Read Chapter 2nd of Class12 Biology

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Chapter
Sexual Reproduction in Flowering Plants

Pre-Fertilisation Events
● Several hormonal and structural changes result in the
development of a flower.
● Inflorescences bear the flower buds, and then the flowers.
● Flowers are the reproductive parts of a plant.
● In the flowers, the androecium (male reproductive part) and
the gynoecium (female reproductive part) develop.

See figure in ncert Figure 2.1

Androecium
● The androecium consists of whorls of stamen.
● The stamen consists of the filament ​(long and slender stalk) and anther​ (bilobed structure).
● Filament is attached to the thalamus or to the petal.
● Anther​:
○ A typical anther is bilobed and each lobe is dithecous
(consists of two theca).
○ Theca are separated by a longitudinal groove running
lengthwise.
○ The microsporangia are located at the corners, two in
each theca. They further develop to form pollen sacs,
which contain the pollen grains.

See figure in ncert Figure 2.2

● Structure of microsporangium
○ The microsporangium is surrounded by four wall layers
(epidermis, endothecium, middle layers, and tapetum).
○ The outer three layers are protective and help in
dehiscence of anther to release the pollen grains. The
tapetum provides nourishment to the developing pollen
grains.
○ In the young anther, the sporogenous tissue forms the
centre of each microsporangium.

See figure in ncert Figure 2.3

Microsporogenesis
● It is the process of formation of microspore from PMC (Pollen
Mother Cells).
● As development occurs in the anther, the sporogenous
tissue undergoes meiosis to form microspore tetrad.
● Each cell of sporogenous tissue has capacity to give rise to a
tetrad. Hence, each cell is a potential pollen or PMC.
● As the anther matures, the microspores get detached from
each other and develop into pollen grains.

Pollen grains
● Represent the male gamete and are spherical, having a
two-layered wall:
○ Exine (outer) − Hard layer made of sporopollenin,
which is extremely resistant and can withstand high
temperatures, acidic and alkaline conditions, and
enzymes
○ Intine (inner) − Thin and continuous layer made up of
cellulose and pectin
● Mature pollen grain contains two cells:

○ Vegetative cell − Large with irregular nucleus, contains
food reserves
○ Generative cell − Small and floats in the cytoplasm of
the vegetative cell

See figure in ncert Figure 2.5

● In 60% of the angiosperms, pollen grains are shed at
2-celled stage while in others generative cell undergoes
mitosis to form two male gametes (3-celled stage).
● The viability of pollen grains after they are shed depends
upon temperature and humidity. It ranges from 30 minutes
to few months.

Gynoecium and Formation of Female Gametophyte
● The gynoecium represents the female reproductive part of a
flower.
● It may be mono-carpellary (one pistil) or multi-carpellary
(many pistils). In multi-carpellary, the pistils may be fused
in one (syncarpous) or free (apocarpous).
● Each pistil consists of:
○ Stigma​ − Receives the pollen grains
○ Style​ − Elongated, slender part below the stigma
○ Ovary​ − Bulged basal part containing the placenta,
which is located inside the ovarian locule (cavity)
○ The placenta contains the megasporangia or ovules.

See figure in ncert Figure 2.7

Megasporangium
● The ovule is attached to the placenta by the funicle​. The junction of the ovule and the funicle is called hilum​.
● Each ovule has one or two protective layers, called
integuments​, which cover the rest of the ovule, except for a small opening called micropyle​.
● The chalaza​ lying on the opposite side of the micropyle end represents the basal part of the ovule.
● Nucellus​ is present within the integuments and contains reserved food. The embryo sac​ or female gametophyte is located within the nucellus.

Megasporogenesis
● The megaspore mother cell (MMC) g ​ ets converted into megaspores by the process of megasporogenesis.
● The MMC is large and contains a dense cytoplasm and aprominent nucleus. It undergoes meiosis to produce fourmegaspores.

Female Gametophyte
● In most flowering plants, only one megaspore is functionalwhile the other three degenerate.
● The single functional megaspore develops into the femalegametophyte. This kind of development is called monosporic
development.
● The nucleus of the functional megaspore divides mitoticallyto form 2 nuclei, which move towards the opposite ends,forming a 2-nucleate embryo sac. Two more mitotic divisionsensue, leading to the formation of 4-nucleate and 8-nucleate
embryo sacs.
● After the 8-nucleate stage, the cell walls are laid down andthe typical female gametophyte (embryo sac) gets
organised.
● Six of the 8-nuclei get surrounded by the cell wall and theremaining two, called polar nuclei​, are situated below theegg apparatus in the large central cell​.
● Three of the six cells are placed at the micropylar end andconstitute the egg apparatus (2 ​ synergids + 1 ​ egg cell​).
● The synergids have special thickenings at the micropylarend. These are together called the filiform apparatus​. It
helps in leading the pollen tubes into the synergids.
● Three cells are at the chalazal end, and are called antipodalcells​.
● A typical angiosperm female gametophyte is 7-celled and8-nucleated at maturity.