2. SEXUAL REPRODUCTION IN FLOWERING PLANTS
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 differentiation & development of the floral
primordium.
-
Inflorescences bear the floral buds
and then the flowers.
STRUCTURE OF A FLOWER
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
thecae (dithecous). Often a longitudinal groove runs lengthwise
separating the theca.
Transverse section
of anther:
-
The anther is a tetragonal structure
consisting of four microsporangia located at the corners (2 in each
lobe).
-
The microsporangia develop to pollen
sacs. They extend longitudinally all through the length of an anther and
are packed with pollen grains.
Structure of a microsporangium:
- A typical microsporangium is near circular in
outline.
-
It is
surrounded by 4 wall layers: epidermis, endothecium,
middle layers & tapetum (innermost layer).
-
The outer 3 layers give protection
and help in dehiscence of anther to release the pollen.
-
The tapetum nourishes the
developing pollen grains. Cells of the tapetum contain dense cytoplasm and
generally have more than one nucleus.
-
In young anther, each
microsporangium has sporogenous tissue at centre. It consists of compactly
arranged homogenous diploid cells (sporogenous cells).
- As the anther develops, each sporogenous
cell (microspore mother 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.
Pollen grain (male gametophyte):
Generally
spherical. 25-50 mm
in diameter. Cytoplasm is surrounded by a plasma membrane.
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 of sporopollenin. Exine exhibits patterns and designs.
o Intine: Inner
wall. It is a thin and continuous layer made up of cellulose and pectin.
A matured pollen grain
contains 2 cells:
o
Vegetative cell: It is bigger, has
abundant food reserve and a large irregularly shaped nucleus.
o Generative
cell: It is small and floats in the cytoplasm of the vegetative
cell. It is spindle shaped with dense cytoplasm and a nucleus.
- Over 60% angiosperms shed their pollen
grains at 2-celled stage. In others, generative cell divides mitotically
to give 2 male gametes. Thus pollen
grains are shed at 3-celled stage.
-
The shed pollen grains have to land on the
stigma before they lose viability. The viability period of pollen grains is
variable. It depends on temperature and humidity.
- Viability of pollen grains of some cereals
(rice, wheat etc.) is 30 minutes. Some members of Leguminoseae, Rosaceae & Solanaceae have viability for months.
Economic importance of pollen grains:
o These are rich in nutrients. Pollen tablets are
used as food supplements. Pollen tablets & syrups increase performance of
athletes and race horses.
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 one pistil (multicarpellary).
-
In multicarpellary, the pistils may
be fused together (syncarpous) or free (apocarpous).
A. Hibiscus pistil.B. Multicarpellary, syncarpous pistil 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 (papaya,
water melon, orchids etc.).
Structure of Megasporangium (Ovule):
-
Ovule is attached to the placenta by a
stalk (funicle).
-
Junction
between the body of ovule and funicle is called hilum.
-
Each ovule has 1 or 2 protective envelopes
(integuments) except at the tip where a small opening (micropyle) is
present.
-
Opposite the micropylar end is the chalaza
(basal part).
-
Enclosed within the integuments, there is
a mass of cells called nucellus. Its cells contain reserve food
materials.
-
Inside the nucellus is embryo sac
(female gametophyte).
-
An ovule generally has a single embryo sac
formed from a megaspore.
Megasporogenesis:
-
It is the formation of megaspores from megaspore
mother cell (MMC).
-
Ovules generally differentiate a single MMC
in micropylar region of the nucellus. It is a large cell containing dense
cytoplasm and a prominent nucleus.
-
MMC undergoes meiosis to produce 4 megaspores.
Formation of Female gametophyte (embryo
sac):
-
In majority of flowering plants, one megaspore
is functional while the other three degenerates.
- he 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 2-nucleate embryo sac.
-
The nuclei again divide two times forming 4-nucleate
and 8-nucleate stages of the embryo sac.
-
These divisions are free nuclear, i.e. nuclear
divisions are not followed immediately by cell wall formation.
-
After the 8-nucleate stage, cell walls are
laid down leading to the organization of the typical female gametophyte.
-
6 of the 8 nuclei are surrounded by cell
walls and organized
into cells. Remaining 2 nuclei (polar nuclei) are
situated below the egg apparatus in the large central cell.
Distribution of cells within the embryo
sac:
A
typical mature embryo sac is 8-nucleate and 7-celled.
o
3 cells (2 synergids + one egg
cell) are grouped at the micropylar end and form egg apparatus.
Synergids have special cellular thickenings at the micropylar
tip called filiform apparatus. It helps to guide the pollen tubes
into the synergid.
o
3 cells (antipodals) at the chalazal
end.
o
A large central cell with two polar
nuclei.
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