All flowering plants (angiosperms) show sexual reproduction. Flowers are the sites of sexual reproduction.
PRE-FERTILISATION: STRUCTURES & EVENTS
- Several hormonal and structural changes result in Microsporogenesis:
differentiation & development of the floral primordium. - As the anther develops, each
- Inflorescences bear the floral buds and then the flowers. sporogenous cell (microspore mother
STRUCTURE OF A FLOWER cell or pollen mother cell) undergoes
meiotic divisions to form microspore
tetrads (microspores arranged in a
cluster of four cells).
- Formation of microspores from pollen mother cell (PMC)
through meiosis is called microsporogenesis.
- As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains.
- Each microsporangium contains thousands of pollen grains.
They are released with the dehiscence of anther.
A typical flower has 2 parts: Androecium & Gynoecium.
Androecium (male reproductive part)
It consists of a whorl of stamens. Their number and length
are variable in different species.
A stamen has 2 parts:
a. Filament: Long and slender stalk. Its proximal end is
attached to the thalamus or the petal of the flower.
b. Anther: Terminal and typically bilobed. Each lobe has 2 Pollen grain (male gametophyte):
thecae (dithecous). Often a longitudinal groove runs Generally spherical. 25-50 µm in diameter. Cytoplasm is
lengthwise separating the theca. surrounded by a plasma membrane.
Transverse section of anther: A pollen grain has a two-layered wall: exine and intine.
o Exine: Hard outer layer. Made up of sporopollenin (highly
resistant organic material). It can withstand high
temperature and strong acids and alkali. Enzymes cannot
degrade sporopollenin.
Exine has apertures called germ pores where
sporopollenin is absent.
Pollen grains are preserved as fossils due to the presence
- The anther is a tetragonal structure consisting of four of sporopollenin. Exine exhibits patterns and designs.
microsporangia located at the corners (2 in each lobe). o Intine: Inner wall. It is a thin and continuous layer made
- The microsporangia develop to pollen sacs. They extend up of cellulose and pectin.
longitudinally all through the length of an anther and are A matured pollen grain contains 2 cells:
packed with pollen grains. o Vegetative cell:
Structure of a microsporangium: It is bigger, has
- A typical microsporangium is near circular in outline. abundant food
- It is surrounded by 4 wall layers: epidermis, endothecium, reserve and a
middle layers & tapetum (innermost layer). large irregularly
- The outer 3 layers give protection and help in dehiscence shaped nucleus.
of anther to release the pollen. o Generative
- The tapetum nourishes the developing pollen grains. Cells cell: It is small
of the tapetum contain dense cytoplasm and generally have and floats in the cytoplasm of the vegetative cell. It is
more than one nucleus. spindle shaped with dense cytoplasm and a nucleus.
- In young anther, each microsporangium has sporogenous - Over 60% angiosperms shed their pollen grains at 2-celled
tissue at centre. It consists of compactly arranged stage. In others, generative cell divides mitotically to give
homogenous diploid cells (sporogenous cells). 2 male gametes. Thus pollen grains are shed at 3-celled stage.
1
, - The shed pollen grains have to land on the stigma before - Inside the nucellus is embryo sac (female gametophyte).
they lose viability. The viability period of pollen grains is - An ovule generally has a single embryo sac formed from a
variable. It depends on temperature and humidity. megaspore.
- Viability of pollen grains of some cereals (rice, wheat etc.) Megasporogenesis:
is 30 minutes. Some members of Leguminoseae, Rosaceae - It is the formation of megaspores from megaspore mother
& Solanaceae have viability for months. cell (MMC).
Economic importance of pollen grains: - Ovules generally differentiate a single MMC in micropylar
o These are rich in nutrients. Pollen tablets are used as food region of the nucellus. It is a large cell containing dense
supplements. Pollen tablets & syrups increase performance cytoplasm and a prominent nucleus.
of athletes and race horses. - MMC undergoes meiosis to produce 4 megaspores.
o They are stored for years in liquid nitrogen (-1960 C). They
can be used as pollen banks in crop breeding programmes.
o Pollen grains of some plants (e.g. Parthenium or carrot
grass) are allergic for some people. It leads to chronic
respiratory disorders (asthma, bronchitis, etc.).
Gynoecium (female reproductive part)
- It may have a single pistil (monocarpellary) or more than
Formation of Female gametophyte (embryo sac):
one pistil (multicarpellary).
