Basic Terms ,
Sexual Reproduction in flowering plants.
by AGRI grovestudies
All flowering plants show sexual reproduction.
“Angiosperm” and
Gymnosperms /--
Angiosperm
Ã
·
A seed is produced by flowering plants & is
enclosed within an ovary.
·
The lifecycle of these plants are seasonal.
·
Has triploid tissue.
·
Leaves are flat in shape..
·
Hardwood type
·
Reproduction rely on animals.
·
Reproduction system present in flowers (unisexual or
bisexual).
Gymnosperms
/-
·
A seed is produced by non-flowering plants and are
unclosed or naked.
·
These plants are evergreen.
·
Has haploid tissue.
·
Leaves are saclike and needle – like in shape.
·
Softwood type.
·
Reproduction rely on wind.
·
Reproduction system present in cones & are unsexual.
Five flowers of ornamental value that are commonly
cultivated at homes & in gardens.
The names of five more flower that are used in social
& cultural celebrations in family these are called floriculture.
Pre-fertilisation
; structures & events /-
The
androecium consists of a whorl of stamens representing the male reproduction
organ & the gynoecium represents the female reproductive organ.
Stamen,
Microsporangium & Pollen Grains /-
The
two parts of a typical stamen-the long and slender stalk called filament.
·
Blooded structure called the anther.
·
The proximal end of the filament is attached to the thalamus or the
petal of the flowers.
The
number & length of stamens are variable in flowers of different species.
A typical angiosperm anther is bilobed with each lobe
having two theca i.e, they are dithecous.
The anther is a
four-sided (tetragonal) structure consisting of four microsporangia located at
the corners, two in each lobe.
Structure
of Microsporangium /-
A
typical micro-microsporangium appears near circular in outline.
·
It is generally surrounded by four wall layers and the
tapetum.
·
The outer three walls layers perform the function of
protection.
·
Help in dihescense of anther.
·
The innermost wall layer is the tapetum.
è
In nourishes the developing pollen grains. Cells of
the tapetum possess dense cytoplasm & generally have more than one nucleus.
The
anther is young, a group of compactly arranged homogenous cells called the
Sporogenous tissue.
Microsporogenesis
Ã
The
anther develops, the cells of the sporogenous tissue undergo meiotic division
to form microspore tetrods.
·
The process of
formation of microspore from a pollen mother cell (PMC) through meiosis is called
microsporogenesis.
·
The microspores, as they are formed, are arranged in a cluster of four cells – the
microspore tetrad.
·
The anthers mature
& dehydrates, the microspore dissociate form each other & develop into
pollen grains.
Pollen
Grains ;- The pollen grains represent the
male gametophytes (Hibiscus).
ü Pollen grains are
generally spherical measuring about 50 micrometers in diameters.
ü The hard outer
layer called the exine is made up of sporopollenin which is one of the most
resistant organic material known.
ü It can withstand
high temperature and strong acids & alkali.
No
enzyme that degrads sporopollenin is for known.
Pollen
grains exine has prominent aperture called germspores.
ü The inner wall of the pollen grain is called the
intine. It is a thin & continuous layer made up of cellular & pectine.
The pollen grain is mature it contains two cells, the
vegetative cell & generation cells.
ü The pollen grain is mature it contains two cells, the
vegetative cell & generative cell.
ü In over 60 percent
of angiosperms, this 2-celled stage. In the remaining species.
ü The two male gamete
before pollen grains are shed (3 – celled stage).
Pollen grains of many species cause serve allergies & bronchial,
disorders – asthma, bronchills, etc. parthenium
or carrot grass that come into India as a contaminant with imported wheat.
The
pistil, Megasporangium ( ovule) & Embryo sac /-
The
gynoecium may consists of a single pistil (monocarpally) or may more than one
pistil (multicellulary).
·
Each pistil has three parts, the stigma, style &
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 ovarian
cavity (locule).
·
The placenta are
the megasporangia, commonly called ovules.
The
Megasporangium (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 funcile in the region
called hilium.
·
Each ovule has one or two protective envelops called
integumnets. The tip where a small opening called the micropyle is organized.
·
The micropylar end, is the chalaza.
·
Enclosed within the integuments is a mass of cell
called the nucellus. The nucellus is the embryo sac or female gametophyte.
Megasporangenesis
;-
The
process of formation of megaspores from the megaspore mother cell is called
megasporogenesis.
Female
Gametophyte /-
One
of the megaspores is functional while the other three degenerate. Only the
functional megaspores develops into the female gametophyte (embryo sac).
·
The formation of the 4 – nucleate & later the 8 –
nucleate stages of the embryo sac.
·
The 8 –
nucleate stage, cell walls are laid down leading to the organization of the
typical female gametophyte or embryo sac.
·
The remaining two nuceli, called polar nuclei are
situated below the egg apparatus in the large central cell.
·
Three cells are grouped together at the metropylar end
constitute the egg apparatus. The egg apparatus, in turn, consists of two
synergids and one egg cells.
Three cells are at the chalazal end & are called
the antipodals.
è
A typical
angiosperm embryo sac, at maturity, through & nucleate is 7 celled.
Pollination
/-
Pollination
is the mechanism to achieve the objective. Transfer of pollen grains (shed from
the anther) to the stigma of a pistil is termed pollination.
Kinds
of Pollination /-- Depending on the source of pollen, pollination can be
divided into three type.
(i)
Autogamy – Pollination is
achieved within the same flower. transfer of pollen grains from the anther of
the stigma of the same flower.
Chasmogamous flower
which are similar to flower of the other species with exposed anthers &
stigma & cleistogamous flowers which do not open at all.
(ii)
Geitonogamy /- Transfer of pollen
grains from the anther to the stigma of another flower of same plant.
