1)The process of fusion of male gamete with the female gamete to form the diploid zygote is called fertilization.
3)Thus fertilization
occurs two times and both the male gametes participate in the so-called double
fertilization.
4)All Angiosperms are characterized by double fertilization. It was discovered by S.G.Nawaschin (1898) in Lillium and Fritillaria
Process of double fertilization:-
1) After pollination
pollen grain forms pollen tube, which grows towards the ovule.
2)Pollen the tube
carries two male gametes, thin cytoplasm, and a tube nucleus at the tip.
3)Non motile male
gametes transfer through a tube called siphonogamy.
4)Pollen the tube enters
in ovule through different ways -
a)Porogamy - Entry of pollen
tube inside the ovule through the micropyle.
b)Chalazogamy - Entry of pollen tube inside the ovule through chalaza.
c)Monogamy- The entry of pollen tube
inside the ovule through integuments i.e. body of ovule collation and bagging
procedure
5)Pollen the tube always
enters the embryo sac near the egg apparatus.
6)Filiform apparatus of
synergids attract the pollen tube towards the egg apparatus.
7)The tip of pollen tube
absorb water from one of the synergids and burst open to release two male
gametes.
8)The tube nucleus degenerates completely.
10)In Triple fusion or
second fertilization -The second male gamete fuses with the diploid secondary nucleus to
form triploid primary endosperm nucleus (PEN).
Significance of double fertilization:-
i) It is a unique
feature of angiosperms. Its parent plants a seed with a food store,
only if the egg is fertilized.
ii)The diploid zygote
which develops into an embryo and then into a new plant.
iii)Syngamy maintain
diploidy in the life cycle.
iv) It restores the
diploid condition by fusion of haploid male gamete with haploid female gamete
v) Triple fusion
produces a triploid PEN which develops into endosperm.
vi) The endosperm
provides nutrition for developing embryo.
vii) Because of double fertilization endosperm and embryo get produced together which increases the viability of seeds.
Development of Endosperm:
1)The triploid primary
endosperm nucleus repeatedly divides, mitotically to form nutritive tissue,
called endosperm.
2)In post-fertilization
changes within the ovule, the embryo, and endosperm are seen to develop
simultaneously.
3) During this
development other cells of the embryo sac get degenerate. The formation of a
triploid endosperm nucleus triggers cell division which leads to the formation
of the endosperm.
4)There are three types
of endospermic
development
i) Nuclear type ii)
Cellular type iii) Helobial type.
i) Nuclear type of Endosperm:
It is the most common
type of endosperm formation found in 161 angiosperms families.
i) The primary
endospermic nucleus repeatedly divides mitotically without a wall formation to
produce a large number of free nuclei.
ii)A big central vacuole
appears in the center of the cell pushing the nuclei towards the periphery.
iii)Between the nuclei
wall is produced, hence multicellular endosperm is formed.
iv)In several cases cell wall formation remains incomplete. e.g. wheat, sunflower, and coconut.
ii) Cellular type of Endosperm:
In some plants, the
division of the triploid primary endospermic nucleus is immediately followed by
wall formation.
It is mostly observed
in 72 families of dicots as in members - Balsam,
Petunia, Adoxa, etc.
iii) Global type of Endosperm:
1)It occurs in the order
of Helobiales of monocotyledons.
2) The first
nuclear division of primary endosperm is followed by a transverse
wall, which divides the cell unequally.
3)The smaller cell is
called the chalaza I cell and the larger cell is the micropylar cell.
5)It is an intermediate type between the nuclear and cellular type e.g. Asphodelus.
1)The process of
development of the zygote into an embryo is called embryogenesis.
2)The embryo is
developed at the micropylar end of the embryo sac.
3)The growth of embryo
triggers only after a certain amount of endosperm is formed. After
fertilization, embryonic development begins.
4)The zygote
or oospore divides transversely to form two cells known
as proembryo; the large cells towards the micropyle is
called basal or suspensor initial cell. and the smaller cell
towards the chalaza is called terminal
or embryonal initial cell.