- In majority of flowering plants, one megaspore is
- In multicarpellary, the pistils may be fused together
functional while the other three degenerates.
(syncarpous) or free (apocarpous).
- The functional megaspore develops into the female
gametophyte. The embryo sac formation from a single
megaspore is called monosporic development.
- Nucleus of the functional megaspore divides mitotically to
form two nuclei. They move to the opposite poles, forming
A. Hibiscus pistil. 2-nucleate embryo sac.
B. Multicarpellary, - The nuclei again divide two times forming 4-nucleate and
syncarpous pistil
8-nucleate stages of the embryo sac.
of Papaver.
C. Multicarpellary,
apocarpous
gynoecium of
Michelia
Each pistil has three parts:
o Stigma: Landing platform for pollen grains.
o Style: Elongated slender part beneath the stigma.
o Ovary: Basal bulged part. It has ovarian cavity (locule)
in which placenta is located. Arising from the placenta are
the ovules (megasporangia). Number of ovules in an
ovary may be one (wheat, paddy, mango etc.) to many - These divisions are free nuclear, i.e. nuclear divisions are
(papaya, water melon, orchids etc.). not followed immediately by cell wall formation.
- After the 8-nucleate stage, cell walls are laid down leading
Structure of Megasporangium (Ovule):
to the organization of the typical female gametophyte.
- Ovule is attached to the placenta by a stalk (funicle). - 6 of the 8 nuclei are surrounded by cell walls and organized
- Junction between the body of ovule and funicle is called into cells. Remaining 2 nuclei (polar nuclei) are situated
hilum. below the egg apparatus in the large central cell.
- Each ovule has 1 or 2
Distribution of cells within the embryo sac:
protective envelopes
A typical mature embryo sac is 8-nucleate and 7-celled.
(integuments) except
o 3 cells (2 synergids + one egg cell) are grouped at the
at the tip where a small
micropylar end and form egg apparatus.
opening (micropyle)
Synergids have special cellular thickenings at the
is present.
micropylar tip called filiform apparatus. It helps to
- Opposite the
guide the pollen tubes into the synergid.
micropylar end is the
o 3 cells (antipodals) at the chalazal end.
chalaza (basal part).
o A large central cell with two polar nuclei.
- Enclosed within the integuments, there is a mass of cells
called nucellus. Its cells contain reserve food materials.
2
PRE-FERTILISATION: STRUCTURES & EVENTS
- Several hormonal and structural changes result in Microsporogenesis:
differentiation & development of the floral primordium. - As the anther develops, each
- Inflorescences bear the floral buds and then the flowers. sporogenous cell (microspore mother
STRUCTURE OF A FLOWER cell or pollen mother cell) undergoes
meiotic divisions to form microspore
tetrads (microspores arranged in a
cluster of four cells).
- Formation of microspores from pollen mother cell (PMC)
through meiosis is called microsporogenesis.
- As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains.
- Each microsporangium contains thousands of pollen grains.
They are released with the dehiscence of anther.
A typical flower has 2 parts: Androecium & Gynoecium.
Androecium (male reproductive part)
It consists of a whorl of stamens. Their number and length
are variable in different species.
A stamen has 2 parts:
a. Filament: Long and slender stalk. Its proximal end is
attached to the thalamus or the petal of the flower.
b. Anther: Terminal and typically bilobed. Each lobe has 2 Pollen grain (male gametophyte):
thecae (dithecous). Often a longitudinal groove runs Generally spherical. 25-50 µm in diameter. Cytoplasm is
lengthwise separating the theca. surrounded by a plasma membrane.
Transverse section of anther: A pollen grain has a two-layered wall: exine and intine.
o Exine: Hard outer layer. Made up of sporopollenin (highly
resistant organic material). It can withstand high
temperature and strong acids and alkali. Enzymes cannot
degrade sporopollenin.
Exine has apertures called germ pores where
sporopollenin is absent.
Pollen grains are preserved as fossils due to the presence
- The anther is a tetragonal structure consisting of four of sporopollenin. Exine exhibits patterns and designs.
microsporangia located at the corners (2 in each lobe). o Intine: Inner wall. It is a thin and continuous layer made
- The microsporangia develop to pollen sacs. They extend up of cellulose and pectin.
longitudinally all through the length of an anther and are A matured pollen grain contains 2 cells:
packed with pollen grains. o Vegetative cell:
Structure of a microsporangium: It is bigger, has
- A typical microsporangium is near circular in outline. abundant food
- It is surrounded by 4 wall layers: epidermis, endothecium, reserve and a
middle layers & tapetum (innermost layer). large irregularly
- The outer 3 layers give protection and help in dehiscence shaped nucleus.
of anther to release the pollen. o Generative
- The tapetum nourishes the developing pollen grains. Cells cell: It is small
of the tapetum contain dense cytoplasm and generally have and floats in the cytoplasm of the vegetative cell. It is
more than one nucleus. spindle shaped with dense cytoplasm and a nucleus.
