SEXUAL REPRODUCTION IN FLOWERING PLANTS

 CHAPTER 2 – SEXUAL REPRODUCTION INFLOWERING PLANT  

 

FLOWERS : modified shoot, Site of sexual Reproduction. Male and female reproductive organs are borne on flowers

PARTS OF A FLOWER: Four whorls-

·    Calyx (sepals): green in colour, protects the bud.

·    Corolla (petals): colourful, attracts insects for pollination.

·    Androecium (Male reproductive organ),

·    Gynoecium (Female reproductive organs) Calyx & corolla: accessory whorl.

Androecium & gynoecium: Essential whorl.

Male Reproductive Organ

·       Androecium consists of Stamens.

·       Stamen consists of anther, filament & connective (when anther is bilobed)

·       Anther: bilobed (two theca) has 4 microsporangia.

MICROSPOROGENESIS: The process of formation of micro spores from pollen mother cell (2n) through meiosis.

Microsporangium is 4 layered:

Epidermis: single outer layer Endothecium: dehiscence of anther Middle layer: 2-4 layered, crushes when sporogenous tissue mature. Tapetum: dense cytoplasm & multinucleate, Nourishes the developing pollen grain & formation of pollen wall,

Microspore mother cell (2n) Meiosis Microspore (n)  Mitosis Pollen grains (n)

 

Pollen grains: Male gametophyte, size-25-50 µm. Two layered: -

·       Exine: Exine is made of sporopollenin. (Hardest natural substance).

·       Intine: Intine is made of cellulose and pectin.

Pollen Maturation: Mature pollen grains have two cells large vegetative cell & small generative cell.

Generative cell forms two male gametes by mitotic division.

Pollen grains shed in 2-celled / 3celled stage

(See Fig 2.5 a and b page 23 NCERT)

 

Gynoecium / carpel (the female reproductive organ)

(Structure of anatropous ovule) / Megasporangium

·       Apocarpus : free carpel

·       Syncarpus : fused carpel

·       Monocarpellary: single carpel

·       Multicarpellary : Many carpel.

·       Each Carpel consists of ovary, style & stigma.

·       Ovules are attached to ovary by placenta.

·       Funicle – stalk of ovule

·       Hilum, a region where funicle is attached

·       Integuments –cover embryo sac.

·       Micropyle – a pore for entry of pollen tube.

·       Embryo Sac: Female gametophyte

·       Nucellus: it covers embryo sac, seed up to maturation.

Megasporogenesis-

·    The formation of megaspore from the megaspore mother cell –MMC (2n) is called megasporogenesis. ·    In megaspore tetrad, 3 degenerate & one functional megaspore develops into female gametophyte (embryo sac). ·    Embryo sac (Polygonum type) - 8 nucleate & 7 celled. ·    Synergid cell have special cellular thickening at the micropylar tip called filiform apparatus, which play an imp. role in guiding the pollen tube into the synergid.

MEGASPOROGENESIS

 

POLLINATION– transfer of pollen from anther to stigma. Agents of pollination –air, water, insect, bat, bird, man.

Type of pollination-

a. Self-pollination (Autogamy)- Auto gamy (Same flower), Geitenogamy Different flowers same plant

b. Cross pollination (Hetrogamy) -Xenogamy (different flowers on different plants of the same species)

Autogamy: Chasmogamous flower-exposed anther and stigma. E.g.- Viola, Commelina, Oxalis. Cleistogamous flower: anther and stigma-closed. It assured seed set in the absence of pollinator.

 e.g.,- Viola, Oxalis.

Adaptation for pollination

 

Wind pollination	Water pollination	Insect pollination
Pollen grains – light, non-sticky, colourless, well exposed stamens, feathery stigma & numerous flower.	Pollen grains protected by mucilaginous covering, long stalk & long coiled style.	Colorful & showy flower, high fragrance, produce nectar, sticky pollen grains & stigma.

 

Special type of Pollination:

 A. Amorphophallus (flower height-6 feet), provides safe place for lay eggs of insects.

 B. A moth deposits eggs in the locule of ovary of Yucca plant & inturn pollinates Yucca.

 

Out Breeding Devices: to promote cross pollination because self-pollination results in inbreeding depression.

·       Heterostyly: Different size of style & stamens eg. Primula (Pin & Thrum flowers)

·       Herkogamy: Anatomical barrier eg. Calotropis procera.

·       Self-Sterility: Pollens do not germinate on stigma of self-flower eg. Malva

·       Protandry: anther mature before pistel eg. Hibiscus rosa sinensis

.·       Protogyny: Pistil matures before anther eg. Aristolochia

Self-Incompatibility: Pollen & stigma of same flower & same plant rejects to each other .

