Sexual Reproduction in Flowering Plants 2 Flashcards
describe the different ways of existence of gynoecium
The gynoecium represents the female reproductive part of the flower. The
gynoecium may consist of a single pistil (monocarpellary) or may have
more than one pistil (multicarpellary). When there are more than one,
the pistils may be fused together (syncarpous) (hibiscus, papaver) or may be
free (apocarpous) ( michelia)
parts of a pistil
Each pistil has three parts (Figure 2.7a),
the stigma, style and ovary. The stigma serves as a landing platform
for pollen grains. The style is the elongated slender part beneath the
stigma. The basal bulged part of the pistil is the ovary. Inside the ovary
is the ovarian cavity (locule). The placenta is located inside the ovarian
cavity.
Arising from the placenta are the megasporangia, commonly
called ovules. The number of ovules in an ovary may be one (wheat,
paddy, mango) to many (papaya, water melon, orchids).
describe the ovule
The ovule is a small
structure attached to the placenta by means of a stalk called funicle.
The body of the ovule fuses with funicle in the region called hilum. Thus,
hilum represents the junction between ovule and funicle. Each ovule has
one or two protective envelopes called integuments. Integuments encircle
the nucellus except at the tip where a small opening called the micropyle
is organised. Opposite the micropylar end, is the chalaza, representing
the basal part of the ovule.
what is present inside the ovule
Enclosed within the integuments is a mass of cells called the nucellus.
Cells of the nucellus have abundant reserve food materials. Located in the
nucellus is the embryo sac or female gametophyte. An ovule generally has
a single embryo sac formed from a megaspore.
what is megasporogensis?
The process of formation of megaspores from the
megaspore mother cell is called megasporogenesis. Ovules generally
differentiate a single megaspore mother cell (MMC) in the micropylar region.of the nucellus. It is a large cell containing dense cytoplasm and a
prominent nucleus. The MMC undergoes meiotic division. Meiosis results in the
production of four megaspores
why si the development of female gametophyte called monosporic
In a majority of flowering plants, one of the
megaspores is functional while the other three degenerate. Only the
functional megaspore develops into the female gametophyte (embryo
sac). This method of embryo sac formation from a single megaspore is termed
monosporic development.
explain first step of formation of embryo sac: free nuclear division
The nucleus of the functional megaspore divides mitotically
to form two nuclei which move to the opposite poles, forming the
2-nucleate embryo sac. Two more sequential mitotic nuclear divisions
result in the formation of the 4-nucleate and later the 8-nucleate stages
of the embryo sac. It is of interest to note that these mitotic divisions are
strictly free nuclear, that is, nuclear divisions are not followed immediately
by cell wall formation. After the 8-nucleate stage, cell walls are laid down
leading to the organisation of the typical female gametophyte
or embryo sac.
Six of the eight nuclei are surrounded by cell walls and
organised into cells; the remaining two nuclei, called polar nuclei are
situated below the egg apparatus in the large central cell.
describe the egg apparatus
Three cells are grouped together at the micropylar end and constitute
the egg apparatus. The egg apparatus, in turn, consists of two synergids
and one egg cell. The synergids have special cellular thickenings at the
micropylar tip called filiform apparatus, which play an important role in
guiding the pollen tubes into the synergid. Three cells are at the chalazal
end and are called the antipodals. The large central cell, as mentioned
earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at
maturity, though 8-nucleate is 7-celled.
define pollination. how does it take place in plants
Transfer
of pollen grains (shed from the anther) to the stigma of a pistil is
termed pollination. Flowering plants have evolved an amazing array
of adaptations to achieve pollination. They make use of external
agents to achieve pollination.
describe autogamy
In this type, pollination is achieved within the same
flower. Transfer of pollen grains from the anther to the stigma of the
same flower (Figure 2.9a). In a normal flower which opens and
exposes the anthers and the stigma, complete autogamy is rather
rare. Autogamy in such flowers requires synchrony in pollen release
and stigma receptivity and also, the anthers and the stigma should
lie close to each other so that self-pollination
can occur.
describe mixed mating system plants
Some plants such as Viola
(common pansy), Oxalis, and Commelina
produce two types of flowers –
chasmogamous flowers which are similar to
flowers of other species with exposed anthers
and stigma, and cleistogamous flowers which
do not open at all (Figure 2.9c). In such flowers,
the anthers and stigma lie close to each other.
When anthers dehisce in the flower buds,
pollen grains come in contact with the stigma
to effect pollination. Thus, cleistogamous
flowers are invariably autogamous as there is
no chance of cross-pollen landing on the
stigma. Cleistogamous flowers produce
assured seed-set even in the absence of
pollinators.
describe geitonogamy
Transfer of pollen grains from
the anther to the stigma of another flower of
the same plant. Although geitonogamy is
functionally cross-pollination involving a
pollinating agent, genetically it is similar to
autogamy since the pollen grains come from
the same plant.
describe xenogamy
Transfer of pollen grains from
anther to the stigma of a different plant (Figure
2.9b). This is the only type of pollination which
during pollination brings genetically different
types of pollen grains to the stigma.
oubtreeding devices ( 1st two)
In some species, pollen
release and stigma receptivity are not synchronised. Either the pollen is
released before the stigma becomes receptive or stigma becomes receptive
much before the release of pollen. In some other species, the anther and
stigma are placed at different positions so that the pollen cannot come in
contact with the stigma of the same flower. Both these devices prevent
autogamy.
self incompatibility
The third device to prevent inbreeding is self-incompatibility.
This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
E.g. Petunia, Tobacco
