sexual reproduction in flowering plants

all flowering plants show sexual reproduction the scent the perfumes the rich colour that attract us are all as an aid to sexual reproduction the diversity of structures of inflorescence flowers and floral parts show amazing range of adaptations to ensure the formation of the end products of sexual reproduction the fruits and seeds

flowers are objects of aesthetic ornamental, social religious and cultural value they have always been used as symbols for conveying important human feelings such as love, affection, happiness,grief , mourning etc flowers are morphological too bright and beautiful however the biological purpose of flower is very different from the way we used and preserve it flower help to reproduce its own kind to a biologist flowers are morphological and embryological marvels and sites of sexual reproduction

floriculture is a branch of horticulture that deals with the cultivation processing and marketing of ornamental plants landscaping and maintenance of gardens so that surroundings may appear aesthetically pleasant

much before the actual flower is seen on a plant the decision that the plant is going to flower has taken place several hormonal and structural changes are initiated which lead to the differentiation and further development of floral primordium inflorescence are formed which bear the floral buds and then the flowers in the flower the male and female reproductive structures the androecium and the gynoecium differentiate and develop Androecium consists of whorl of stamens representing the male reproductive organ and the gynoecium represent the female reproductive organ

stamens are male reproductive unit of angiosperm each stamen composed of Long and slender stalk called the filament it's proximal end is attached to the thalamus or the petal of the flower terminal generally bilobed structure called the anther the number and length of the stamens are variable in flower of different species

a typical angiosperm anther is bilobed each lobe having two theca (dithecous) often a longitudinal groove runs lengthwise seperating the theca the anther is a four -sided(tetragonal) structure consist of four microsporangia located at the corners two in each lobe the microsporangia develop further and become pollen sacs they extend longitudinally all through the length of an anther and are packed with pollen grains

a typical microsporangia appears near circular in outline in transverse section it is generally surrounded by four wall layers- the epidermis, endothecium,middle layers and the tapetum

the outer 3 layers give protection and help in dehiscence of anther

the innermost wall layer is the tapetum cells of the tapetum have dense cytoplasm and generally multinucleated it nourishes the developing pollen grains

In young anther each microsporangium has compactly arranged homogeneous diploid cells called sporogenous tissue/cells

as the anther develops each sporogenous cell(microspore mother cell or pollen mother cell) undergo meiotic divisionsbto form microspore tetrads(microspore arranged in a cluster of four cells)each cellof the sporogenous tissue is capable of giving rise to a microspore tetrad the process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogensis the microspores asthey are formed are arranged in a cluster of four cells- the microspore tetrad as the anthers mature and dehydrate the microspores dissociate from each other and develop into pollen grains inside each microsporangium several thousands of microspores of pollen grains are formed that are released with the dehiscence of anther

tetrad are formed as a result of meiosis thus they have half the ploidy of the initial or the parental cell here microspore mother cell is diploid under go meiosis and form four microspores hence the ploidy of microspore tetrad is haploid

the pollen grains represent the male gametophytes pollen grains are generally spherical measuring about 25-50 micrometers in diameter it has a prominent two layered wall Exine and intine

the hard outer layer of pollen wall called Exine which is made up of a substance called sporopollenin sporopollenin is one of the most resistant organic material known it can withstand high temperatures and strong acids and alkali no enzyme that degrades sporopollenin is so far known pollen grains are well preserved as fossils because of the presence of sporopollenin pollen grain Exine has prominent apertures called germ pores where sporopollenin is absent the elongation of the pollen tube takes place from the germ pores when the pollen grain lands on the stigma hence the function of the germ pores is the initiation of the pollen tube and release of male gametes

the inner wall of the pollen grain is called the intine it is a thin and continuous layer made up of cellulose and pectin the cytoplasm of pollen grain is surrounded by a plasma membrane when the pollen grain is mature it contains two cells the vegetative cell and generative cell the vegetative cell is bigger has abundant food reserve and a large irregular shape nucleus the generative cell is small and floats in the cytoplasm of the vegetative cell it is spindle shaped with cytoplasm and a nucleus

in over 60 percent of angiosperms pollen grains are shed at 2 celled stage in the remaining species the generative cell divides mitotically to give rise to two male male gametes before pollen grains are shed (3-celled stage) once they are shed pollen grains have to land on stigma before they lose viability if they have to bring about fertilisation pollen viability refers to the ability of the pollen to perform it's function of delivering male gametes to the embryo sac the period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperature and humidity in some cereals such as rice and wheat pollen grains lose viability within 30 minutes of their release and in dome members of Rosaceae Leguminoseae and Solanaceae they maintain viability for months

pollen grains are rich in nutrients pollen products in the form of tablets and syrups are available in the market pollen consumption has been claimed to increase the performance of athletes and race horses it is possible to store pollen grains of a large number of species for years in liquid nitrogen (-196°C) such stored pollen can be used as pollen banks similar to seed banks in crop breeding programmes

pollen grains of many species cause severe allergies and bronchial afflictions in some people often lead ling to chronic respiratory disorders -asthma, bronchitis etc it may be mentioned that Parthenium ir carrot grass that came to India as a contaminant with imported Wheat has become ubiquitous in occurance and causes pollen allergy