- In young anther, each microsporangium has sporogenous - Over 60% angiosperms shed their pollen grains at 2-celled
tissue at centre. It consists of compactly arranged stage. In others, generative cell divides mitotically to give
homogenous diploid cells (sporogenous cells). 2 male gametes. Thus pollen grains are shed at 3-celled stage.
1
, - The shed pollen grains have to land on the stigma before - Inside the nucellus is embryo sac (female gametophyte).
they lose viability. The viability period of pollen grains is - An ovule generally has a single embryo sac formed from a
variable. It depends on temperature and humidity. megaspore.
- Viability of pollen grains of some cereals (rice, wheat etc.) Megasporogenesis:
is 30 minutes. Some members of Leguminoseae, Rosaceae - It is the formation of megaspores from megaspore mother
& Solanaceae have viability for months. cell (MMC).
Economic importance of pollen grains: - Ovules generally differentiate a single MMC in micropylar
o These are rich in nutrients. Pollen tablets are used as food region of the nucellus. It is a large cell containing dense
supplements. Pollen tablets & syrups increase performance cytoplasm and a prominent nucleus.
of athletes and race horses. - MMC undergoes meiosis to produce 4 megaspores.
o They are stored for years in liquid nitrogen (-1960 C). They
can be used as pollen banks in crop breeding programmes.
o Pollen grains of some plants (e.g. Parthenium or carrot
grass) are allergic for some people. It leads to chronic
respiratory disorders (asthma, bronchitis, etc.).
Gynoecium (female reproductive part)
- It may have a single pistil (monocarpellary) or more than
Formation of Female gametophyte (embryo sac):
one pistil (multicarpellary).
- In majority of flowering plants, one megaspore is
- In multicarpellary, the pistils may be fused together
functional while the other three degenerates.
(syncarpous) or free (apocarpous).
- The functional megaspore develops into the female
gametophyte. The embryo sac formation from a single
megaspore is called monosporic development.
- Nucleus of the functional megaspore divides mitotically to
form two nuclei. They move to the opposite poles, forming
A. Hibiscus pistil. 2-nucleate embryo sac.
B. Multicarpellary, - The nuclei again divide two times forming 4-nucleate and
syncarpous pistil
8-nucleate stages of the embryo sac.
of Papaver.
C. Multicarpellary,
apocarpous
gynoecium of
Michelia
Each pistil has three parts:
o Stigma: Landing platform for pollen grains.
o Style: Elongated slender part beneath the stigma.
o Ovary: Basal bulged part. It has ovarian cavity (locule)
in which placenta is located. Arising from the placenta are
the ovules (megasporangia). Number of ovules in an
ovary may be one (wheat, paddy, mango etc.) to many - These divisions are free nuclear, i.e. nuclear divisions are
(papaya, water melon, orchids etc.). not followed immediately by cell wall formation.
- After the 8-nucleate stage, cell walls are laid down leading
Structure of Megasporangium (Ovule):
to the organization of the typical female gametophyte.
- Ovule is attached to the placenta by a stalk (funicle). - 6 of the 8 nuclei are surrounded by cell walls and organized
- Junction between the body of ovule and funicle is called into cells. Remaining 2 nuclei (polar nuclei) are situated
hilum. below the egg apparatus in the large central cell.
- Each ovule has 1 or 2
Distribution of cells within the embryo sac:
protective envelopes
A typical mature embryo sac is 8-nucleate and 7-celled.
(integuments) except
o 3 cells (2 synergids + one egg cell) are grouped at the
at the tip where a small
micropylar end and form egg apparatus.
opening (micropyle)
Synergids have special cellular thickenings at the
is present.
micropylar tip called filiform apparatus. It helps to
- Opposite the
guide the pollen tubes into the synergid.
micropylar end is the
o 3 cells (antipodals) at the chalazal end.
chalaza (basal part).
o A large central cell with two polar nuclei.
- Enclosed within the integuments, there is a mass of cells
called nucellus. Its cells contain reserve food materials.
2
