In the life history of angiospermic plant there are
two stages, the sporophyte and gametophyte which alternate with each other.
Sporophytic
stage:
This stage is derived from the diploid zygote, so it
is a diploid stage in angiospermic plants. The plant body belongs to this stage.
Reproductive organs or flowers develop on this plant.
Gametophytic
stage:
This is the highly reduced haploid stage of
angiospermic plant. This stage is derived from a spore which is a product of
meiotic division of spore mother cells.
In angiosperms, spore mother cells are of two types : the microspore
mother cells and megaspore mother cells.
Microspore mother cells develop in an anther while megaspore mother cell
develops from the nucellus of the ovule. Each of these mother cells in turn
gives rise to respective microspore and megaspores after undergoing meiotic
divisions. Consequently the microscope
and megaspores which are haploid structures give rise to male and female
gametophytes respectively. The male
gametophyte gives rise to male gametes and female gametophyte forms egg
cell. Fusion of male gamete and female
gamete results in the formation of diploid zygote that later becomes embryo in
the seed.
When the seed germinates the embryo develops into a
sporophytic plant.
Structure
of Anther and Microsporogenesis
Structure
of Anther
Pollen grains which contribute the male gametes, are
formed within an anther. A typical (dithecous) anther consists of two anther
lobes, between which there is a column of sterile tissue called the connective.
Each anther lobe in turn bears two pollen chambers or pollen sacs. Each pollen
chamber represents a microsporangium. Thus, a typical dithecous anther
comprises of four pollen sacs or microsporangia.
In some plants, such as Moringa and Wolffia, each
anther lobe has only one microsporangium. Such anthers are called monothecous.
A very young
anther comprises of a homogenous mass of cells bound by a well-defined
epidermis. During its development the anther assumes a four-lobed, and rows of
hypodermal cells become differentiated in each lobe by their larger size,
radial elongation, and more conspicuous nuclei. These form the archesporium.
The archesporial cells divide in a plane parallel to
the outer walls of the anther lobe (periclinal division) to form a primary
parietal layer toward the outside and a primary sporogenous layer toward the
inside.
The cells of the parietal layer undergo a series of
periclinal and anticlinal divisions to form 2-5 concentric layers composing the
wall of the anther. The primary sporogenous cells either directly or after a
few divisions, function as microspore mother cells.
Each anther at maturity consists of sporogenous
tissue covered by an anther wall.
Anther
Wall:
The anther wall consists of epidermis, endothecium,
middle layers and tapetum
Epidermis:
It is the outermost layer of the anther, it is one
celled thick and useful for protection. The epidermal cells undergo repeated
anticlinal divisions in order to cope up with the enlargement of the anther.
The epidermal cells present between the pollensac
are thin walled and constitute ‘stomium’ which is useful for the dehiscence of
pollen sac. The stomium degenerates by the time the spore mother cells undergo
divisions.
Endothecium:
The layer of cells lying immediately beneath the
epidermis is the endothecium. Its maximum development is attained at the time
when the pollen grains are about to be shed. The cells are radially elongated
and from their inner tangential walls thickened fibrous bands develop. These thickenings
are hygroscopic.
The presence of fibrous bands, differential
expansion of the outer and inner tangential walls, and the hygrospic nature of
the endothecial cells help in the dehiscence of anthers at maturity.
Middle
Layers:
Next to the endothecium there are usually one to
three middle layers. They are ephemeral in nature and are crushed during
meiosis of pollen mother cells.
Tapetum:
The innermost layer of anther wall is called as
tapetum which of considerable physiological significance, for all the food
material entering into the sporogenous cells must pass through it. The cells of
tapetum are large, with thin cell walls, abundant cytoplasm and prominent
nuclei.
Based on behaviour, the tapetum is of two types:
1. Amoeboid (
also called invasive or periplasmodial):
This type of tapetum is
characterized by breakdown of the inner and radial walls of the tapetal cells.
The protoplasm moves or protrude into the anther cavity, where they coalesce to
form a continuous mass called the tapetal periplasmodium, closely investing the
pollen mother cells or the microspores.
This type of tapetum is
found in Alisma, Butomus,
Tradescantia,Typha
2. Secretory or Glandular
:
In this type, the tapetal cells
remain in their position throughout the microspore development. Subtances are
contributed by secretion from the inner faces of the cells. Toward the close of
the meiotic divisions in the microspore mother cells, the tapetal cells begin
to lose contact with each other and finally cells are entirely absorbed at the
time when the microspores begin to separate from one another.
This type of tapetum is common
occurrence in angiosperms
A common feature of tapetum , amoeboid as well as
secretory is the presence of more than one nucleus.
The tapetum plays a significant role in the
development of pollen. Tapetum stores starch and protein during early stages of
anther development and such tapetal reserves are used by pollen grains. During
post-meiotic period, tapetum exhibits nutritional value in concern with the
pollen wall formation.
During meiosis it transport the nutrients to the
inside of the anther locule since tapetum is the only channel through which
material can reach the meiocytes.
