Pollen grains are very fine in size, usually ranging between 10 and 200 microns, and resemble dust particles. They consist of a microscopic cell whose wall is made up of three layers.
The coat of the pollen grains is made hard material known sporopollenin. It protects the gametophytes during its movement from the anther to the stigma of flowering plants, or from the male cone to the female cone of gymnosperms. Pollen germinates and produces pollen tube, if lands on a compatible stigma or female cone. The produced pollen tube transfers the sperm to the ovule in the pistil.
Pollen is small
enough to be seen with naked eye. It needs magnification to see detail. The science
of study of pollen is called palynology. This science is highly useful in
archaeology, paleoecology, paleontology and forensics. In cross pollination the
pollen transfers from the anther of a single flower to the stigma of another on
the same plant or other plant. In self-pollination, this process the pollen
transfers from the anther of a flower to the stigma of the same flower.
Pollen grains are used
as food and food supplement. However, they are often contaminated by agricultural pesticides.
The outer layer (Exine)
The outer layer is
composed of a solid substance called Sporopollenin, which is considered the
hardest substance in existence. The outer layer is the most resistant plant
material to weather and chemical factors. The strongest acids do not affect the
outer layer. It enables the pollen grain to remain among the rocks without
being affected. It retains the shape, features and germination holes on pollen
grain. The appearance of the outer layer varies in terms of shape, composition
and thickness. They are either very thick or very thin, and they are either
granular in shape and have no special structure, or they are composed of stacked
columns perpendicular to the surface of the leaf, known as bacula. These bacula
may end in structures known as heads. Capita), and these heads may join
together to form an external dermis called the tegillum, which is penetrated by
germination holes. This dermis may be formed in many shapes according to the
arrangement of the baculas. Some of them are reticular in appearance, and some
are arranged in parallel rows. The reticulate shape varies greatly from one
pill to another depending on the width or narrowness of the mesh eyes. The
outer layer protects the contents of the grain from drying out and from
external environmental influences. It is noted that this layer is thin above
the germination holes. This enables the germination tubes to penetrate the
outside easily
Middle class (Medline)
This layer is present
in all pollen grains to varying degrees. This layer is characterized by its
pectin-gelatinous nature, and its intermediate structure and function between
the outer and inner layers. Its function is limited to protecting the
germination holes from drying out, and to prevent the pollen grain from tearing
during its expansion and contraction. As a result of absorbing water in humid
weather and losing it in dry weather; this is due to the nature of its composition
of successive layers of two materials different in composition and hardness.
Also, it is noted that this layer is thick under the germination holes, unlike
the outer layer
The inner layer (Intine)
The inner layer is a
thin cellulose layer that completely surrounds the protoplasm. When a pollen
grain germinates, the inner layer grows outward, forming a tube, known as the
pollen tube. It protrudes outward through thin, weak cracks or openings, known
as germ pores, found in the external layer. Sometimes the germination holes are
covered with a distinctive cover, which opens by pushing the pollen tube that
grows from the inner layer. At first, each pollen grain contains only one
nucleus, which is later divided into two nuclei: the larger one is known as the
vegetative nucleus or tube nucleus, while the smaller one is known as the
reproductive nucleus). As the pollen tube grows, the tube nucleus and the
reproductive nucleus move to its apex. Then the tube nucleus advances to the
reproductive nucleus. The reproductive nucleus divides immediately; giving rise
to two male nuclei, known as the male gametes and the nucleus of the pollen
tube decomposes.
Watch more about the pollen
grain structure in the attached link [x].
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