Download sexual-reproduction-in-plants-2

SEXUAL REPRODUCTION IN FLOWERING PLANTS:
Phylum Angiospermae consists of flowering plants, and these plants produce both male and
female gametes for sexual reproduction.
The sexual reproduction in plants takes place in the following steps:
a) The reproductive structure of the angiosperms is located in the flower.
b) The male reproductive structure is the stamen, which produces the male gametes
which are present in the pollen grains of the plant.
c) The female reproductive structure is the carpel which produces the female gametes
in ovules in the plant.
d) The male gamete present in the pollen grain fertilizes the female gamete present in
the ovule to produce a zygote, which undergoes further divisions to form an embryo.
e) The fertilized ovules grow and become seeds.
f) The seeds give rise to new plants when they germinate.
Parts of a flower: The flowers are usually bisexual ie, both male and female reproductive
structures are present in the same flower. The flower is attached to the plant by the pedicel
or the stalk.
The main parts of the flower are – sepals, petals, stamens, and carpel / pistil
Sepals are usually green leaf like parts forming the outermost whorl of the flower and it
protects the flower in the earlier stage of its development as bud.
Petals are the colorful parts of the flower, which forms the inner whorl of the sepals. They
assist the flower in the process of pollination by attracting insects to the flower. They also
provide protection to the reproductive structures of the flower.
Stamen and carpel form the reproductive parts of a flower.
Stamen / Androecium is the male reproductive organ of the plant and each flower has a
number of stamens in it. Each stamen is made up of a filament which is a thin stalk like
structure and a bilobed anther which at the tip of the filament. Each lobe of the anther has
two pollen sacs, which contain numerous pollen grains which are very light and yellow in
color.
Carpel / Gynoecium is the female reproductive structure of the plant and it is present in the
centre of the flower. It consists of three parts- the stigma, style and ovary.
Stigma is the terminal, sticky part of the carpel, which facilitates the pollen grains to stick to
the stigma during pollination, when the pollen are released from the anthers
Style is the slender, elongated middle portion of the carpel which connects the stigma to the
ovary.
Ovary is the swollen lower portion of the carpel, which contains the ovules and each ovule is
an egg cell/ female gamete. Carpel in a flower is usually surrounded by a no. of stamens.
Flowers can be of two types –
Unisexual flowers are those which contain either the stamens or the carpel, but not both.
Eg. Watermelon, papaya
Bisexual flowers are those which have both the stamens and carpel. Eg. Hibiscus,
mustard.
Pollination: is a process in which the pollen grains are transferred from the anther of the
stamen to the stigma of the carpel, by agents such as insects, birds, wind and water.
Pollination can be of two types: self pollination and cross pollination.
Self – pollination is the transfer of pollen grains from the anther of the flower onto the
stigma of the same flower or another flower of the same plant.
Advantages of self pollination:
a) Self pollination in bisexual flowers ensures continuity of the species.
b) It helps to preserve the parental characters in the next generations as both the
gametes are from the same plant.
c) It is not necessary for the plant to produce brightly colored flowers, nectar or scent to
facilitate pollination.
Disadvantages of self pollination:
a) New varieties cannot be obtained from self pollination.
b) Genetic defects if any, gets passed on to several generations.
c) Repeated self pollination leads to loss of vigor and vitality in the species.
Cross – pollination is the transfer of pollen grains from the anther of the flower to the
stigma of another flower of a different plant of the same kind. This is a common method of
pollination in most of the flowering plants. Cross pollination is facilitated by biotic agents like
birds and insects, and abiotic agents like wind and water.
Advantages of cross pollination:
a) It results in production of healthier off springs.
b) Seeds produced by cross pollination have better germinating ability.
c) More abundant and viable seeds are produced.
d) Variations in the species results in the evolution of better varieties.
Disadvantages of cross pollination:
a) Plants become dependant on external agents like birds, insects, wind and water for
pollination.
b) Plants would have to produce large quantity of pollen grains to ensure pollination.
Zygote: the cell which is formed by the fusion of the male gamete with the female gamete is
called the zygote i.e. it is the fertilized egg / ovum.
Embryo: the zygote, after fertilization, undergoes further cell division and gets enclosed in
the seed once the cell division stops. This stage of development which is intermediary of the
zygote stage and that of a young plant is called an embryo.
Fertilization: is defined as the fusion of the male gamete with the female gamete to form a
zygote during sexual reproduction.
Fertilization in plants:
a) Pollination is followed by the process of fertilization.
b) Once the pollen lands on a suitable stigma, it has to reach the ovary to fuse with the
female gamete
c) The pollen tube grows out of the pollen grain and makes its way through the style,
towards the ovary.
d) The male gamete undergoes division while passing through the pollen tube to form
two nuclei.
e) It enters the ovary from the micropylar end, and one of the nuclei fuses with the
female gamete inside the ovule to form the zygote. The second nucleus moves
upwards in the embryo sac and fuses with the two polar nuclei, to form a fusion of
three nuclei. These nuclei later divide and develop into endosperm/ the nutritive
material for the embryo to develop further.
f)
Since there is fusion of gametes twice between two sets of nuclei - once with the
female gamete present in the ovule and the second fusion with the polar nuclei, the
process is called double fertilization.
Post fertilization changes in a flower:
g) After fertilization, the zygote divides several times to form an embryo within the ovule.
h) The ovule then develops a tough seed coat and gets converted into a seed.
i) The seed contains the future embryo which develops into a seedling when the seed
is provided with suitable conditions for germination.
j) The petals, style and stigma, shrivel and fall off after the fertilization process, as they
are not required.
k) The ovary develops and ripens to form a fruit.
What is the significance of reproduction?
a) Reproduction allows perpetuation of species
b) It increases the population of species
c) It plays an important role in evolution by transmitting favorable variations from one
generation to another generation.
What is DNA copying?
a) Reproduction at its most basic level involves making blueprints of body design of
organisms.
b) DNA in the cell nucleus is the information source for making proteins and different
proteins lead to different designs
c) A basic event in reproduction is the creation of a DNA copy. (Replication)
d) DNA copying is accompanied by cell division giving rise to two cells
e) DNA copying always involves some variation; hence the DNA copies generated are
similar but not identical.
f) This tendency of variation during reproduction leads to evolution.
What is the importance of variation in organisms?
a) Organisms fill well defined places or niches in the ecosystem by reproduction.
b) Organisms having the same body design occupy the same niche.
c) If a niche suitable for a population is drastically changed, the population may be
wiped out completely.
d) But if some variations are there in a few individuals of these populations, there could
be chances of survival, and the better adapted organisms will establish themselves
and perpetuate.
e) Thus, variation is important for survival of species.