SEXUAL REPRODUCTION IN FLOWERING PLANTS PREPARED BY-D.KANAKA LAKSHMI

LEARNING OBJECTIVES

• To know the LS of a complete flower.

• Learn the process of microsporpgenesis and formation of pollen grains.

• To describe the structure of pollen grains

• Learn the process of megasporogenesis and formation of female gametophyte.

• Describe different types of pollination.

• List agents of pollination and their adapations.

• Explain double fertilization

• To learn differ stages in embryo development

• Describe different types of fruits.

• Learn the concept of polyembryony and apomixes with suitable examples.

IMPORTANT TERMS AND CONCPTS

Flower

• The sexual reproductive part of angiosperms is called flower.

• Gynoecium is the female reproductive part which contains carpels.

• Androecium is a male reproductive part which contains stamens.

MALE REPRODUCTIVE UNIT

Stamen

• A stamen consists of two parts- Filament and Anther

Anther

• It is composed of two lobes separated by a connective tissue.

• Each anther is bilobed and each lobe consists of two theca separated by a septum.

• Each anther consists of four microsporangia which get transformed into pollen sacs

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STRUCTURE OF MICROSPORANGIUM

Microsporangium is surrounded by the following

• Epidermis

• Endothecium

• Middle layer

• Tapetum

• The centre of each microsporangium is filled with closely arranged similar cells called sporogenous cells.

• At maturity, pollen grains is formed by fusion of two microsporangium in each lobe.

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MICROSPOROGENESIS

• The formation of microspore from a pollen mother cell is called microsporogenesis.

• Each cell of tissue in a microsporangium acts as a potential pollen mother cell or microspore mother cell.

• PMC undergoes meiotic divisions to form microspore tetrad.

• On maturity anther dehydrates and form pollen grains.

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POLLEN GRAINS

• They develop from PMC by meiotic division.

• Each pollen grain has a two layered wall- the outer exine made of sporopolonnin and the inner intine made of pectin and cellulose.

• The exine has an aperture where sporopollonnin is absent, called germ pore.

• A mature pollen grain has two cells-a vegetative cell and a generative cell.

FEMALE REPPRODUCTIVE UNIT

• Gynoecium represents female reproductive unit.

• The gynoecium may have single pistil called monocarpellary, more than two pistils called multicarpellery, fused called syncarppous or free pistils called apocarpous.

STRUCTURE OF MEGASPORANGIUM

• The ovule is stalked and is attached to the placenta by funicle.

• The junction between ovule and funicle is called hilum.

Megasporogenesis

• A single megaspore mother cell is differentiated in the mycopylar region of the nucellus of an ovule, it undergoes meiosis and forms a cluster of four haploid cells, called megaspore tetrad.

• Of these ,soon three degenerate and only one megaspore becomes functional.

MEGAGAMETOGENESIS

• The female gametiphyte develops from a single megaspore and hence ,its development is escribed as monosporic development.

POLLINATION

Depending on the source of pollen gains, pollination is of the following types:

Devices for cross pollination

• Unisexuality

• Dichogamy

• Protandry

• Protogyny

• Self-sterility

• Chasmogamous flowers

Agents of pollination

ADAPTATIONS OF WATER AND WIND POLLINATION

Pollen-pistil interaction

• stigma tillthe pollen tube enters the ovule are collectively referred to aThe stigma or pistil has the ability to recognise the compatible

pollen of the same species, it is the result of interaction between the chemical components of the pollen and those of the stigma.

• The events from the deposition of pollen on the s pollen pistil interaction.

DOUBLE FERTILISATION

• Since two fusions ie;triple fusion and syngamy occur in an embryo sac, it is known as double fertilisation and it is unique to angiosperms.

ARTIFICIAL HYBRIDISATION

• Commonly used technique in plant breeding programmes to obtain desirable charecters.

• Anthers are removed from the bisexual flowers using foreceps.This is emasculation and is done before the anther dehisces.

• The emasculated flower is covered with a paper bag to prevent contamination from unwanted pollen.This is called bagging.

Endosperm and embryo development

• ENDOSPERM-It develops from PEN.It preceeds embryo development.

• EMBRYO-The formation of embryo starts after a certain amount of endosperm is formed and this ensures assured supply of nutrition to the embryo.

STRUCTURE OF DICOT EMBRYO AND MONOCOT EMBRYO

SEED

• It is the fertilised ovule which is the final product of sexual reproduction.

• It consists of the following parts:

Seed coat

Cotyledons

Embryonal axis

STRUCTURE OF SOME SEEDS

ADVANTAGES OF SOME SEEDS

• Seeds possess better adaptive strategies for dispersal .

• The hard seed coats protection against injury.

• The reserve food of seed support the growth of seedlings.

• Seeds provide genetic recombination and variation.

• Depending on the viability of seeds , they are used to grow crops in favourable season.

FRUIT

• The ovary matures to form fruit.

• Fruits can be of two types:True fruits and false fruits

APOMIXIS

• The phenomeno of asexual reproduction that imitates sexual reproduction by formation of seed without fertilisation is called apomixes.

• Apomictic seeds may be formed in one or more of the following ways:

• --A diploid egg cell may develop into an embryo without fertilisation.

• --Cells of the nucellus surrounding the embryo sac protrude into the embryo sac and develop into embryos.

Ex:Citrus, Mango

POLYEMBRYONY

• The occurance of more than one embryo in a seed is called polyembryony.Ex:Orange

• Ways of forming polyembryonic seeds:

(i)Formation of more than one egg in an embryo sac.

(ii)Development of cells like synergids,nucellus,integument into an embryo.

(iii)Formation of more than one embryosac in an ovule.