulose and pectin.

Cytoplasm

·         Rich in starch and unsaturated oils

·          Surrounded by a thin plasma membrane

Pollen Germination / Development of Male Gametophyte

Male gametophyte begins to develop while the pollen grain is still in the microsporangium or pollen sac. This type of germination is known as precocious germination.

Microspore undergoes only 2 mitotic divisions:

·         1^st : forms a larger vegetative cell and a smaller generative cell

·         2^nd:  The generative cell divides to form two male gametes. Thus a three cell stage is formed.

The 2^nd division may occur after pollination i.e. during the growth of pollen tube into stigma. In such plants the pollen is shed at 2 cell stage. This occurs in 60% of the plants. In some other plants the male gametes are formed prior to pollination and the pollen is shed at 3 cell stage.

The vegetative cell is bigger, has abundant food reserve and has large irregularly shaped nucleus.

The genitive cell is smaller, floats in cytoplasm of vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus.

Post-pollination changes occur in the pollen once it lands on the stigma:

·         Pollen grain absorbs water and nutrients from the stigma secretions through its germ pores.

·         Intine protrudes out through one of the germ pores or through a germ furrow.

·         Generative nucleus divides to form two male nuclei which appear as distinct male gametes.

·         Protruded intine forms pollen tube.

Pollen grain must remain viable when they fall on stigma. The period of viability of pollen grains is variable. It depends on temperature, and humidity. In certain plants it loses viability within 30 minutes e.g. cereals such as rice and wheat. In some others such as members of leguminoceae, Solanaceae, Rosaceae; they maintain viability for months. Like semen, it is possible to store pollen grains of large number of species for years in liquid nitrogen at -196^degC. Such stored pollen can be used as pollen banks similar to sperm banks for plant breeding programmes.

Harms due to Pollen

Pollens of many species cause allergies and bronchial problemsin some people leading to respiratory disorders such as asthma, bronchitis etc. e.g. Parthenium or carrot grass that came to India as contaminant with wheat, has spread everywhere and is responsible for pollen allergy.

Uses of Pollen

Rich in nutrients hence used as food supplements in form of pollen tablets and syrups. Available in these forms in markets, in western countries. Consumption of pollen has been shown to increase performance of athletes and race horses.

Pistil, Megasporangium (Ovule) and Embryo Sac

Pistil

Gynoecium or pistil represents the female reproductive part of the flower. It is composed of 1 or more carpels/pistil.

If single pistil: monocarpellary

If more than 1 pistil: multicarpellary

Multicapellary with all pistils fused: syncarpous e.g. Petunia, Papaver

Multicarpellary with all pistils free: apocarpous e.g. Ranunculus, Aconitum

Parts of Pistil: 3 distinct parts:

·         Stigma

·         Style

·         Ovary

Pistil is a highly modified leaf folded along the midrib. A cushion like ridge develops on the ventral side of modified leaf that forms the placenta on which ovules are borne.

Stigma: Upper generally knobbed. Stigma serves as landing platform for pollen garins

Style: elongated slender part. Raises the stigma to expose it, and thus increases the chances of pollination.

Ovary: Basal bulged part of pistil. Inside the ovary is the ovarian cavity or locule. Depending on the number of locules or chambers, the ovary is known as unilocular, Bilocular, trilocular etc.

Placenta located on the inside of ovary cavity produces different types of placentation, depending on the attachment of megasporangium (ovule) inside the ovarian cavity.

Number of ovules in the ovarian cavity may be 1 (e.g. wheat, paddy, mango); or many (papaya, water melon, orchids)

Megasporangium (Ovule):

Ovule is small structure attached to the placenta by means of a stalk called as funicle.

The point on the ovule where funicle is attached is known as Hilum. Hilum represents the junction between ovule and funicle.

Each ovule is enclosed in 1 or 2 protective envelops known as integuments.

The integuments leave a small opening called micropyle at the tip of the ovule.

Opposite the micropyle end, is the chalaza, which represents the base of the ovule. The integuments arise from chalaza.

Inside the integuments is a layer of parenchymatous tissue known as nucellus, which is nutritive in function. The parenchyma cells of nucellus are rich in reserve food materials.

Depending on the nature of nucellus and integuments, ovule is classified in the following groups:

On the basis of type of Nucellus: Tenuinucellate (nucellus thin), & crassinucellate (Nucellus thick)

On the basis of integument number: Unitegmic ( Ovule with 1 integument); and Bitegmic (ovule with two integuments)

On the basis of relationship of funicle with ovule and the orientation of the ovule; ovule is classified as follows:

Orthotropous or upright ovule

Anatropous or inverted ovule.

