Pre-Fertilisation : Structures and Events

Microsporogenesis

The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis.

Microsporogenesis is well studied under following heads :

(1) Structure of anther : The fertile portion of stamens is called anther. Each anther is usually made up of two lobes connected by a connective. In turn each anther lobe contains two pollen chambers placed longitudinally. Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores.

The pollen sacs are surrounded by following 4 layers :

(i) Epidermis : This is the outermost single layered and protective. In Arceuthobium, cells of epidermis develops a fibrous thickening and the epidermis is designated as exothecium.

(ii) Endothecium : Inner to epidermis, there is a single layer of radially elongated cells. Cells of endothecium develop fibrous thickening (made up of cellulose with a little pectin and lignin) which help in the dehiscence of anther. In between these cells, a few cells without thickening are also present. These thin walled cells collectively form the stomium.

(iii) Middle layer : Three to four layers of thin walled cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells.

(iv) Tapetum : This is the innermost layer of the wall. The cells are multinucleate(undergo endopolyploidy) and polyploid. Tapetal cells are nutritive.

In these cells the Ubisch bodies are present which help in the ornamentation of microspore walls. A compound sporopollenin is secreted in the exine of microspore wall. According to Periasamy and Swamy (1966), developmentally the tapetum has dual nature.

The tapetum is of two types

(a) Amoeboid or Periplasmodial tapetum.

(b) Secretory or Glandular tapetum.

(2) Development of anther and formation of microspores (Pollen grains) : The young anther consists of homogenous mass of paranchymatous cells surrounded by epidermis. It soon becomes four lobed. In each of the four lobes, some of the hypodermal cells begin to act as archesporial initials. Each archesporial initial divides into an outer primary parietal cell and an inner primary sporogenous cell. The primary parietal cell divides to form 3-5 wall layers, i.e., endothecium, middle layers and tapetum. The primary sporogenous cells divide to produce a mass of sporogenous cells or microsporocytes.

Each microspore mother cell divides meiotically to form four haploid microspores or pollen grains and remains arranged in tetrads. The arrangement in the tetrads can be tetrahedral, isobilateral, linear, T-shaped and decussate.

Now the microspores are separated from tetrad. In Drosera, Typha, Elodea, Hydrilla, etc. all the four pollen grains do not separate and thus form compound pollen grains. In the members of the family Cyperaceae (Cyprus), out of 4 pollen in a tetrad, 3 degenerate and one remains alive. So one meiosis produces one pollen. Sometimes more than four pollens are produced from one microspore mother cell. It is called as polyspory e.g., Cuscuta. In Calotropis (Asclepiadaceae) and some orchids all the pollen grains of an anther lobe form a typical structure called pollinium.

(3) Development of male gametophyte (Microgametogenesis) : Microspore or pollen grain is the first cell of male gametophyte (partially developed). It is unicellular and haploid. The shape varies from oval to polyhedral. The wall of the pollen grain is made of two layers.

The outer layer is called exine. It is made up of sporopollenin (derived from carotenoid). It is thick and ornamented. At certain places, exine remains unthickened or missing and these places are known as germ pores. Sporopollenin is resistant to physical and biological decomposition. So pollen wall preserved for long periods in fossil deposits. The inner intine is thin, delicate and is made of cellulose and pectose.

In insect pollinated flowers, the exine of the pollen grain is covered with a yellowish, viscous and sticky substance called pollenkitt. This is perhaps the protective envelope which also sticks to the body of the insects and thus helps in pollination. It is chiefly made up of lipids and carotenoids. In monocots germ pores are absent and there is one germinal furrow. The development of male gametophyte from pollen grain is called microgametogenesis.

(4) Pre-pollination development : Microspores start germinating in situ (i.e., while enclosed inside the microsporangium or pollen sac) and is called precocious. Microspores may be best defined as partially developed male gametophyte. Microspore nucleus divides mitotically to form a smaller generative cell lying next to spore wall and a much larger vegetative cell (or tube cell). A callose layer is deposited around the generative cell. The generative cell loses its contact with the wall of microspore and becomes free in the cytoplasm. The callose layer than dissolves. The pollen grains are shed from the anther at this bicelled stage (rarely three celled).

