Sexual reproduction in plants

Group three(3) members:

Petrina bailey

Tavoy Campbell

Teacher:

Mrs. Haughton

What Is SEXUAL REPRODUCTION?Plant reproduction is the production of new individuals or

offspring in plants, which can be accomplished by both sexual or asexual means.

Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from the parent or parents.

Each gamete, one from male parent and the other from the female parent, has the haploid number of chromosomes and the gamete formed has the diploid number of chromosomes.

Development of PollenInside an anther are four pollen

sacs. They can be seen in the diagram below of a young anther

The central region of each pollen sac is made up of microspore mother cells.

Surrounding the mother cells is the tapetum which provides nutrition to the mother cells

Each microspore mother cell undergoes meiosis to produce a tetrad of haploid cells

Each haploid cell undergoes mitosis to become a pollen grain (microspore)

The two cells produced by this mitotic division are:

generative nucleuspollen tube nucleus

The pollen tube nucleus will produce the pollen tube

The generative nucleus is the gametophyte - it will undergo a mitotic nucleus to produce two male gametes (exactly when this occurs depends on the species)

Surrounding the nuclei are two walls; the inner intine and the outer exine

When the pollen grains mature the anther will burst releasing the pollen grains 

Dehisced (pollen grains released) anther 

Development of OvuleThe ovary is contained within the carpelFrom a structure known as the placenta an outgrowth

called the nucellus developsInside the nucellus is a megaspore mother cellThe megaspore mother cell undergoes meiosis to produce

four haploid megasporesOne of these four cells undergoes three mitotic divisions

to produce an eight-celled embryo sac. The embryo sac is the female gametophyte

Two layers of cells, called the integuments, grow around the embryo sac. They do not completely enclose the embryo sac - a small gap (the micropyle) is left.

The eight cells arrange themselves with:three cells at the micropylar end - these

are the egg apparatus. The middle cell is the female gamete (the egg cell) while the other two are the synergids

two cells in the middle - the polar nucleithree cells at the other end - the

antipodal cells (these have no known function)

What is pollination? Pollination is the transfer of pollen grains from

anthers to stigmas.

There are two types:Cross pollination Self pollinationCross pollination is the transfer of pollen grains between

two plants of different genetic make up. (Two different species).

Self pollination is the transfer of pollen grains between flowers of identical genetic constitution.(same species).

Pollination - and adaptations for wind and insect pollination

Pollination is the transfer of the male pollen grains to the female stigma

Depending on the species this can be:self-pollination, where pollen is transferred to

a stigma on the same plant (this obviously reduces genetic variability)

cross-pollination where pollen is transferred to a stigma on a different plant

Pollination can be brought about by:wind-pollination - where the pollen are

blown around and a small fraction land on a stigma

insect-pollination - where the pollen are attached to an insect which then releases them on to the stigma of another flower

Typical characteristics of wind- and insect-pollinated plants include:

Feature Wind-pollinated Insect-pollinated

position of flowers above leaves above leaves

petalssmall, inconspicuous or absent

large, conspicuous, brightly coloured

nectaries absent present

scent not scented scented

stamens pendulous (hang outside flower) inside flower

anthers move freelyfixed - positioned to come into contact with insect

pollenlarge quantities, light, smooth grains

fewer produced, not smooth to aid attachment to insect

stigmalarge, often feathery, hang outside flower

small, within flower, positioned to come into contact with insect

Genetic Consequences of Sexual Reproduction in Plants

Because of the fusion of two gametes, the zygote may have some genetic consequences.

Some of these genetic consequences with sexual reproduction by self pollination may include:

Passing down of genetic mutations; if present in parents.Variety in genetic make up will be limited.Species tend too die off since genetic make-up has not

varied/adopted to fend off against genetic diseases.

Genetic Consequences Cont’dThrough cross pollination, there may also be some genetic

consequences.

