Sexual Reproduction of the Flowering Plant

Review the plant life cycle

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2

pollen is transferred

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After fertilizationflower withers

seeds disperse and germinate into new plant

seeds develop in ovary4

Structure of the flower

Carpel

Petal

Anther

FilamentStamen

Stigma

Style

Ovary

OvuleSepal

Structure of the flower

Function of floral parts

Sepal : To protect the flower (and to prevent it from drying out

Petals : To attract insects to the flower for pollination

Function of floral parts

Stamen : To produce the pollen grains in the anthers. (Each pollen grain produces two male gametes, one of which can fertilise an egg cell)

Function of floral parts -Stamen

Anther• Produces pollen

Filament• Holds the anther in place

Function of floral parts

Carpel : To produce the ovules (Each ovule contains an egg cell inside an embryo sac)

Function of floral parts - Carpel

Stigma• Where pollen lands after

pollination

Style• Pollen travels down this

Ovary• Contains ovules

Angiosperm anther

Dithecous - anther is bilobed with each lobe having two theca

A longitudinal groove runslengthwise separating the theca.

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Structure of an Anther11

T.S. OF AN ANTHER12

T.S. of an anther

Usually the anther is Dithecous - bilobed nature Anther - Two microsporangia in each lobe develop into

pollen sacs. Microsporangium: microsporangium is surrounded by four

wall layers -- the epidermis, endothecium, middle layers and the tapetum.

The outer three wall layers perform are for protection and help in dehiscence of anther to release the pollen.

The innermost wall layer is the tapetum. It nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus. Nucleus divides – without cytoplasmic division- polyploidy

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MICROSPOROGENESIS

Cells

of

sporangeous

tissue

Four cells

Haploid

Stay together

Tetrad cells

separates

Meiosis

Micro-

sporogenesis

Anthers mature,

Dehydration

Dissociate

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Structure of a Pollen grain The pollen grains represent the male

gametophytes. Sizes, shapes, colours, designs-

according to different species Very resistant because of the

presence of sporopollenin(Tough Protein)

Cryopreservation(-196 0C ) used in crop breeding

Pollen food nutritional value – performance of athletes and race horses

Pollen Allergy-asthma, bronchitis eg Parthenium(notorious invasive Sp)

Viability of pollen grains depends on temperature and humidity. Viable from few mins to several months according to species.

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The cytoplasm of pollen grain is surrounded by a plasma membrane. A mature pollen grain – ( inside intine and exine )

During development from microspore mitosis – 2 cells

vegetative cell / tube cell --bigger, has abundant -food reserve and a large irregularly shaped nucleus.

generative Cell -- small , spindle shaped with dense cytoplasm & a nucleus. It floats in the cytoplasm of the vegetative cell.

60 % angiosperms, pollen grains are shed at 2-celled stage.

In others generative cell divides mitotically - the two male gametes before pollen grains are shed (3-celled stage).

Stages of microspore maturing into a pollen grain

Stigma

Style

Ovary

TYPES OF PLACETATION-attachment of ovules to placenta

Cells in the nucellus near micropylar region.

Four cells Haploid 3 near micropyle

degenerrate1 functional megaspore

Meiosis

Mega-sporogenesis

Functional megaspore enlarges, mitotic division 2 nuclei – move opposite poles – 2 mitoic division- 4 nuclei at each poleWall formation only after this stageChalaza end – 3 form antipodalsMicropylar end – 3 form egg apparatus

Remaining 2 nucei Polar nuclei in large central cell.

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Megaspore mother cells MMCDeploid

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Megasporangium / Ovule (Anatropus)