MAKEUP

 

                                                   Basic Terms ,

Sexual Reproduction in flowering plants.

by AGRI grovestudies 

 

All flowering plants show sexual reproduction.

“Angiosperm”  and Gymnosperms /--

Angiosperm à

·        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 à

                                    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.