(5) Post-pollination development : The liberated pollen grains are transferred to the receptive surface of the carpel (i.e., stigma) by the process called pollination. On the stigma, the pollen grain absorbs water and swells within a few minutes. It releases the wall-held recognition factors. These factors determine whether the pollen grain will germinate on the stigma or not. Subsequent to mutual recognition, the vegetative (or tube) cell enlarges and comes out through one of the apertures in the form of a pollen tube. The wall of pollen tube is the extension of intine. The tube secretes exogenous pectinases and other hydrolytic enzymes to create a passage for its entry. It absorbs nourishment from the transmitting tissue of the style. Gradually, the vegetative and generative nuclei are carried by the pollen tube, the farmer lying at its tip. The generative cell divides to form two non-motile male gametes. The tube nucleus has no important function and may disintegrate.

Megasporogenesis

The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule.

Megasporogenesis can be studied under following heads :

(1) Structure of ovule (Megasporangium) : Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe. The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus. 

The residual part of nucellus in the mature seed is called perisperm. In the centre of the nucellus is situated a female gametophyte known as embryo sac.

Following are the conditions seen in ovule in relation to integuments :

(i) Unitegmic : Ovule with a single integument, e.g., sympetalous or gamopetalous dicotyledons.

(ii) Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons.

(iii) Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg, etc.

(iv) Caruncle : It is formed as an outgrowth of the outer integument in the micropylar region. Caruncle is common in the ovules of Euphorbiaceae. e.g., Castor (Ricinus).

(v) Ategmic : In some parasites like Loranthus, Viscum, Santalum etc., there is no integument. Such an ovule is called ategmic.

(2) Kinds of ovules : Depending upon the shape and orientation, the ovules of angiosperms are classified into following types :

Fig : 3 Different forms of the ovule in longitudinal section

(a) Orthotropous, (b) Anatropous, (c) Hemianatropous,

(d) Campylotropous, (e) Amphitropous, (f) Circinotropous

(i) Orthotropous or Atropus : The micropyle, chalaza and funicle are in straight line. This is most primitive type of ovules. e.g., Betel, Piper, Polygonum.

(ii) Anatropous : The body of the ovule is completely inverted (turn at 180^o angle ) so that micropyle and hilum come to lie very close to each other. e.g., 82% of angiosperm families.

(iii) Hemianatropous : Ovule turns at 90^o angle upon the funicle or body of ovule is at right angle to the funicle e.g., Ranunculus.

(iv) Campylotropous : Ovule is circled more or less at right angle to funicle. Micropylar end is bent down slightly. e.g., in members of Leguminosae and Cruciferae.

(v) Amphitropous : Curvature of ovule is more and embryo sac becomes curved like horse shoe e.g. Lemna, Poppy, Alisma.

(vi) Circinotropous : The ovule is initially orthotropous but becomes anatropous due to unilateral growth of funicle. The growth continues till the ovule once again becomes orthotropous. As a result funicle completely surrounds the body of the ovule e.g., Opuntia (prickly pear).

(3) Formation of megaspore : The ovule or the megasporangium develops as a small protuberance of the placental tissue. In the very young ovule a single hypodermal cell is differentiated as archesporium cell. The archesporial cell may directly function as megaspore mother cell (tenuinucellate ovule) or may divide periclinally to form an outer parietal cell and an inner sporogenous cell (crassinucellate ovule). The sporogenous cell directly behaves as megaspore mother cell (or megasporocyte). The diploid megaspore mother cell enlarges in size and divides by meiosis to form a linear tetrad of four haploid megaspores. Occasionally T-shaped or inverted T-shaped ($\perp$) tetrads are also formed. Megaspore is the first cell of female gametophyte.

Of the linear tetrad, three megaspores towards the micropyle degenerate. The lowermost, i.e., the chalazal megaspore enlarges and remains functional. It later produces an embryo sac.

(4) Development of female gametophyte (Megagametogenesis) : The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.

(i) Monosporic type (Polygonum) : In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac. The functional haploid megaspore enlarges in size and by means of three successive mitotic divisions, gives rise to an 8-nucleate embryo sac. Of these, four nuclei occur at micropylar end and the other four at the chalazal end. Three nuclei at the micropylar end form egg apparatus and the fourth migrates from the both pole to the centre and form polar nucleus.

A fully developed typical or polygonum type of embryo sac is large and oval structure consisting of seven cells and eight nuclei.