Pollen Pistel Interaction: Stigma have the ability to recognize the right type of pollen it rejects the pollen grains of other species & also the incompatible pollens of the species.

Artificial Hybridization: it is a major approach to crop improvement programme. It is achieved by -

i)  Emasculation: Removal of anthers from the flower bud of a bisexual flower before the anther dehisces using a pair of forceps.

ii)  Bagging - covering the emasculated flowers with a bag of suitable size to protect them from contamination with unwanted pollen.

iii). Rebagging : Mature pollen grains collected from anther of the male plant & dusted on stigma, and the flowers are rebagged and the fruits allowed to develop.

Double fertilization

     Fertilization is the process of fusion of male & female gametes (n+n) to form a diploid (2n) zygote.

First fertilization: Fusion of male gamete with egg is called SYNGAMY & develops embryo (2N) Second fertilization: Fusion of polar nuclei with 2^nd male gamete is called TRIPLE FUSION & develops PEN (3N).

Since two types of fusions, syngamy & triple fusion takes place in an embryo sac the phenomenon is termed as double fertilization.

C. Post fertilization changes:

1.   Endosperm: 3 type of endosperm development

i). Nuclear: PEN successive nuclear divisions to give rise free nuclei

    eg. Coccus , Areca catechu (betal nut).

    ii). Cellular: Wall formation occurs after every division of PEN eg. Annona squamosa, Adoxa.

iii)  . Helobial : both type of development is present (nuclear & cellular) eg. Monocots.

Embryo development:

1.  Zygote divides by mitosis into suspensor & embryo cells 2.    Suspensor cell forms a globular basal cell which remains embedded in the endosperm & a multicellular suspensor bearing the embryo 3.Globular embryo becomes heart-shaped & then mature embryo with radicle, plumule & Cotyledons. Scutellum : Monocot embryo
DICOT EMBRYO	MONOCOT EMBRYO GRASS

Difference between:

EPICOTYL	HYPOCOTYL
·       Area of embryo axis between the plumule & cotyledonary node. ·       In hypogeal germination epicotyls elongates & seeds remain underground.	·       Area of embryo axis between the radicle & cotyledonary node. ·       In epigeal germination, hypocotyls elongate & push the seed out of soil.

 

COLEOPTILE	COLEORHIZA
·       Covering of plumule in monocots. ·       Coleoptile breaks grain covering and elongates. ·       It turns in green. ·       It protects plumule during emergence from soi.	·       Covering of radicle in monocots. ·       It breaks grain covering but stop further growth ·       It remains inside soil. ·       No such function.

 

PERISPERM	PERICARP
·       It is a part of seed. ·       It is Remnant of nucellus. ·       Dry ·       No significant importance ·        Eg. Black pepper, beet	·       It is a part of fruit. ·       It is ovary wall (epicarp, mesocarp & endocarp). ·       Dry / fleshy ·       Protection, dispersal and nutrition of seed. ·        Eg. Mango

TRUE FRUITS	FALSE FRUITS
True Fruit develops only from the ovary, e.g. mango, tomato	False Fruit develops from parts of the flower other than the ovary e.g. apple, peach etc

 

Albuminous Seed	Non albuminous seed
·       Seed retains endosperm as it is not completely used up during embryo development. ·       Eg. Wheat, maize, barley, sunflower, castor.	·       Seeds do not retain endosperm as it is completely utilized during embryo development. ·       Eg. Pea, groundnut.

 

Apomixis	Parthenogenesis
·  It is a asexual reproduction which mimics sexual reproduction where seeds are formed without fertilization from diploid egg cell and other cells of nucellus.	·       Development of haploid female gamete (egg cell) into an adult without fertilization.

 

Significance of fruit formation:

·       The fruits protect the seeds from unfavorable climatic conditions.

·       Both fleshy and dry fruits help in the dispersal of seeds to distant places.

·       source of many chemicals like sugars, protein, oil, organic acids, vitamins and minerals.

·       provide nutrition to the developing seedlings.

Importance of Apomixis:

Since apomictic seed are produced asexually so they maintain their agronomic characters. So they are used in hybrid seed industry, floriculture, horticulture.

Polyembryony- Occurrence of more than one embryo in a seed is called polyembryony. It is due to formation of more than one egg in embryo sac, formation of more embryo-sac, synergid cell, integument and nucellus cells may also develops into embryo. e.g., Orange, lemon, groundnut, mango, onion etc

Click here to see important Key points of Sexual reproduction in flowering plants

REPRODUCTION IN ORGANISMS

 REPRODUCTION IN ORGANISMS

§  Life Span :Period from birth till natural death. Every organism live only for a certain period of time. 

