Class 10, Science, Chapter-7, Lecture-2, Sexual Reproduction in Plants (Notes)
UNISEXUAL ORGANISMS:
An organism which has only one kind of sex organ (male or female) in its body is called a unisexual organism.
BISEXUAL ORGANISMS OR HERMAPHRODITES:
An organism which has both kind of sex organs (male and female) in its body is called a bisexual organism or hermaphrodite.
Examples: Plants, tapeworm, earthworm, starfish.
FERTILISATION:
The process of fusion of male gametes and female gametes to produce a zygote is called fertilisation.
Two Types – External fertilisation ( in fishes, amphibians )
Internal fertilisation ( in reptiles, birds and mammals )
ADVANTAGE OF SEXUAL REPRODUCTION:
It promotes diversity of characters in the offspring leading to genetic variation which is necessary for evolution.
PARTS OF A FLOWER:
1.Sepals: green
2.Petals: coloured
3.Stamen:
(i) Filament
(ii) Anther – Produces pollen grains each of which produces two male gametes.
4.Carpel (Pistil):
(i) Ovary – Contains ovules each of which contains one female gamete and few polar nuclei.
(ii) Style
(iii) Stigma
POLLINATION:
The transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower of the same plant or of a different plant of the same species through various agents is called pollination.
Two Types:
(i) Self Pollination:
The transfer of pollen from the anther of a flower to the stigma of the same flower is known as self-pollination.
(ii) Cross Pollination:
The transfer of pollen from the anther of one flower to the stigma of another flower of the same plant or of a different plant of the same species through various agents is known as cross-pollination.
FERTILISATION IN PLANTS:
1.Pollen grains deposited on the stigma of carpel moves along pollen tube through the style. It germinates to form a haploid generative cell and a tube nucleus. The generative cell travels behind the tube nucleus and undergoes mitosis to form two haploid sperm cells.
2.Inside each ovule:
A diploid megasporocyte (megaspore mother cell) undergoes meiosis and produces 4 haploid 3 of the megaspores degenerate and the remaining one undergoes 3 rounds of mitosis to produce 8 haploid nuclei.
Inside this multinucleated structure (embryo sac):
3 nuclei become antipodal cells present opposite the micropyle.
3 nuclei move towards micropyle and form one egg and 2 synergids.
2 remaining nuclei in the embryo sac are called polar nuclei.
3.Double fertilisation:
The pollen tube enters through the micropyle, digests one of the synergid and releases the two sperms.
The other synergid degenerates.
SYNGAMY: One of the sperm cell fertilise with the egg cell to produce diploid zygote which further develops into embryo
TRIPLE FUSION: Another sperm cell fuses with the polar nuclei to form a triploid cell called triple fusion nucleus which develops into endosperm for supplying food to embryo.