(iii)
Xenogamy /- transfer of pollen
grains from anther to the stigma of a different plant.
Agents of
Pollination /-
Plants
use two abiotic (wind & water) & one biotic (animals) agents to achieve
pollination.
·
Pollination by wind is more common amongst abiotic
pollinations. Wind pollination also requires that the pollen grains are light
& non-sticky so that they can be transported in wind currents.
·
Pollination by water is quite rare in flowering plants
and is limited to about 30 genra, mostly monocotyledons.
Example
of water pollination plants are Vallisneria
and Hydrilla which grow in fresh
water & several marine sea-grassess such as Zostera.
·
Majority of flowering plants use a range of animals
are pollinating agents, bees, butterflies, files, beetles, wasps, ants, moths,
birds (sunbirds & humming birds) & bats are the common pollinating
agents.
·
Majority of insect – pollinated flowers are large,
colourful, fragments & rich in nectar.
Outbreeding
Devices /-
Majority
of flowering plants produce hermaphrodite flower & pollen grains are likely
to come in contact with the stigma of same flower.
·
Contiued self – pollination result in inbreeding
depression.
·
If both male & female flowers are present on the
plant such as caster & maize (monoecious).
·
It prevents autogamy but not geitonogamy.
·
In several species such as papaya, male & female
flowers are present on different plants, that is each plant is either male or
female (dioecy).
Pollen
– pistil Interaction /-
Pollination
does not guarantee the transfer of the right type of pollen ( compatible pollen
of the same species as the stigma).
·
Pollen tube grows through the through the tissue of
the stigma & style and reaches the ovary.
·
Some plants, pollen grains are shed at two – celled
condition ( a vegetative cell & generative cell ).
·
The generation cell divides & froms the two male
gamete during the growth of pollen tube in the stigma.
In
plants, which shed pollen in the three celled conditions.
Artificial
Hybridisation /-
One
of the major approaches of crop improvement programme. In such crossing
experiments it is important to male sure that only the desired pollen is
protected from contamination (from unwanted pollen).
·
The female, parent bears bisexual flowers.
·
The flower bud before the anther dehisces using a pair
of forceps is necessary. The step is emasculation. Emasculated flowers have to
be covered with a bag of suitable size, generally made up of butter paper this
process is called bagging.
·
If the female produces unsexual flower, there is no
need for emasculation.
Double
– Fertilisation /-
The
male gametes move towards the egg cells & fuses with its nucleus thus
completing syngamy.
·
The central cell & fuses with them to produce the
triploid primary endosperm nucleus.
·
The fussion of
three haploid nuclei in termed triple fusion.
·
Since two types of fusions, syngamy & the fusion
take place in an embryo sac the phenomenon is termed double fertilisation.
·
The central cell after triple fusion becomes the
primary endosperm cell (PEC) into the endosperm while the zygote develops into
a embryo.
Post-fertilisation
// Structures & Events /-
Double
fertilisation, events of endosperm & embryo development, maturation of
ovules(s) into seeds(s) & ovary into fruit, an collectively termed
post-fertilisation events.
Endosperm /-
Endosperm
development preceeds embryo development.
·
The primary endosperm cell divides repeatly and forms
a triploid endosperm tissue.
·
The cells of this tissue are filled with reserve food
material.
·
The nutrition of developing.
Embryo
/-
Embryo
develop at the micropylar end of the embryo sac where the zygote is situated.
·
The early stages of embryo development (embryogeny)
are similar in both monocotyledons & dicotyledonous embryo.
·
The zygote give rise to the proembryo &
subsenquently to the globular, heart- shaped & mature embryo.
·
A typical dicotyledonous embryo, of an embryonal axis
and two cotyledons.
·
The portion of embryonal axis above the level of
cotyledons.
·
The level of cotyledons is hypocotyl.
·
Terminate with the plumule or stem tip.
·
The level of cotyledons is the hypocotyl.
·
Terminate at its lower end in the radicle or root tip.
The root tip is covered with a root cap.
·
The grass family the cotyledon is called scutellum.
Radical and root cap enclosed in undifferentiated sheath called coleorhiza.
·
Epicotyl has a
shoot apex and a few leaf primordia enclosed in a hollow follar structure, the
coleoptile.
Seed
/-
The
embryo are simple structure, generally thick and swollen due to storage of food
reserves los in legumes. Mature seeds may be non-albuminous or ex-albuminous.
·
In some seeds such as black paper and beet, remnants
of nucellus are is the perisperm.
·
The embryo may enter a state of inactivity called
dormancy.
·
The wall of the ovary develops into the wall of fruit
called pericarp.
The thalamus also contributes to fruit formation. Such
fruits are called false fruits.
Most fruits however develop only from the ovary &
are called true fruits.
The
species, fruits are the result of fertilisation, there are a few species in
which fruits develop without fertilisation.
Such
fruits are called parthenocarpic fruits.
Banana is one such example. Parthenocarpy can induced
through the application of growth hormones & fruits are seedless.
Apomixis
and Polyembryony /-
In
generals are the products of fertilisation, a few flowering plants such as some
species of Asteracecae and grasses.
·
Special mechanism
to produce seeds without fertilisation called apomixes.
·
One of the embryo of a seed is referred to a
polyembryony.
·
Hybrid varieties of several of our food and vegetable
crops are being extensively cultivated.
Cultivation of hybrids has tremendously increased
productivity. One of the problems of hybrids is that hybrid seeds have to be
produced every year.
·
The farmers can keep on using the hybrids seeds to
raise new copy year after year & does not have to buy hybrid seeds every
year.
·
The world to understand the genetics of apomixes &
to transfer apomietic gene into hybrid varitites.
