XII BIOLOGY
CH-1 Reproduction in lower and higher plants
PART-PART- 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 embryo sac gets degenerate. The formation of
triploid endosperm nucleus triggers cell division which leads to the formation
of endosperm.
4.
There are three
types of endospermic development
i) Nuclear type ii) Cellular type iii) Helobial type
1)Nuclear
type of Endosperm:
It is the most common type of endosperm formation
found in 161 angiospermic families.
i) The primary endospermic nucleus repeatedly
divides mitotically without wall formation to produce large number of free
nuclei.
ii)A big central vacuole appears in the centre of
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. v)Coconut has multicellular
endosperm in the outer part and free nuclear as well as vacuolated endosperm in
the centre.
ii)
Cellular type of Endosperm:
1.
In some plants,
division of triploid primary endospermic
nucleus is immediately followed by wall formation.So that the endosperm is
cellular right from the beginning.
2.
It is mostly
observed in 72 families of dicots
as in members - Balsam, Petunia, Adoxa,
etc .
iii) Helobial type of Endosperm:
1.
It occurs in the
order Helobiales of monocotyledons.
2.
The first
nuclear division of primary
endosperm is followed
a transverse wall, which divides
the cell unequally.
3.
The smaller cell
is called chalaza I cell and larger cell is the micropylar cell.
4.
Then the
nuclei in each cell divide by free
nuclear divisions and
then walls develop between nuclei
in micropylar chamber .
5.
It is an
intermediate type between the nuclear and
cellular type e.g. Asphodelus
Endosperm containing
tissues of two different types is called mosaic endosperm.
In plants like corn the
endosperm contains patches of two different colours.
It forms a sort of
mosaic pattern.
Salient features of
Endosperm
1.
Endosperm
development takes place from the
triploid cell (3n) in the embryo sac. The second male gamete fuses with the
diploid secondary nucleus (formed by 2 polar nuclei) to form primary endosperm
nucleus (PEN) and a triploid cell. The process is known as triple fusion
2.
Endospermic cells are
mostly triploid but in some plants, they may be diploid (water lily) or
polyploid (up to 15n)
3.
n some of the dicots,
mature seeds contain food stored in the endosperm, they are called endospermic
or albuminous seeds, e.g. castor
4.
In beans, peas, gram
seeds, the endosperm is completely absorbed during development and food is
stored in two cotyledons. They are called exalbuminous seeds
5.
Endosperm mostly
contains starch as a food reserve, but it may contain fats, e.g. castor
6.
Endosperm provide nutrients
to seeds during dormancy
7. Endosperm also
contains certain hormones like cytokinins and helps in cell differentiation
8.
In grains, like wheat,
maize, barley, corn, the endosperm is the main source of food
9.
Coconut water is the
example of a liquid endosperm
10.The white flour, used to prepare bread, only
contains endosperm of wheat seeds
11.The outer layer of endosperm is the aleurone
layer. It secretes the amylase enzyme, which breaks down the starch contained
in the endosperm to sugars for utilisation by seedlings
Types of Endosperm
How the primary
endosperm nucleus (PEN) develops into endosperm differs in various plants.
Endosperms are divided into three main types based on their development
pattern.
1. Nuclear Endosperm:
It is the most common type of endosperm found.
Here PEN divides repeatedly (mitotic division) without cytokinesis. It results
in the formation of a large number of free nuclei in the cell. A large central
vacuole is formed and nuclei get arranged at the periphery. There are more
nuclei at the chalazal and micropylar end, compared to the sides.
At this stage, cell
wall formation takes place from the periphery towards the centre and
multicellular endosperm is formed. Examples: maize, rice, wheat, cotton,
sunflower
In coconut, the cell
wall formation is incomplete resulting in the outer multicellular solid
endosperm and inner multinucleated (having free nuclei) liquid endosperm
(coconut milk)
2. Cellular Endosperm:
It is not very common.
In this type of development, division of PEN (karyokinesis) is followed by
cytokinesis and two cells are formed due to transverse division, giving rise to
the chalazal and micropylar chamber. Further division is similar, which leads
to the formation of the cellular endosperm. Examples: Petunia, Balsam, Datura.
3. Helobial Endosperm:
This type of endosperm
development is common in monocotyledons. The first division is similar to
cellular endosperm and results in a large micropylar cell and small chalazal
cell. The chalazal cell mostly does not divide further and functions as a base
cell. The micropylar cell divides further, similar to nuclear endosperm. So
helobial is an intermediate type, a combination of both nuclear and cellular
endosperm. Examples: Eremurus
- Endosperm is important for the growth of an embryo and stores the food reserve, They supply nutrients and provide protection to the developing embryo.
- Endospermic tissues have shown to regulate gene expression and seed germination.
- Endosperm induces signals according to environmental conditions and regulates embryonic growth.
- The endosperm contains cytokinin, which regulates cellular differentiation,It may induce abortion of seeds from the genetically mismatched cross.