5)The suspensor cell
divides transversely to form a row of seven to eight cells called suspensor.
6) The first cell of the suspensor towards the micropylar end becomes swollen and function as a haustorium.
The plants are to create offsprings for the next generation. One of the ways that plants can produce offsprings is by forming (making) seeds. The flowers must be pollinated to produce seeds and fruit.
Seed Development:
1) Seed development is initiated by fertilization. The integuments of the fertilized ovule persist and get transformed into the seed coat of the mature seed
2) Seed sometimes
consists of two distinct coverings i.e. outer testa and the inner thin
tegmen.
3) In addition to these
two the nucellus in the ovule may remain as a thin, papery layer, the
perisperm, e.g. some genera like black pepper and beet.
4) In some seeds, the food reserves in the endosperm are partially used up in the development of an embryo, in such seeds the endosperm remain conspicuous and fills a greater part of the seed. Thus, the resultant seed is called endospermic or albuminous. e.g. Castor, Coconut, etc
5) Embryo absorbs reserves food from the endosperm
completely during its developmental stages.
1)Fruit development is
triggered by hormones produced by developing seeds.
2)After fertilization
the zygote is formed and the ovary begins to differentiate into the fruit and
the ovary wall develops into a pericarp.
3)The pericarp is
basically three-layered like epicarp, mesocarp & endocarp which get
differentiated in the fleshy fruit like mango, coconut, etc.
1)Structural or
physiological adaptive mechanism for survival is called Dormancy.
2)Mature and viable
seeds will not germinate even in the presence of favorable conditions. Seeds
get dispersed at different places during dormancy.
3)Viable seeds germinate
only after completion of the dormancy period.
1)It is a phenomenon of the formation of embryo(s) through the asexual method of reproduction without Formation of games and the act of fertilization.
2)Formation of seed without fertilization is called apomixes, and the seeds are called apomictic seeds. 3)It is the type of asexual reproduction that was their no meiosis and syngamy. The embryo develops in the ovule and the ovule develops to form a seed.
4)In apomixis, when a
gametophyte organ or cell produces an embryo-like structure without
fertilization, it is termed as apogamy.
5)When a diploid
sporophyte cell produces a diploid gametophyte without undergoing meiosis is
called Apospory. e.g. Orange, Mango
The main categories of
apomixis are:
a)Recurrent apomixis:
3)Apospory- the nucellar
cells give rise to an apomictic embryo sac.
b) non-recurrent
apomixis:
1)Megaspore mother cell
undergoes usual meiotic division and a haploid embryo sac is
formed.
2)The embryo arises
either from the egg by parthenogenesis or from some other haploid cells of
gametophyte through apogamy.
3)Plants produced by
this method are generally sterile i.e. do not reproduce sexually, e.g.
Nicotiana
c. Adventive
Embryonic:
1)Embryos may develop
from somatic nucellus or integuments along with normal zygotic embryo. 2)It
common in Mango, Orange, Lemon,
etc.
3)It gives rise to a
condition called polyembryony Genetically identical plants can be
produced effectively and rapidly by apomixes
1)This term is coined by
Noll (1902).
2)Fruit Development
without fertilization is called as parthenocarpy.
3)In these plants the
placental tissue in the unfertilized ovary produces auxin IAA
(Indole-3 Acetic Acid) which convert the ovary into a fruit.
1) Development of more than one embryo,
inside a seed, is called polyembryony.
2)It was first noticed
by Leeuwenhoek in the seeds of the Citrus genus.
3)It will result in the
termination of multiple seedlings.
4)Development of more
than one embryo, inside a seed, is called polyembryony.
5)It was first noticed
by Leeuwenhoek (1719) in the seeds of the Citrus genus.
6)It will result in the
termination of multiple seedlings.
7)Parthenogenesis is the
development of an embryo directly from an egg cell or a male gamete. It is a
kind of apogamy.