Reproduction –Producing young-ones of their kind, generation after generation. Two types:

·  Asexual reproduction :single parent capable of producing offspring. also known as somatogenic reproduction

·   Sexual reproduction :two parents are involved in producing offspring.

Asexual Reproduction	Sexual Reproduction
Single parent involved	Two parents involved
Fusion & formation of gametes is absent	Fusion & formation of gametes is present
Only mitotic divisions	Involves both (mitosis and meiosis)
Progeny genetically identical (clones)	Progeny with variations (Offsprings.)

Modes of Asexual Reproduction:

§ Binary fission: parent body divides into two halves, genetically identical to parent. Eg.-Amoeba, Paramecium.

§ Multiple fission: parent body divides into many daughter organisms. Eg. Plasmodium.

§ Budding: daughter organisms grow from small buds arising in parent body. eg. Hydra, Yeast.§ Gemmule in sponge.

§ Conidia: non-motile, exogenous spores in chains eg. Fungi (Penicillium).

§ Zoospores: microscopic, haploid, motile structures eg. Algae (Chlamydomonas).

§ Sporulation : Under unfavourable conditions, When the products of  multiple  fission  become individually surrounded by resistant coats, the cyst walls before their release from the parent,  the  process is known as sporulation and the encysted products are termed spores. Eg- Amoeba

§ Fragmentation : Filamentous organism divided into two or more individuals eg. Spirogyra, Spirulina.

§ Regeneration: it is the process of renewal, restoration and growth. Eg. Hydra, Planarian flatworm, lizard tail & human liver.

Vegetative Reproduction in plants

§  Vegetative reproduction frequently used instead of asexual reproduction, units of vegetative propagation called vegetative propagules. Eg.- runner (Grass, Oxalis), rhizome (ginger), sucker (Mentha), tuber (Potato), offset (Eichhornia), bulb (Onion), Bulbil (Agave).

Sexual Reproduction : PHASES OF LIFE SPAN.

§  Juvenile phase: The phase of growth before reproductive maturity. In plants- Vegetative Phase.

§  Reproductive phase: Reproductive maturity.

§  Senescent phase: Phase between reproductive maturity and death. 

Special Flowering : Bamboo- once in life, generally after 50-100 years.

      Strobilanthus kunthiana (Neelakuranji) – flowers once in 12 years, (In Sep-Oct 2006,blue stretched mass flowering attract tourists in Kerala, Karnataka and Tamil Nadu).

The main events of sexual cycle are: Pre-fertilization, Fertilization & Post Fertilization.

A.  Pre-fertilisation events:

a.     Gametogenesis: The process of formation of male and female gametes by meiosis (cell-division)

.·       Homogamete (Isogamete): - gametes similar eg. Algae

·       Heterogamete (anisogamete): - morphologically dissimilar gamete, male gamete (antherozoid or sperm), female gamete (egg or ovum) eg. Human.

·       Sexuality in organisms: In plants Bisexual term is used for Homothallic and Monoecious plants

·       Monoecious: Reproductive organs at different positions in same plant eg. Chara, Maize.·       Hermophrodite : Reproductive organs at different positions in same animal eg. Earthworm.

·       Unisexual term used for Heterothallic and Dioecious plants.

·       Male and female reproductive-structure on different plants.

·       Flowering plants: male flower (staminate flower) & female flower (pistillate flower) eg. papaya and date, palm.

·       Animals – Bisexual term is used for Hermaphrodite animals-eg. Earth-worm, Tape-worm, Leech,

·       Unisexual animals have male & female sexes in separate individuals-e.g. insects, frogs, human beings

Cell division during gamete formation:

Haploid-parent (n) produces haploid gametes (n) by mitotic division, eg. Monera, fungi, algae and bryophytes.

Diploid parent (2n) produces haploid gametes (n) by meiosis division (possess only one set of chromosomes) and such specialized parent cell is called meiocyte or gamete mother cell (2n).

·       Example-

Name of organism	Meiocyte (2n)	gamete          (n)
Human	46	23
Housefly	12	6
Ophioglossum (fern)	1260	630
Potato	48	24

b)  Gamete transfer:- to facilitate fusion.

·    Male gametes - motile & female- non-motile, exception few fungi and in algae.

·    Gamete transfer through water : algae, Bryophytes, Pteridophytes & amphibians.

·    Higher plants, pollen-grains are transferred by pollination. 

B.  Fertilization : fusion of male and female gamete.

·    Syngamy : Process of fusion of male and female gamete.

Two type of Fertilization: external and internal . 

EXTERNAL FERTILISATION	INTERNAL FERTILISATION
Fertilization takes place outside the body	Fertilization takes place inside the body
External medium is required- water	No external medium required
Large no. of gametes produced.	Lesser no. of gametes produced.
Offsprings –less protected	Offsprings- well protected.
Eg. Algae, fish, amphibians	Gymnosperm, angiosperm, aves, mammals.