(a) Egg apparatus : This is a group of 3 cells situated at the micropylar end. The centrally located cell is called egg cell. On its sides are present two synergids. Egg cell has a large vacuole at its upper end and a prominent nucleus near its lower end. Synergids show a filiform apparatus attached to their upper wall. It is known to attract and guide the pollen tube. Each of the synergids has a vacuole at its lower end and the nucleus at its upper end.

(b) Polar nuclei : These are situated in the centre of the embryo sac representing a large binucleate central cell. Generally, both the polar nuclei fuse before fertilization and form a single diploid nucleus called secondary nucleus or definitive nucleus.

(c) Antipodals : The three cells situated at the chalazal end are called antipodals. These cells generally degenerate soon after fertilization.

$\bullet$ Polygonum type occurs in about 70% of angiosperms and is the common type.

(ii) Bisporic type : In this type two megaspore nuclei take part in embryo sac formation.

(iii) Tetrasporic type : This type of embryo sac develops from four megaspore nuclei.

Pollination

The process of transfer of pollen grains, from an anther to the stigma of the same flower or of different flower. It is of two types :

(1) Self pollination : This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. It is of two types :

(i) Autogamy : It is a kind of pollination in which the pollen from the anthers of a flower are transferred to the stigma of the same flower.

(ii) Geitonogamy : It is a kind of pollination in which the pollen from the anthers of one flower are transferred to the stigma of another flower borne on the same plant. It usually occurs in plants which show monoecious condition (unisexual, male and female flowers are borne on the same plant). Geitonogamy involves two flowers but these belong to the same parent plant.

Merits 

$\bullet$ Pollen grains are not wasted.

$\bullet$ The purity of the generation is maintained.

Demerits 

$\bullet$ New and healthier varieties are not formed

$\bullet$ It results in weaker progeny, producing weaker seeds and plants.

Contrivances for self pollination : The major contrivances or adaptations which favours self pollination are : 

(a) Bisexuality : Flowers should be bisexual or hermophrodite.

(b) Homogamy : Anthers and stigma of the bisexual flowers of some plants mature at the same time. They are brought close to each other by growth, bending or folding to ensure self pollination. This condition is called homogamy. e.g., Mirabilis (Four O, clock), Catharanthus (= Vinca), Potato, Sunflower, Wheat, Rice, etc.

(c) Cleistogamy : Some plants never open to ensure complete self-pollination. This condition is called cleistogamy, e.g., Commelina bengalensis, Oxalis, Viola, etc. The cleistogamous flowers are bisexual small, inconspicious, colourless and do not secrete nectar.

(2) Cross pollination : Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. It is also called xenogamy.

Merits  

$\bullet$ Seeds are more and viable.

$\bullet$ Progenies are healthier.

$\bullet$ Adaptability is better.

$\bullet$ New varieties can be produced.

Demerits

$\bullet$ The process is not definite because plants depend on agencies.

$\bullet$ Large amount of pollen grains are wasted.

Contrivances for cross pollination : Nature favours cross pollination. All unisexual flowers and a large number of bisexual flowers are naturally cross pollinated.

The main contrivances ensuring cross pollination are as follows :

(i) Diclincy or Unisexuality : In unisexual flowers stamens and carpels are found in different flowers. Unisexuality can be of two types :

$\bullet$ Monoecious plant : When male and female flowers are borne on the same plant. e.g., Maize, Cucurbits, Castor.$\bullet$ Dioecious plant : When male and female flowers are borne on different plants. e.g., Carica papaya, Cannabis.

(ii) Dichogamy : In bisexual flowers, when two sexes mature at different intervals and thus avoid self pollination is known as dichogamy. When stamens mature earlier than the stigma, it is known as protandry and the flowers are called protandrous e.g., Coriander, Jasmine, Sunflower, Lady’s finger, etc. When stigma matures earlier than the stamens, it is known as protogyny and the flowers are called protogynous. e.g., Rose, Tobacco, Crucifers, etc.

(iii) Heterostyly : The plants of some species in which flowers are dimorphic. Thus facilitate cross pollination. Some of them possess a long style but short stamens and are known as pin-eyed while others have short style and long stamens. These are known as thrum-eyed. e.g., Oxalis.