Besides these, there are variously inclined ovules such as campylotropous, amphitropous etc.

Inside the nucellus is the embryo sac or femnale gametophyte. An ovule ususally has only one embryo sac formed from 1 megaspore through reduction division.

Megasporogensis:

Porcess of formation of megaspore from megaspore mother cell (MMC)

In the nucellus, towards the micropylar end, [1]a distinct microspore mother cell is differentiated. It is a large cell contaning dense cytoplasm and distinct nucleus.

The MMC undergoes meiosis to form four haploid megaspore. Of the four megaspores, only one (the megaspore at the chalazal end) remains as the functional megaspore. The remaining three degenerate.

With the formation of  haploid megaspore, the sporophyte phase of plant’s life cycle ends and the gametophyte phase begins.

Development oif female gametophyte:

The functional megaspore develops in to the female gametophyte (embryo sac). This method of development of embryo sac from a single megaspore is known as monosporic development.

The nucleus of megaspore divides mitotically in to 2 daughter nuclei. These 2 move at opposite poles of embryo sac. The next mitotic division leads to formation of 4 nucleate embryo sac and later 8 nucleate stage. These mitotic divisions are free nuclear, i.e. the karyokinesis is not followed by cytokinesis.

One nucleus from each pole migrates to the centre of cell to form two polar nuclei. At this stage, cell walls are formed leading to formation of typical female gametophyte or embryo sac.

A typical female gametophyte contains 8 nuclei:

• 3 at the chalazal end: antipodals. They provide polarity to embryo sac.
•  3 at the micropylar end: egg apparatus comprising of 2 synergids and 1 egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform appartus, which play an important role in guiding the pollen tubes inside the synergids.
• 2 at the centre: Polar nuclei

The nuclei at the two poles form their own cell walls; while the two at the centre or polar nuclei lie in the large central cell.

Thus a mature female gametophyte (embryo sac) is 8 nucleate and 7 cell structure.

POLLINATION

Transfer of pollen grain from anther to stigma.

Both male and female gametes in angiosperms are non motile in angiosperms, thus plants have developed a number of adaptations to enable pollination. They use a number of external agents for this purpose.

Types of Pollination

On the basis of source of pollen, pollination can be of many types:

              I.      Plants are said to be self pollinated when the pollen is transferred from anther to stigma of same flower or of another flower on the same plant.

Self pollination can be of following types:

1. Autogamy:  Pollination is achieved within the same flower. Thus the flower in this case is bisexual.

For a successful pollination through autogamy, ecrtain conditions must be met:

• Homogamy or the anther and stigma in bisexual flower should attain maturity at the same time. Pollen maturity and stigma receptivity should occur at the same time.
• The anther and stigma should lie close to each other.

Even if these conditions are met, autogamy is not an exclusive (only) method of pollination in bisexual flower. There is always the possibility of stigma receiving pollen from another flower of same plant or of another plant; since the flowers are open and stigma is exposed.

Certain plants such as Viola (common pansy), Oxalis, Commelina; ensure autogamy as the only method of pollination by producing two types of flowers:

ü  Chasmogamous Flowers or the flowers that are normal bisexual, with stigma exposed and anther avaiable for dispersal by external agencies.

ü  Cleistogamous Flower or the flowers that do not open at all, so that the anthers and stigma remain enclosed within the flowers and are not at all exposed.

In such flowers anthers and stigma lie close to each other so that when the anther dehisces, mature pollen grain is released within the flower and directly comes in contact with its own stigma. Thus pollinbation is achieved.

In absence of exposure, cleistogamous flowers are essentially autogamous. They are also able to produce seed essentially, even in absence of an external pollinator.

2. Geitonogamy: Transfer of pollen grain from stigma to anther of another flower on the same plant.

Gitonogamy exhibits characters similar to both autogamy and xenogamy.

It is similar to xenogamy since it requires agents of pollination

It is similar to autiogamy genetically; since pollen grains and stigma belong to same plant; resulting in similar gene combination in seed.

            II.      Cross Pollination refers to transfer of pollen from anther of a flower to stigma of a flower of another plant. It is also known as Xenogamy. In this type of pollination, pollen and oule of geentically different types are brought together.