Sporogenous
Tissue:
The tissue present inside the anther wall is called
sporogenous tissue. It is diploid in nature. The sporogenous cells may directly
function as microspore mother cells or pollen mother cells or they may undergo a few mitosis to add up
to their number before entering meiosis.
Each pollen mother cells by a meiotic division, give
rise to a group of four haploid microspores or pollen tetrads. Cytokinesis
during the meiotic division is of two types in the pollen mother cells, they
are –
1. Successive type
: the nuclear divisions are accompanied by wall formation. After the first
meiotic division a wall is formed separating the two nuclei. This leads to the
formation of a dyad. After second division walls are again fomed resulting in a
tetrad.
2. Simultaneous type
: in this type of cytokinesis, the first meiotic division is not followed by
wall formation. Consequently, binucleate cell is formed after meiosis – I;
there is no dyad stage. The two nuclei synchronously undergo the second meiotic
division. The walls are formed after the second meiotic division, giving rise
to a tetrad.
After the completion of the meiotic division in the
microspore mother cell, different types of tetrads are formed. These are –
i. Tetrahedral type:
this type of arrangement is associated with simultaneous type of cytokinesis in
the pollen mother cell. In the tetrad three microspores are seen in one surface
while the fourth one appears on the backside.
ii.
Isobilateral
type:
four microspores are arranged in the four corners of the square. All the four
spores are visible in the same surface. This is formed due to successive type
of cytokinesis in the microspore mother cell. E.g., Zea mays.
iii. Decussate type:
microspores are arranged in two pairs. One pair of microspores is at right
angles to the other. E.g., Magnolia,
Atriplex
iv.
T
Shaped type: two microspores are arranged side by
side, while the other two appear one above the other. Thus, giving a T shaped
appearance. E.g., Aristolochia.
v. Linear type:
four microspores are arranged one above the other in a single row.
Usually, the microspores soon separate from one
another but in some plants they adhere in tetrads to form the so called
‘compound‘ pollen grains. In the Mimosaceae there are larger units composed of
8 to 64 cells and in genera belonging to the Asclepiadaceae all the microspores
in a sporangium remain together to form a single mass called the pollinium.
In angiosperms, the microspores represent the
beginning of the male gametophytic generation. It is the first cell of male
gametophyte.
They possess a haploid nucleus and dense cytoplasm.
Soon after it’s release from the tetrad the microspore or pollen grain rapidly
increases in volume and accompanying formation of a vacuole. The nucleus is
displaced from the centre to a place adjacent to the wall.
The first division of the microspore gives rise to
the vegetative and generative cells. The generative cell is small and lens
shaped.
Pollen grains are usually dispersed from the anther
in 2-celled condition. In certain plants the generative cell divides in the
pollen grain while it is in the anther itself giving rise to two male gametes
or sperm cells. The pollen grains are thus 3-celled condition in these plants.
The pollen grains or male gametophyte reach the
stigma either in 2-celled or 3-celled condition, through various pollinating
agents.
By absorbing water available in the exudates of
stigma, the pollen germinates and the intine of the pollen grain comes out and
forms a pollen tube. Generally only one pollen tube develops from a pollen
grain (Monosiphonous). But in Malvaceae and Cucurbitaceae more number of pollen
tubes may form from a single pollen grain (Polysiphonous).
The vegetative and generative nuclei enter into the
pollen tube along with cytoplasm. The vegetative and generative cells change
their position during their travel in the pollen tube. The generative cell
undergoes mitotic division and two haploid male gametes are formed. This is
called as spermatogenesis as the male gametes are equivalent to the ciliated
sperms of the lower groups of plants.
Normally, the number of nuclei in a mature pollen
grain is two or three. Occasionally, grains with more nuclei resulting in
abnormal type of the male gametophyte occurs. In 1898 Nemec noted that in
anther of Hyacinthus orientalis the pollen grains some time form large eight
nucleate structures showing a resemblance to embryo sacs.
He believed that they arose as the result of
degeneration of the generative nucleus and three divisions of the vegetative
nucleus.
The microspores increase in size to form large
sac-like bodies. Later the nucleus undergoes three successive divisions to form
8 daughter nuclei. Of these, 3 lie at the end where the exine is still intact,
3 at the opposite end and 2 in the middle. Cells found at the exine end
represent egg and synergids, the opposite end nuclei form the antipodals. The
tow central nuclei represent the polar nuclei.
Androgenic
Embryoids (Haploid plants)
Guha and Maheshwari working with anther cultures of
Datura innoxia reported that embryo like structures called embryoids develop
from the pollen grains. The embryoids give rise to haploid plants.
The anther is excised from the flower bud of
Nicotiana and cultured on a suitable medium. Pollen grains inside the anther by
repeated divisions form a multicellular tissue within the parent wall.
Ultimately the pollen wall bursts, releasing the tissue mass. This tissue mass
is capable to grow directly into an ambryoid and germinates to from a haploid
plant.
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