(iv) Herkogamy : In some bisexual flowers where the stigma and anthers mature at the same time, self pollination is avoided by some sort of barrier. The flowers show following contrivances :

$\bullet$ The male and female sex organs lie at some distance from each other.

$\bullet$ In some flowers corolla has peculiar forms which act as barrier in self pollination. e.g., Aristolochia.

$\bullet$ In some other flowers, the pollens are held together to form pollinia which can only be carried away by insects. e.g., Orchids and Calotropis.

(v) Self sterility or Incompatibility : When pollen grain of an anther do not germinate on the stigma of the same flower, then such flower is called self sterile or incompatible and this condition of flower is called self sterility, intraspecific incompatibility or self incompatibility. In these flowers cross pollination is the only means for fertilization and production of seeds.

Agents for cross pollination : Cross pollination involves external agents for the transfer of pollen grains of one flower to the stigma of another flower. There are two main groups of agents : (i) Abiotic agents like wind and water (ii) Biotic agents which include animals of different types such as insects, birds, bats, snails, etc.

(i) Abiotic agents

(a) Anemophily : When flowers are pollinated by wind agency, the phenomenon is known as anemophily. Anemophilous flowers are small and inconspicuous with long and versatile stamens. e.g., Sugarcane, Maize, Wheat, Bamboo, Pinus, Papaya, Grasses, Typha, Datepalm, Coconut, Mulberry, Chenopodium, etc. This type of pollination mainly observed in Graminae.

(b) Hydrophily : When the pollination takes place through the agency of water, it is known as hydrophily. All aquatic plants are not hydrophilous some are anemophilous e.g., Potamogeton, Myriophyllum or Entomophilous e.g., Alisma, Lotus. Hydrophily is of two types :

$\bullet$ Hypohydrophily : Plants which are pollinated inside the water e.g., Zostera, Ceratophyllum, Najas, etc.

$\bullet$ Epihydrophily : Plants which are pollinated outside the water. e.g., Vallisneria (Ribbon weed).

(ii) Biotic agents

(a) Entomophily : When pollination is brought about by the agency of insects, it is known as entomophily or insect pollination. About 80% pollination occurs by insects like moths, beetles, butterflies, wasp, etc. All the flowers pollinated by insects are brightly coloured, have a sweet smell and produce nectar. Entomophilous flowers produce a small amount of pollen which has a spinous and sticky exine due to presence of pollenkitt. The stigmas of such flowers are long rough and sticky. Salvia is excellent example of insect pollination is which pollination occurs by lever or turn pipe mechanism. Other examples of insect plants are Yucca (by Tageticula moth), Orchid Ophrys speculum (by Colpa aurea a hairy wasp), Ficus (by Blastophega), etc. Yucca is pollinated by Pronuba (= Tegaticula) yuccasella which passes its larval stage inside the ripening ovary. The flower of orchid ophrys resemble in shape colour and odour to female wasp of colpa aurea (mimicry). The male wasps pollinate the flowers mistaking them as female (pseudocopulation).

(b) Ornithophily : When flowers are pollinated by birds, the phenomenon is known as ornithophily. The most common bird pollinators are Sun bird, Humming bird, Crow, Bulbul, Parrot, Mynah, etc. The birds visit a large variety of flowers such as Bombax (red silk cotton), Erythrina (Coral tree), Callistemon (Bottle brush), Bignonia, Agave, etc. Flowers are brightly coloured and produce plenty of nectar and large quantities of pollen. Humming bird pollinates while hovering over the flowers and sucking nectar. The bird can derive about half of its body weight of nectar in a single day. The nectar is chiefly made of sugars and provides a sweet drink to the bird.

(c) Chiropterophily : It is a mode of pollination performed by bats. The flowers they visit are large, dull-coloured and have a strong scent. Chiropterophilous flowers produce abundant pollen grains. These flowers secrete more nectar than ornithophilous flowers and open at night emit a good fragrance. e.g., Kigelia pinnata (Sausage tree), Adansonia (Baobab tree), Bauhinia megalandra, Anthocephalus (Kadam tree), etc.

(d) Malacophily : Pollination by slugs and snails is called malacophily. Land plants like Chrysanthemum and water plant like lemna shows malacophily. Arisaema (aroid snake plant) is often visited by snails.

(e) Myrmecophily : Pollination by ants. e.g., Anemone nemarosa (fruit).