Agents Of Pollination

2 abiotic agents: wind and water

1 biotic agent: animal

Adaptations in flowers for Pollination

I. Wind Pollination

pollen grains :– light, non- sticky, winged

anther :- well exposed 

stigma :- large and feathery

flower :- one ovule, arranged as inflorescence

         e.g. corn cob, cotton, date palm

II. Water Pollination
                   - Bryophytes, Pteridophytes, Algae

pollen grains : protected by mucilaginous covering

          e,g, : Fresh water plants- Vallisneria, Hydrilla
                 Sea grass- Zostera 

Main features of wind and water pollinated plants
         - produce pollen grains in large no.
         - do not produce nectar

III. Insect Pollination
       - Flowers : large, colourful, fragrant, rich in nectar
       - Pollen grains : sticky
       - Stigma : sticky

Certain rewards to pollinators:

nectar and (edible) pollen grains as foods

 provide safe place for laying eggs

Ex : Amorphophallus, Yucca

Outbreeding Devices

Continued self - Pollination leads to inbreeding depression. Plants develop various mechanisms to avoid self pollination and ensure cross pollination.

Ways to avoid Self-pollination :
(i) Pollen release & stigma receptivity – not synchronized

Protandry: Pollen matures first

Protogyny: Ovule matures first
(ii) Stigma and anther – placed at different positions
(iii) Self-incompatibility: genetic mechanism that prevents self pollen from fertilizing the ovules by inhibiting pollen germination or growth of pollen tube in pistil.
(iv) Production of unisexual flowers

       Eg: castor, maize (prevents autogamy)
              papaya (prevents autogamy & geitonogamy)

Pollen – Pistil Interaction
All events – from deposition of pollen on stigma till the pollen tube enters the ovule are called Pollen-pistil interaction.
•        Recognition of compatible pollen
•        Germination of pollen grains
•        Development of Male Gametophyte

Artificial Hybridization

Crossing different varieties of species. Hybrid individual with desirable characters of the parent plants are produced. This involves:

• Collection of desired pollen grains for pollination
• Stigma protected from contamination

Emasculation : removal of anther

Bagging : flower covered- bag made up of butter-prevent contamination of stigma from unwanted pollen

Bagged flower- attains receptivity - mature pollen grains- dusted on the stigma – rebagged - fruits allowed to develop

Fertilization

The process of fertilization was discovered by Strasburger in 1884. After pollination the intine of pollen grain forms pollen tube through weak areas on exine (germ pore). The growth of pollen tube is stimulated by the sugary substances produced in stigma.

The pollen tube with two male gametes and tube nucleus runs through the style and finally turns towards the micropylar end of the ovule in the cavity of ovary. The length of pollen tube depends on the length of styles.

On piercing the nucellus, the pollen tube penetrates the embryo sac. Its tip penetrates in the embryo sac and reaches the egg apparatus passing either between the egg and synergids or between one synergid and wall of the embryo sac. Ultimately, the tip of the pollen tube bursts and two male gametes are discharged.

The tube nucleus disorganises before bursting of pollen tube. One of these male gametes fuses with the egg cell or oosphere (syngamy) causing fertilization, as a result of which diploid oospore or zygote is formed. The other gamete fuses with the secondary nucleus (triple fusion) forming the triploid endosperm nucleus which later on gives rise to endosperm. 

Thus the process of fertilization which occurs twice in the same embryo sac at a time by the two male gametes (syngamy and triple fusion) is called double fertilization. The process of double fertilization was discovered by S.G. Nawaschin (1897) in Lilium and Fritillaria species

Double Fertilisation / Syngamy 

Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms). This process involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm).
       - pollen tube releases male gametes into synergids
       - fusion of 1 of male gametes and egg cell
       - fusion of 2nd male gamete and polar nuclei =Triploid endosperm nucleus- PEN (Triple Fusion)
       - PEN – now called Primary Endosperm Nucleus – Endosperm
Significance of double fertilization:

Double fertilization is found in angiosperms only. In angio­sperms, female gametophyte abruptly stops its growth at 8 nucleate stages. Further growth of embryo sac occurs only when the zygote has been formed and primary endosperm nucleus has been created by triple fusion.

The triple fusion initiates the formation of endosperm. The endosperm is formed only when it is needed. The need arises after fertilization because the endosperm provides nutrition for the simultaneously developing embryo.

If fertilization fails no endosperm will be formed. Thus, there will be wastage of energy in the development of endosperm. There is no such provision in gymnosperms. There is, therefore, no wastage of energy on this account in angiosperms.

Post- fertilization Events      
All events that occur in a flower, after double fertilization is called Post- fertilization events

Major events are :
   (i)            Development of endosperm
  (ii)             Development of embryo
 (iii)             Maturation of ovule into seed
 (iv)             Maturation of ovary into fruit

Endosperm
Endosperm development precedes embryo development since endosperm accumulates food and acts as source of nutrition to the developing embryo.
†     Two types of endosperm development :
          (i) Free nuclear type (common method) during which few initial nuclear development is not followed by development of cell wall.