C.  Post -fertilization events- formation of zygote and embryo.

Zygote. Single celled , diploid, vital link between two generations. In fungi & algae, zygote develops a thick walled that is resistant to dessication and damage.

Haplontic life cycle- zygote (2n) divides by meiosis to form haploid (n) spores.

Diplontic life-cycle- zygote (2n) divides mitotically, develops into embryo (2n).

Embryogenesis: development of embryo from zygote by cell division (mitosis) and cell differentiation.

  On based of Zygote development, animals are grouped into two types-

 

Oviparous Animals	Viviparous Animals
They ley fertilized or unfertilized egg.	They give birth to young ones.
Fertilized eggs have calcareous shell	No calcareous shell
Development of embryo outside female body	Development of embryo inside female body
Less chance of survival.	More chance of survival.
Eg. Reptiles and Birds	Eg. Mammals

 

Cell- division increases the number of cells in the developing embryo

Cell differentiation - groups of cells undergo certain modifications for the formation of different kinds of tissues and organs.

In flowering plants- zygote formed inside ovule.

Changes occur in flowering plants:

Sepal, petals, stamens,	Fall off
Zygote	Embryo
Primary endosperm nucleus	Endosperm (3 N)
Synergid, antipodal cells	Disintegrate
Ovary	Fruit
Ovule	Seed
Ovary wall	Pericarp (epicarp + mesocarp + endocarp)
Outer Integument	testa
Inner Integument	tegmen

Parthenogenesis: Female gamete develops into new organism without fertilization . eg- Honey bee, turkey, lizard, rotifers (Protozoans).

Seedless fruits formed by parthenogenesis

Clone: A group of individuals of the same species that are morphologically and genetically similar to each other & their parents.

Chapter 1: REPRODUCTION IN ORGANISMS QUESTIONS

Q) What is meiocyte?

Ans) It is the gamete mother cells which undergoes meiosis.

Q) Why is date palm referred to as dioecious?

Ans) In date palm the male and female flowers are present in different plants.

Q) What is special in flowering in bamboo?

Ans) Bamboo flowers only once in their life time generally after 50-100 years .

Q) Define clone?

Ans) It is a group of individual of the same species that are morphologically and genetically similar.

Q) What is vegetative propagule?

Ans) The units of vegetative propagule that gives rise to a new plant is called vegetative propagule. Examples: Runner and rhizome.

Q) Diagrammatically represent asexual reproduction in yeast?

 Ans) See Figure 1.2 (page-5)

Q) Cucur bit are called monoecious. Justify?

Ans) They have male and female reproductive structures in different flowers.

Q) If chromosomes number in meiocyte in rat, elephant and rice is 42,56,24 what will be the chromosomes number in their gamets?

Ans) Rat 21, elephant 28,rice 12

3 marks

Q) Why higher organisms have resorted to sexual reproduction in spite of its complexity? Ans) 1. It brings about variations.

2.  Variation helps in better adaptation in nature.

3.  Variation is the basis of evolution.

Q) What is parthenogenesis. Give Example?

Ans) Development of a new individual from an unfertilized egg is called parthenogenesis. It is simpler easier and rapid mode of replication. But there is no chance of any variation. So it does not play any role in evolution. Eg.- Honey bee and some lizards

Q Mention the reason for difference in ploidy of zygote and primary endosperm nucleus in angiosperm Ans Zygote is formed by fusion of haploid male gamete and haploid egg so it is diploid.

Whereas the PEN is formed by fusion of haploid male gamete with two haploid polar nuclei so it is triploid.

5 Marks.

Q) What are vegetative propagules. Name any four of them and give example? Eg.- rhizome of ginger, leaf bud of bryophyllum, bulb of onion, offset of pistia.

HOTS.

Q)Amoeba is immortal. Explain?

Ans) Amobea has no natural death. It undergoes binary fission and forms two new daughter cells.

Q) Which is the first diploid single cell which begins life? 

Ans) Zygote.

Q What is the fate of zygote in organism which show 1 Haplontic life cycle and Diplontic life cycle

Ans In haplontic life cycle zygote undergoes meiosis during its generation In diplontic life cycle zygote divides mitotically during embryogenesis

Q) What type gamete are formed in staminate and pistillate flowers? 

Ans) Pollen grains and egg cell respectively.

Q) Name the organisms with respect to their sexuality?

i)  Monoecious animal

ii)  Dioecious animal

iii)  Monoecious plant

iv)  Dioecious plant

Ans :- i) Earthworm ii) Cockroach iii) Chara iv) Marchantia.