Flower – A fascinating Organ of Angiosperms : • Flowers have ornamental, aesthetic and cultural values. • Flowers are the sites of sexual reproduction
PREFERTILISATION : • Structures and Events – • Several hormonal and structural changes leads to the differentiation and further development of floral primordium. • Flower in turn contains male and female reproductive structures that is Androecium and gynoecium respectively.
STAMEN, MICROSPORANGIUM AND POLLEN GRAIN : • Stamen consists of two parts – long slender stalk called filament and the terminal bilobed structure called anther • Anther is a bilobed structure and each lobe is dithecous , having two theca or chambers, thus the anther becomes a four sided structure with four microsporangia at the four corners. Microsporangia later becomes pollen sacs.
STRUCTURE OF MICROSPORANGIUM : • It is circular in transverse section . • It is generally surrounded by four wall layers- • Epidermis, • endothecium, • middle layers • tapetum • Epidermis , endothecium and middle layers help in the protection and dehiscence of the anther to release pollen grains • Innermost wall layer tapetum possess dense cytoplasm and gives nourishment to the developing pollen grains • Young anther at the centre occupies sporogenous tissue
MICROSPOROGENESIS : • When the anther matures , sporogenous tissue undergo meiosis to form microspore tetrads is called Microsporogenesis
POLLEN GRAIN : • Mature microspore or pollen grain represents Male gametophytes. • pollengrins are generally spherical measuring about 25-50 micrometeres in diameter. • It has a two layered wall , outer layer called exineand inner layer called intine. • Exine is made up of sporopollenin the most resistant organic material. It is resistant to high temperature , acids and alkali.Because of sporopollenin pollen grains are well preserved as fossils. • At some places in the exine , there are germ pores, for the entry of pollen tube. • Intine is a thin and continuous layer made up of cellulose and pectin. • Mature pollen grain consists of two cells , the vegetative cell and generative cell.
Vegetative cell is bigger with abundant food reserve and irregular shaped nucleus. • Generative cell is small floats in the cytoplasm of vegetative cell, it is spindle shaped with dense cytoplasm and a nucleus. • In 60% of angiosperms pollen grains are released at this two celled stage. • while in others generative cell divides further to two male gametes to form 3- celled stage .
Pollen grains are in significant the effect of causing allergies and respiratory disorders – asthma, bronchitis etc.eg : Parthenium or carrot • Pollen grains are rich in nutrients. Pollen products are available in the market as Pollen tablets and syrups. Pollen consumption has been claimed to increase the performance of athletes and horses.
Pollen viability : for effective fertilization to occur pollen should reach at stigma before viability is lost. The time period for which pollen grains have the capacity to produce male gametes for effective fertilization to occur is the Pollen viability. It is variable in different . eg: 30 minutes for rice pollen . • Pollen banks : Pollen can be stored in liquid nitrogen at -196 degree Celsius for future plant breeding purposes
THE PISTIL, MEGASPORANGIUM ( OVULE ) AND EMBRYOSAC : • The gynoecium represents the reproductive part of the flower. • Monocarpellary Pistil : Gynoecium with single pistil • Multicarpellary Pistil : Gynoecium with many pistil • Syncarpous : Many pistils present are seen fused • Apocarpous :More than one pistils that are free
Pistil : • Each pistil has three parts the stigma , style and ovary. • stigma is the landing space for the pollen grains. • Style is the slender elongated middle part . the basal swollen part is called ovary. • Placenta is located inside the ovarian cavity.
Megasporangium ( Ovule ) : • Ovule is a small structure attached to the placenta by means of a stalk called funicle. • The entire ovule fused with funicle at a point called Hilum. • Each ovule has one or two protective envelops called integuments. • Integument encircles the ovule except at the tip at a small opening called micropyle. • The region opposite to micropyle is the chalazal pole. • Integument is enclosed by a mass of cells called the Nucellus, abundant with food reserves. Mature ovule develops Embryosac inside nucellus from the single megaspore
Megasporogenesis: • The process of formation of megaspore from megaspore mother cell is called Megasporogenesis . • Single megaspore mother cell differentiate from nucellus with dense cytoplasm and prominent nucleus. • Female Gametophyte : One functional megaspore of the four develops into female gametophyte or embryosac . • Three repeated mitotic division of the megaspore results in the formation of 7-celled or 8-nucleate embryosac. • Six of the eight nuclei are organized at the two poles . • Three cells grouped at micropylar pole forms egg apparatus and at the chalazal pole forms antipodal cells. The large central cell at the centre has two polar nuclei.
Pollination : • The transfer of pollen grain from the anther to the stigma of a pistil is called Pollination. • Depending on the source of pollen pollination is of three types, autogamy , geitonogamy and xenogamy. • Transfer of pollen grain from the anther to the stigma of the same flower is called autogamy. • In some plants ( Viola, Oxalis, Commelina ) there are two types of flowers, Chasmogamous and Cleistogamous. • Chasmogamous flowers are normal open flowers with exposed anther and stigma. • Cleistogamous flowers are closed flowers with stigma and anther lie close to each other. It is invariably autogamous which show assured seed set.
Geitonogamy : Tranfer of pollen grain from the anther of one flower to the stigma of another flower in the same plant is called Geitonogamy. It is functionally similar to cross pollination and genetically to autogamy. • Xenogamy : Tranfer of pollen grain from the anther to the stigma of a different plant. This induce variation as pollination brings genetically different pollen grains to stigma
AGENTS OF POLLINATION • Plants use two abiotic agents and one biotic agents for pollination . • Wind pollination (Anemophily ) : It is more common among abiotic pollinations . eg : maize • Characteristics of wind pollinated flowers –light and non sticky pollen grains – well exposed stamens – large and feathery stigma for easy trapping of pollen grains – single ovule in single ovary – numerous flowers packed into inflorescence
Wind-pollinated flowers may have: Large stigmas outside the flower Little or no fragrance Light non-sticky pollen Lack showy petals
WATER POLLINATION ( HYDROPHILY ) : • It is rare limited to about 30 genera. As the female flower reach the surface of water, by long stalk, male flowers or pollen grains are released on to the surface of water. Eg : Vallisneria, Hydrilla, Zostera. • Characteristics of water pollinated flowers ;long ribben like pollen garins – pollen grain protected by mucilaginous covering – female flower with long stalk.
Animal pollination ( Zoophily) :A range of animals are used as pollinating agents eg:- Wasps, ants, moths, birds , lemur, rodents, reptiles etc. • Characteristics of insect pollinted flowers : large , colourful, fragrant and rich in nectar.—-Small flowers are clustered into inflorescence —– sticky pollen grains
Flies (sapromyiophily) maroon / brown in color foul smelling (like rotting flesh)
Mutualism of pollination :- • By pollination , both the species involved in pollination getting the benefits. • sometimes flower provide space for laying eggs and in turn pollination of the flower takes place by moth. eg:- moth and amorphophallus, • moth and yucca flower ( both the species cannot complete their life cycle without each other , moth deposits its eggs in the locule of the ovary, and the flower in turn , gets pollinated by the moth. The larvae of the moth come out of the eggs as the seeds start developing )