The nuclei so formed either remain free in the cytoplasm of embryo sac or form walls later.

In coconut the cell wall formation of endosperm never occurs. The tender coconut water is thus thousands of nuclei present in the surrounding kernel, which is cellular endosperm.
          (ii) Cellular type: Cytokinesis follows each nucleus division.

†      Cells of endosperm– store food materials- used for developing embryo
†      Non - Albuminous / Non-Endospermic seeds- endosperm completely utilized - before maturation of seeds. Ex: pea
†      Albuminous / Endospermic seeds- a portion of endosperm remain in mature seeds. Ex: castor

Embryo
†     Embryogeny – early stages of embryo development
†      Zygote   →  Proembryo   →   Mature embryo (globular/heart-shaped)

Embryo develops at the micropylar end of the embryo sac.

Early stages of embryo development is similar in both monocots and dicots.

Embryo consists of:
                 - embryonal axis
                 - cotyledons
                 - plumule
                 - radicle
 The portion of embryonal axis above the cotyledons is known as epicotyle. This forms plumule or stem tip.

The portion of embryonal axis below the cotyledons is known as hypocotyle. This forms radicle or root tip. The root tip is covered by root cap.
Monocotyledonous Seed: Has only one cotyledon or scutellum.
-          Scutellem = Cotyledon. Present on lateral side of embryonal axis.
-           Coleorrhiza: undifferentiated sheath covering radical & root cap
-           Coleoptile: sheath covering plumule 

Seed
-          Fertilized and mature ovule develops into seed.

Seed consists of:
-          cotyledon(s)
-          embryonal axis
-          Integuments of ovary harden as tough protective seed coat which is double layered and formed by integuments. The integuments are:     

• Testa (outer coat)
• Tegmen (inner coat)

-          Micropyle: - small opening on seed coat, it facilitates entry of H2O & O2 into seeds (for germination)
-          Hilum:- scar on seed coat
-          Seed     - Albuminous / Non-Albuminous 

Non albuminous seeds do not have residual endosperm. The endosperm is completely consumed during embryo development. E.g. pea and groundnut

Albuminous seeds retain part of endosperm as some of it is only used during embryo development e.g. wheat, maize barley etc.
-          Perisperm : remnants of nucellus that is persistent. Ex: Black pepper
-          Dormancy:  state of inactivity

Advantages of Seeds

To plants

    (i) Seeds - reserve food materials- nourish seedling
    (ii) Seed coat- protection to young embryo
    (iii) Seeds of large no of species –live for several years
    (iv) Seeds - better adaptive strategies- dispersal to new habitats- better survival

To mankind

      (i) used as food - throughout the year
      (ii) seed - basis of agriculture

Fruit

As ovule matures in to seeds, ovary develops in to fruit. The wall of ovary develops in to fruit wall or pericarp.
-          True fruit : - Fruit formed from the ovary
-          Parthenogenesis:  If ovary transform to fruit without fertilization. Ex : Banana
-          Parthenocarpy – induced with gibberellins & auxins without fertilization.
-          False fruit: any part other than ovary- forms the fruit. Ex: Apple


Apomixis & Polyembryony
Other modes of reproduction

Apomixis
-          Form of asexual reproduction- mimics sexual reproduction- seed formed without fertilisation
-          Formation of apomictic seeds :
·         diploid cell (formed without meiosis) - develop into embryo without fertilization
·         cells of nucellus (2n) surrounding embryo sac- protrude into embryo sac - develop into embryos. Ex. Citrus and Mango.

Polyembryony
-          Occurrence of more than one embryo in a seed
-          Often associated with apomixes.  Ex: Citrus, groundnut

Hybrid seeds: cultivation of hybrid seeds is preferred for increased productivity, rich nutrition, better quality and disease resistance. But one of the major problems of cultivation of hybrid seeds is that if seeds obtained from hybrid plants are sown, plants in progeny will segregate and do not maintain the hybrid characters. Thus cost of cultivation from hybrid seeds is very high.

But if apomictic plants are grown from hybrid seeds, the hybrid characters are preserved and the farmer can grow hybrid plants repeatedly year after year.

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[1] In the nucellus, towards the micropylar end, a distinct hypodermal, archesporial cell is formed. It undergoes periclinal division to form outer primary parietal cell and inner primary sporogenous cell. Primary parietal cell divides to form parietal tissue of nucellus and primary sporogenous cell forms megaspore mother cell.