Sexual Reproduction in Flowering Plants pdf Flashcards
how do we know that a plant will flower or not
Much before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place. Several hormonal and structural changes
are initiated which lead to the differentiation and further development of
the floral primordium. Inflorescences are formed which bear the floral buds
and then the flowers. In the flower the male and female reproductive
structures, the androecium and the gynoecium differentiate and develop.
what does a stamen consist of
The stamen is the unit of androecium ( male reproductive whorl)
the two parts of a typical stamen – the long and slender
stalk called the filament, and the terminal generally bilobed structure
called the anther.
The proximal end of the filament
is attached to the thalamus or the petal of the flower or perianth. The distal end is attached to the anther.
The number and length of stamens are variable in
flowers of different species.
what are the features of anther of a plant
A typical angiosperm anther is bilobed with
each lobe having two theca, i.e., they are dithecous
(Figure 2.2 b). Often a longitudinal groove runs
lengthwise separating the theca.
The bilobed nature of an anther is
very distinct in the transverse section of the anther.
The anther is a four-sided (tetragonal) structure
consisting of four microsporangia located at the
corners, two in each lobe, ie, it is tetrasporangiate.
what develop into the pollen sacs?
The microsporangia develop further and
become pollen sacs. They extend longitudinally
all through the length of an anther and are packed
with pollen grains.
The theca, microsporangia and pollen sacs refer to the same cavity.
It is called theca when it encloses sporogenous tissue. it is called microsporangia when it encloses microspores while it is called pollen sacs when it bears pollen grains.
describe the structure of microsporangia
In a transverse
section, a typical microsporangium appears near
circular in outline. It is generally surrounded by four wall layers – the epidermis, endothecium, middle layers and the tapetum.
The
outer three wall layers perform the function of protection and help in
dehiscence of anther to release the pollen.
The innermost wall layer is
the tapetum. It nourishes the developing pollen grains. Cells of the
tapetum possess dense cytoplasm and generally have more than one
nucleus.
Young anther will possess all 4 layers while mature anthers will possess only epidermis and endothecium.
what is sporogenous tissue
When the anther is young, a group of compactly arranged homogenous
cells called the sporogenous tissue occupies the centre of each
microsporangium.
define microsporogenesis? how does it take place?
As each cell of the sporogenous tissue is capable of giving rise to a
microspore tetrad. Each one is a potential pollen or microspore mother
cell. The process of formation of microspores from a pollen mother cell (PMC)
through meiosis is called microsporogenesis. The microspores, as they
are formed, are arranged in a cluster of four cells–the microspore tetrad
what happens to the microspore tetrad?
As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains (Figure 2.3 b).
Inside each microsporangium several thousands of microspores or pollen
grains are formed that are released with the dehiscence of anther
what represents male gametophyte
The pollen grains represent the male gametophytes.Pollen grains are generally spherical measuring about
25-50 micrometers in diameter. It has a prominent two-layered
wall.
explain outer wall or exine
The hard outer layer called the exine is made up of
sporopollenin which is one of the most resistant organic material
known. It can withstand high temperatures and strong acids
and alkali. No enzyme that degrades sporopollenin is so far
known. Pollen grain exine has prominent apertures called germ
pores where sporopollenin is absent. Pollen grains are wellpreserved as fossils because of the presence of sporopollenin.
The exine exhibits a fascinating array of patterns and designs.
what is the inner wall or intine
The inner wall of the pollen grain is
called the intine. It is a thin and continuous layer made up of
cellulose and pectin ( also c/a pectocellulose). The cytoplasm of pollen grain is
surrounded by a plasma membrane
explain how generative and vegetaitve cells are formed
the microspore first becomes big due to vacuolisation. Then it undergoes asymmetrical division to form one larger cell called vegetative cell and one small cell called generative cell.
The generative cell undergoes mitosis to form 2 male gametes
what are vegetative and generative cells
When the pollen grain is
mature it contains two cells, the vegetative cell and generative
cell (Figure 2.5b). The vegetative cell is bigger, has abundant
food reserve and a large irregularly shaped nucleus. The
generative cell is small and floats in the cytoplasm of the
vegetative cell. It is spindle shaped with dense cytoplasm and
a nucleus. In over 60 per cent of angiosperms, pollen grains
are shed at this 2-celled stage. In the remaining species, the
generative cell divides mitotically to give rise to the two male
gametes before pollen grains are shed (3-celled stage).
what are pollen allergies
Pollen grains of many species cause severe allergies and bronchial
afflictions in some people often leading to chronic respiratory
disorders– asthma, bronchitis, etc. It may be mentioned that Parthenium
or carrot grass that came into India as a contaminant with imported wheat,
has become ubiquitous in occurrence and causes pollen allergy.
why are pollen supplements used?
Pollen grains are rich in nutrients. It has become a fashion in recent
years to use pollen tablets as food supplements. In western countries, a
large number of pollen products in the form of tablets and syrups are
available in the market. Pollen consumption has been claimed to increase
the performance of athletes and race horses
