Sexual Reproduction in Flowering Plants-1
Sexual Reproduction in Flowering Plants-1
Sexual Reproduction in Flowering Plants-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.
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.
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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
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.
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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) gets converted into megaspores by the process of
megasporogenesis.
The MMC is large and contains a dense cytoplasm and a prominent nucleus. It undergoes
meiosis to produce four megaspores.
Female Gametophyte
In most flowering plants, only one megaspore is functional while the other three
degenerate.
The single functional megaspore develops into the female gametophyte. This kind of
development is called monosporic development.
The nucleus of the functional megaspore divides mitotically to form 2 nuclei, which move
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towards the opposite ends, forming a 2-nucleate embryo sac. Two more mitotic divisions
ensue, leading to the formation of 4-nucleate and 8-nucleate embryo sacs.
After the 8-nucleate stage, the cell walls are laid down and the typical female gametophyte
(embryo sac) gets organised.
Six of the 8-nuclei get surrounded by the cell wall and the remaining two, called polar
nuclei, are situated below the egg apparatus in the large central cell.
Three of the six cells are placed at the micropylar end and constitute the egg apparatus
(2 synergids + 1 egg cell).
The synergids have special thickenings at the micropylar end. 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 antipodal cells.
A typical angiosperm female gametophyte is 7-celled and 8-nucleated at maturity.
Pollination
It is the process of transfer of pollen grains from the anther to the stigma.
Depending on the source of pollen, pollination can be divided as follows:
Autogamy It is the transfer of pollen grains from the anther to the stigma of the same
flower. Autogamy requires the anther and the stigma to lie close. It also requires synchrony
in the pollen release and stigma receptivity.
Plants like Viola, Oxalis, etc., produce two kinds of flowerschasmogamous flowers
(with exposed anther and stigma) and cleistogamous flowers (which do not open at all
and only autogamy occurs).
Geitonogamy It is the transfer of pollens from the anther of one flower to the stigma of
another flower in the same plant. Genetically, it is similar to autogamy, but it requires
pollinating agents.
Xenogamy It is the transfer of pollen grains from the anther to the stigma of a different
plant. Pollination causes genetically different types of pollens to be brought to a plant.
Agents of Pollination
Plants use air, water (abiotic agents) and animals (biotic agents) for pollination.
Pollination by wind
It is the most common form of abiotic pollination.
Plants possess well-exposed stamens and large, feathery stigma.
Pollens should be light and non-sticky to be carried easily by winds.
Wind-pollinated flowers often have single ovule in the ovary and numerous flowers packed
in an inflorescence.
It is common in grass.
Pollination by water
It is rare in flowering plants, except for some aquatic plants like Vallisneria and Hydrilla.
In most water-pollinated plants, the pollen grains are long and ribbon-like, and are
protected from wetting by mucilaginous covering.
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Post-Fertilisation Events
It includes development of endosperm and embryo, and maturation of ovules into seeds
and ovaries into fruits.
Formation of Endosperm
The endosperm develops before the embryo because the cells of the endosperm provide
nutrition to the developing embryo.
The primary endosperm nucleus repeatedly divides to give rise to free nuclei. This stage of
development is called free nuclear endosperm.
Cell wall formation occurs next, resulting in a cellular endosperm.
The endosperm may be either fully consumed by the growing embryo (as in pea and
beans) or retained in the mature seed (as in coconut and castor).
Development of Embryo
The embryo develops at the micropylar end of the embryo sac where the zygote is
situated.
The zygote gives rise first to the pro-embryo, and then to the globular, heart-shaped,
mature embryo.
A typical dicot embryo consists of an embryonal axis and two cotyledons.
The portion of the embryonal axis above the level of cotyledons is called epicotyl. It
contains the plumule (shoot tip). The portion below the axis is called hypocotyl. It contains
the radicle (root tip). The root tip is covered by the root cap.
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as
and
Development of Fruits
The ovary of a flower develops into a fruit.
The walls of the ovary transform into the walls of the fruit (pericarp).
Fruits may be fleshy, as in mango and orange, or can be dry, as in groundnut and mustard.
In some plants, floral parts other than the ovary take part in fruit formation, as in apple and
strawberry. In these, the thalamus contributes to fruit formation. Such fruits are called
false fruits. Fruits that develop from the ovary are called true fruits.
Some fruits develop without fertilisation, and are known as parthenocarpic fruits
(example: banana).
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