GENETIC CONSEQUENCES WITH SEXUAL reproduction BY CROSS FERTILIZATION MAY INCLUDE:

HELPFUL GENES FROM ONE PARENTS MAYBE MASKED/ DEACTIVATED BY UNHELPFUL GENE FROM THE OTHER.

PARENT GENES MAY CLASH, CAUSING ERROR IN GENETIC MAKE UP OF OFFSPRINGS REASULTING IN MUTATIONS OR IN FERTILE OFFSPRINGS.

Fertilization

This is the process by which two gametes; both of the n number of chromosomes, come together, whether of the same plant as in self fertilization, or of two different plants as in cross fertilization, to produce a zygote with the 2n number of chromosomes.

The events occurring during fertilization are seen in this diagram of a flowering plant's lifecycle.

The pollen grain is deposited on the stigma (this is pollination) and germinates to produce a pollen tube.

The pollen tube grows down through the style, this growth is controlled by the tube nucleus.

The pollen grain is able to penetrate the style because of the secretion of digestive enzymes

The pollen tube enters the micropyle (by this time the generative nucleus has undergone its mitotic division so there are two male nuclei [gametes] present)

The male nuclei enter the embryo sac

one fuses with the egg cell to form a diploid zygote - this will give rise to the embryo

the other fuses with the two polar nuclei to form a triploid endosperm nucleus - this will give rise to the endosperm that will nourish the developing embryo.

This process is known as a double fertilization because two fusions occur.

Double FertilizationDouble fertilization was first discovered by a Russian

scientist namely; S. G. Navashin in 1898 in two species of plants – Lily Lilum martagon and the Fritillaria Orientalis.(both are sperm making plants).

Double Fertilization is the process by which, two sperms all enter the pollen tube where one sperm fertilizes the mature egg/ovule and the other the embryo sac. The nucleus of both sperm fuses with the respective nucleus of the ovule and the nuclei of the embryo sac.

Development of seeds and fruits

Immediately after fertilization the ovule becomes the seed and the ovary the fruit. The following changes takes place:

The zygote grows by mitotic divisions to become a multicellular embryo consisting of a first shoot (plumule), first root (radicle) and either 1 or 2 seed- leaves called cotyledon.

The triploid primary endosperm nucleus undergoes repeated mitotic divisions to form the endosperm. in some seeds this remains as the food store, as in cereals eg: wheat and maize.

If the cotyledon acts as food store they grow at the expense of the endosperm, which may disappear altogether. Some seeds do have both as food storage.

As growth of the embryo continues the surrounding nucellus breaks down supplying nutrients for growth.

The testa develops from the integument, a thin but strong protective layer.

The micropyle remains a small pore in the testa through which oxygen and water enters when the seed germinates.

The final stage in development of the seed involves a reduction in the water content of the seed from 90% to 10-15% by mass. The potential for metabolic activity is greatly reduce and is an essential step ensuring seed dormancy.

While the seeds develop the ovary becomes a mature fruit, with its wall know as pericarp.

Advantages of reproduction by seed:

Plant is independent of water for reproduction and better adapted for land environment.

The seed protects the embryo.

The seed is usually adopted for dispersal.

The seed contains food for embryo.

The seed can remain dormant and survive adverse conditions.

Disadvantages of reproduction by seeds

Seeds are large because of extensive food reserves. Dispersal is more difficult than by spores.

Seeds are eaten by animals.

There is a reliance on wind, insect and water for pollination.

There is a large wastage of seeds because the chances of survival of a given seed are limited.

Food supply for seeds is limited whereas in vegetative reproduction food is available from parent plant until the daughter plant is established.

Types of SeedsEndospermic (maize; monocots) - The endosperm is

present in the mature seed and serves as food storage organ. Testa and endosperm are the two covering layers of the embryo.

Non-endospermic (pea; dicot) - The cotyledons serve as sole food storage organs as in the case of pea (Pisum sativum). During embryo development the cotyledons absorb the food reserves from the endosperm. The endosperm is almost degraded in the mature seed and the embryo is enclosed by the testa.

Types of Seeds

Germination

Types of Fruits

Types of Fruits