MALE
GAMETOPHYTE
Androecium
is the male reproductive organ of the plant. It consists of stamens or
microsporophyll. Each stamen consists of three parts- filament, anther and
connective. The filament is the slender stalk of the stamen, which bears a
2-lobed structure –the anther at its tip. The filament and the anther are
joined together at connective.
Structure of anther
Each anther
usually consists of two lobes. Each lobe of the anther contains two pollen sacs
or microsporangia, so, we can find four chambers in each chamber. But in some
cases, we may find only two and sometimes even one chamber. Within each pollen
sac pollen gains or microspores are produced in large quantities in the form of
fine, powdery or granular mass of cells.
Development of anther
Each stamen
arises as a small papillate outgrowth of meristematic tissue from the floral
primordium. It grows actively and soon gets differentiated into an apical
broader portion, the anther and the lower slender part, the filament.
A very young
anther is a homogeneous mass of cells bound by an epidermis. When we cut a
transverse section, at a later stage, we find a four lobed shape. In each lobe
a few cells of hypodermal region become differentiated by their large size,
radial growth, dense cytoplasm and conspicuous nuclei. These are the
arschesporial cells. In majority of the members, the arschesporium consists of
several vertical rows of cells which appear as a plate of cells in a transverse
section.
The
archesporial cells divide periclinally forming a primary parietal layer towards
the epidermis and a primary sporogeneous layer towards the interior. The
primary parietal layer divides to form two secondary parietal layers. Of these,
the inner one forms the tapetum, while the other divides again periclinally to
form two layers of cells. Of these the outer one forms the endothecium, while
the inner one may or may not further divide and form middle layer. The cells of
the primary sporogeneous layer may either function directly as pollen mother
cell or divide to form a large number of cells.
Anther wall-
A transverse section of a fully developed
anther shows four groups of microspore mother cells surrounded by a tapetum,
middle layer, an endothecium and an epidermis.
1. Epidermis -
It is the
outermost layer of the anther. During development, it divides anticlinally only
to keep pace with the enlargement of inner tissues. The cells become greatly
stretched and flattened.
2. Endothecium –
The next
layer is the endothecium or fibrous layer. The walls of its cells become
radially elongated. Their inner tangential walls develop fibrous bands of
callose which run toward and upward ending near the outer wall of each cell.
These thickenings are for dehiscence of anthers. As the thickenings are
hygroscopic, they absorb water, and cause a pressure over endothecium thereby
causing dehiscence along the thin walled cells. The opening through which the
pollen grains are discharged from the pollen sac is known as stomium.
3. Middle layers-
It is 1-2 layered structure. Cells of the
middle layer lack the capacity to divide anticlinally and hence it can not
adjust with the developing inner mass of cells. As a result, it gets crushed.
Sometimes, this layer acts as storage centers for starch (eg. Ranunculus).
4. Tapetum-
Tapetum is
the innermost single layered structure with cells having dense cytoplasm and a
prominent nucleus. Based on the behaviour of the cells at the time of
sporogenesis, the tapetum is of two types-
(A) Glandular tapetum-
It
is also known as secretory type of tapetum.In this type, the cells of tapetum
remains unaltered throughout the development of microspores and secretes
nutritional materials to the anther sac from the inner sides until their total
breakdown at the time of pollen maturity. It is found in 87% of angiosperms.
(B) Amoeboid tapetum-
It is also
known as invasive or plasmodial type of tapetum. Here, the tapetal cells
breakdown, and the protoplasts “wander” into the anther cavity encircling the
pollen mother cells.
Role of tapetum
The tapetum
plays an important role in microsporogenesis and matuaration of microspores
into pollen grains. If it is not properly utilized, the pollen grains either
become non-functional or sterile. It may perform following functions –
1. During
microsporogenesis, the nutrients are chanelled to microspores, through tapetum.
2. Tapetal periplasmodium appears to have
callase activity. This callase helps in the dissolution of callose walls
linking the pollen tetrads.
3. Faulty functioning of tapetum leads to the
formation of cytoplasmic male sterile pollen grains.
4. Tapetum produces sporopollenin through
Ubisch bodies which is essential for wall formation of pollen grains. This
sporopollenin is highly resistant to physical and biological decomposition.
5. It also syntheses and releases an oily
matrial pollen kitt, which gives a coating to the pollen grain (particularly of
entomophilous species). It comprises of carotenes and flavonoids.
6. The tapetum also form pollen wall protein,
which play an important role in pollen-stigma interaction during pollen
germination.
Microsporogenesis
In the
meantime, the primary sporogeneous layer divides for a number of times to form
microspore mother cells or pollen mother cells. These cells then enlarge in
size, become spherical in shape and get loosely arranged.
Most of
these PMCs are functional and divide meiotically to form four pollen grains or
microspores. Rest of the PMCs
degenerate and serve as food material for functional cells. The four pollen
grains formed after Meiosis I & II have haploid nucleus. The cytokinensis
may be of two types-
Successive
Type- Here, the cytokinensis or wall formation takes place just after nuclear
division. It is common in monocots.
Simultaneous Type- In this type, after meiosis
I, no walls are laid down and the two nuclei remain in a cell, divide
simultaneously. The wall formation takes place after meiosis II, resulting in a
pollen tetrad.
Generally
the pollen grains are arranged in tetrads but there may be other arrangements
as tetrahedral, isobilateral, cross, rhomboidal, desussate, T-shaped and
linear.
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