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

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