introduction of an ovule

8/7/2019 Introduction of an Ovule

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INTRODUCTION OF AN OVULE :

An Ovule is a small egg or seed in an early stage of development.

Ovule is a structure that is found in seed plants gymnosperms and angiosperms thatdevelops into a seed after fertilization of an egg-cell within it.

Ovule consists of an embryo sac containing the female gamete (ovum or egg cell),

surrounded by a nutritive tissue called NUCELLUS. Outside this there will be one or twocoverings called Integuments that provide protection, developing into the testa or seedcoat, following fertilization.

In ANGIOSPERMSthat is flowering plants the ovule will be within an ovary, but inGYMNOSPERMS ovules are born on the surface of an ovuliferous scale, usually withina cone, and are not enclosed by an ovary.

Ovules are attached to ovaries by a cushion shaped structure Placenta. These Ovulesare connected to ovary by means of a stalk called FUNICLE.

Structure of an Ovule



Components of an OvuleMICROPHYLE: The gap the integuments leave is microphyle.

CHALAZA: Basal part of ovule where two integuments arise is chalazal end.

EMBRYO SAC: From the cells of nucellus , one cell differentiates as megaspore mother

cell (2n)which is diploid. This megaspore mother cell divides by meiosis to form four megaspores out of which only one develops as embryosac.

Mature embryo sac contains 7 cells at the time of fertilisation. This is haploid and iscalled female gametophyte.They are:

y EGG CELL: This is female gamete .it is in haploid (1n) condition. At the time of fertilization this egg cell fuses with male gamete and forms zygote. This is the first

fertilisation. y SYNERGIDS : They are 2 in number. These synergids are also called helper cells as

they direct the growth of pollen tube towards the egg. They have beak like structureand are situated at microphylar end. It is also in haploid (1n) condition.

y ANTIPODALS: They are 3 in number. There are situated at the chalazal end . Theylive for a short duration. They are in haploid (1n) condition.

y CENTRAL NUCLEUS : It is also called secondary nucleus.inside the mature embryosac only this central nucleus is in diploid (2n) condition. This is due to the fusion of two nuclei to form a single large secondary nucleus.At the time of fertilization theother male gamete fuses with secondary nucleus to form endosperm nucleus whichwill be in triploid (3n) condition.

Micropyle as Constituent of OvuleIn plants which are seed bearing the ovule is known to develop into reproductive cells of flower and is known to contain integuments, nucellus, and megaspore derived femalegametophyte.



It is referred that mega gametophyte or megaspore derived female gametophyte isknown as embryo sac and is responsible for production of egg which later gets fertilizedand results in formation of seed.

The integuments, micropyle and chalaza are the key structures found in ovule of seedbearing plants. Integuments are represented as outer cell layers which cover the ovule.

It is known that gymnosperms contain a single integument layer and angiospermscontain two integument layers.

The integuments generally do not cover the nucellus in total and leave a small openingat the apex region which is generally referred as micropyle. The formation of micropyleis observed to allow the pollen tube entry into the ovule for the purpose of fertilization.

At the opposite of micropyle, chalaza is found to be located from where nucellus isfound joined to integuments. The chalaza helps in transfer of nutrients from the phloemand reaches the ovule.

Function of MicropyleMicropyle in case of Pinus plants is known to play pivotal role in pollination where inpollen is transported via micropylar canal to reach the surface of nucellus. Themicropylar fluid is known to be secreted during pollination and helps in transportation of pollen to the nucellus.

Sexual Reproduction in Flowering Plants (Angiosperms)

Sexual reproduction is the formation of a new individual following the union of twogametes. In plants, the gametes are egg and sperm and the structures that producethese gametes are located within the flower.

Flower Structure



Sexual reproduction in flowering plants centres around the flower. within a flower, thereare usually structures that produce both male gametes and female gametes.

Development of the Ovule and Female GameteInside the ovary theer may develop one or more ovules. each ovule begins life as asmall projection into the cavity of the ovary. As it grows and develops it begins to bendbut remains attached to the ovary wall by a placenta.

At the start, the ovule is a group of similar cells called the nucellus. as it develops, themass of cells differentiates to form an inner and an outer integument, surrounding andprotecting the nucellus within, but leaving a small opening called the micropyle.

At the centre of the ovule is an embryo sac containing the haploid egg cell (the femalegamete).



Development of Male Gameteeach anther contains four pollen sacs. many pollen grains develop inside each pollensac. It begins with a mass of large pollen mother cells in each pollen sac. All are diploid.

In each pollen grain the wall thickens and forms an inner layer (the intine) and an oftenhighly sculptured outer layer (the exine). the surface pattern is different on pollen grains



from different species. When the pollen grains are mature, the anther dries out andsplits open (a process called dehiscence) and the pollen is released.

PollinationMany plants favour cross pollination,so pollen must be transferred to the stigma of another plant if sexual reproduction is to take place. Some flowers rely on the wind tocarry pollen grains other rely on insects.

Self pollination is where the pollen is transferred to the stigma of another flower on thesame plant. self pollination is obviously more reliable, particularly if the nearest plant isnot very close.

FertilisationIf the pollen grain lands on a compatible stigma, a pollen tube will grow so thateventually the egg cell, hidden away in the embryo sac, can be fertilised. A tube

emerges from the grain, its growth being controlled by the tube nucleus at the tip of thetube. It may grow downwards in response to chemicals made by ovary (a responseknown as chemotropism).

During the growth and extension of the tube, the generative nucleus, behind the tubenucleus, divides by mitosis to produce 2 male gametes. the pollen tube enters the ovulethrough the micropyle and penetrates the embryo sac wall. The tip of the tube burstsopen, the tube nucleus dies and what follows is called double fertilisation.



y 1 male gamete fuses with the egg cell to produce a diploid zygote

y 1 male gamete fuses with both the polar nuclei to produce the triploid primaryendosperm nucleus.

Immediately after fertilisation, the ovule is known as seed.

GerminationWhen the conditions are right, the seed will take up water through the micropyle byimbibition.This triggers the beginning of the growth of the seed.

The cell swells and the testa splits. With the addition of water, large molecules of carbohydrate, protein and fat can be hydrolysed (broken down) to produce substancefor respiration.The water activates such enzymes (amylases) to catalyse digestion.

The growing embryo releases a hormone called gibberellic acid and some enzymes areproduced and released in response to this.

The soluble products of digestion are delivered to the cotyledons, root and shoot. Theyrespire aerobically and grow in size.

By the time the food store has been used up, the shoot has grown enough to push the

first leaves into the sunlight. Photosynthesis can then start.

introduction of an ovule

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