Pre-fertilisation | Structures and Events
Much before the actual flower is seen on a plant, the decision that the plant is going to flower has already taken place. Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium. Inflorescences are formed which bear the floral buds and then the flowers. In the flower, the male and female reproductive structures, the androecium and gynoecium, differentiate and develop. The androecium consists of a whorl of stamens representing the male reproductive organ, while the gynoecium represents the female reproductive organ.
Stamen, Microsporangium and Pollen Grain
A typical stamen has two parts—the long and slender stalk called the filament, and the terminal, generally bilobed structure called the anther. The proximal end of the filament is attached to the thalamus or the petal of the flower. The number and length of stamens are variable in flowers of different species. If stamens from different flowers and species are collected and arranged on a slide, the large variation in their size can be appreciated. Careful observation under a dissecting microscope also shows considerable variation in the shape and attachment of anthers in different flowers.
A typical angiosperm anther is bilobed, with each lobe having two theca; therefore, it is dithecous. Often, a longitudinal groove runs lengthwise separating the two theca. In transverse section, the bilobed nature of the anther is clearly visible. The anther is a four-sided or tetragonal structure consisting of four microsporangia, located at the corners, two in each lobe. The microsporangia develop further and become pollen sacs. They extend longitudinally through the length of the anther and are packed with pollen grains.
Structure of Microsporangium
In transverse section, a typical microsporangium appears nearly circular in outline. It is generally surrounded by four wall layers:
The outer three wall layers perform the function of protection and help in the dehiscence of the anther to release the pollen. The innermost wall layer is the tapetum. It nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus. When the anther is young, a group of compactly arranged homogeneous cells called the sporogenous tissue occupies the centre of each microsporangium.
Microsporogenesis
As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads. Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad. Each one is therefore a potential pollen or microspore mother cell (PMC). The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis. The microspores, as they are formed, remain arranged in a cluster of four cells called 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 or pollen grains are formed and are released with the dehiscence of the anther.
Pollen Grain
The pollen grains represent the male gametophytes. If the opened anthers of Hibiscus or another flower are touched, a yellowish powdery deposition of pollen grains can be observed on the fingers. These grains can be placed on a drop of water on a glass slide and observed under a microscope.
Pollen grains show a remarkable variety of architecture, including differences in size, shape, colour and surface designs, among different species. Pollen grains are generally spherical and measure about 25–50 micrometres in diameter. They have a prominent two-layered wall. The hard outer layer is called the exine. It is made up of sporopollenin, one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkalis. No enzyme that degrades sporopollenin is known so far.
The exine has prominent apertures called germ pores, where sporopollenin is absent. Pollen grains are well preserved as fossils because of the presence of sporopollenin. The exine exhibits a fascinating array of patterns and designs. 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 the 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 possesses a large, irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped and contains dense cytoplasm and a nucleus.
In over 60 per cent of angiosperms, pollen grains are shed at the two-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before the pollen grains are shed, resulting in the three-celled stage. Pollen grains of many species cause severe allergies and bronchial afflictions in some people, often leading to chronic respiratory disorders such as asthma and bronchitis. Parthenium or carrot grass, which came to India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy.
Pollen Products
Pollen grains are rich in nutrients. In recent years, the use of pollen tablets as food supplements has become popular. In western countries, a large number of 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.
Pollen Viability
Once pollen grains are shed, they have to land on the stigma before they lose viability if they are to bring about fertilisation. 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 about 30 minutes of their release. In some members of Rosaceae, Leguminosae and Solanaceae, they maintain viability for months.
Semen or sperms of many animals, including humans, can be stored for artificial insemination. Similarly, pollen grains of a large number of species can be stored for years in liquid nitrogen at −196°C. Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.
The Pistil, Megasporangium (Ovule) and Embryo Sac
The gynoecium represents the female reproductive part of the flower.
The gynoecium may consist of a single pistil, called monocarpellary, or may have more than one pistil, called multicarpellary. When there is more than one pistil, the pistils may be fused together, called syncarpous, or may be free, called apocarpous. Each pistil has three parts:
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 or locule. The placenta is located inside the ovarian cavity. The number of ovules in an ovary may vary from one in plants such as wheat, paddy and mango to many in plants such as papaya, watermelon and orchids.
The Megasporangium (Ovule)
The ovule is a small structure attached to the placenta by means of a stalk called the funicle. The body of the ovule fuses with the funicle in the region called the hilum. Thus, the hilum represents the junction between the ovule and funicle. Each ovule has one or two protective envelopes called integuments. The integuments encircle the nucellus except at the tip, where a small opening called the micropyle is present. Opposite the micropylar end is the chalaza, representing the basal part of 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.
Megasporogenesis
The process of formation of megaspores from the megaspore mother cell (MMC) is called megasporogenesis. Ovules generally differentiate a single megaspore mother cell 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.
Female Gametophyte
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 or embryo sac. This method of embryo sac formation from a single megaspore is termed monosporic development.
Formation of the Embryo Sac
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. These mitotic divisions are strictly free nuclear, that is, nuclear divisions are not immediately followed 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 become surrounded by cell walls and are organised into cells. The remaining two nuclei, called the polar nuclei, are situated below the egg apparatus in the large central cell. There is a characteristic distribution of cells within the embryo sac. Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus consists of:
• Two synergids • One egg cell
The synergids have special cellular thickenings at the micropylar tip called the filiform apparatus. These play an important role in guiding the pollen tubes into the synergids. Three cells are present at the chalazal end and are called the antipodals. The large central cell contains the two polar nuclei. Thus, a typical angiosperm embryo sac at maturity, though 8-nucleate, is 7-celled.
Pollination
The male and female gametes in flowering plants are produced in the pollen grain and embryo sac, respectively. Both types of gametes are non-motile and therefore have to be brought together for fertilisation to occur. Pollination is the mechanism to achieve this objective. The 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 and make use of external agents for this process.
Kinds of Pollination
Depending on the source of pollen, pollination can be divided into three types:
(i) Autogamy
In autogamy, pollination is achieved within the same flower. It involves the transfer of pollen grains from the anther to the stigma of the same flower. In a normal flower which opens and exposes the anthers and stigma, complete autogamy is rather rare. Autogamy in such flowers requires:
Some plants such as Viola (common pansy), Oxalis and Commelina produce two types of flowers: Chasmogamous flowers are similar to flowers of other species, with exposed anthers and stigma. Cleistogamous flowers do not open at all. In cleistogamous flowers, the anthers and stigma lie close to each other. When the anthers dehisce in the flower buds, pollen grains come in contact with the stigma and bring about 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.
(ii) Geitonogamy
Geitonogamy is the 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.
(iii) Xenogamy
Xenogamy is the transfer of pollen grains from the anther to the stigma of a different plant. It is the only type of pollination which brings genetically different types of pollen grains to the stigma.
Agents of Pollination
Plants use two abiotic agents—wind and water—and one biotic agent—animals to achieve pollination. The majority of plants use biotic agents for pollination. Only a small proportion of plants use abiotic agents. Pollen grains coming in contact with the stigma is a chance factor in both wind and water pollination. To compensate for this uncertainty and the associated loss of pollen grains, flowers using abiotic agents produce an enormous amount of pollen compared with the number of ovules available for pollination.
Pollination by Wind
Pollination by wind is more common amongst abiotic pollinations. Wind pollination requires pollen grains to be light and non-sticky, so that they can be transported in wind currents. Wind-pollinated plants often possess well-exposed stamens, allowing pollen grains to be easily dispersed into wind currents. They also have large, often feathery stigmas which can easily trap airborne pollen grains. Wind-pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence. A familiar example is the corn cob. The ears seen in a corn cob are actually the stigma and style, which wave in the wind to trap pollen grains. Wind pollination is quite common in grasses.
Pollination by Water
Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons. In contrast, water is a regular mode of transport for male gametes among lower plant groups such as algae, bryophytes and pteridophytes. Particularly in some bryophytes and pteridophytes, their distribution is believed to be limited because of the need for water for the transport of male gametes and fertilisation. Examples of water-pollinated flowering plants include Vallisneria and Hydrilla, which grow in fresh water, and several marine sea-grasses such as Zostera. Not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind, as in most land plants. In Vallisneria, the female flowers reach the surface of water by their long stalk. The male flowers or pollen grains are released onto the surface of water. They are carried passively by water currents, and some of them eventually reach the female flowers and stigma.
Pollination by Different Agents
| Pollination by | Type | Scientific Term |
|---|---|---|
| Pollination by Wind | Abiotic | Anemophily |
| Pollination by Water | Abiotic | Hydrophily |
| Pollination by Insects | Biotic | Entomophily |
| Pollination by Bees | Biotic | Melittophily |
| Pollination by Butterflies | Biotic | Psychophily |
| Pollination by Moths | Biotic | Phalaenophily |
| Pollination by Flies | Biotic | Myophily |
| Pollination by Beetles | Biotic | Cantharophily |
| Pollination by Wasps | Biotic | Sphecophily |
| Pollination by Birds | Biotic | Ornithophily |
| Pollination by Bats | Biotic | Chiropterophily |
| Pollination by Snails | Biotic | Malacophily |
| Pollination by Lizards | Biotic | Saurophily |
| Pollination by Ants | Biotic | Myrmecophily |
| Pollination by Rodents | Biotic | Therophily |
| Pollination by Mammals | Biotic | Mammophily |
| Pollination by Primates | Biotic | Primatophily |
| Pollination by Cockroaches | Biotic | Blattophily |
| Pollination by Thrips | Biotic | Thripsophily |
| Pollination by Mosquitoes | Biotic | Culicidophily |
| Pollination by Midges | Biotic | Cebrellophily |
| Pollination by Slugs | Biotic | Malacophily |
Read Also
Pollen–Pistil Interaction | Reproduction in Flowering Plants