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Let us make an in-depth study of Modes of Reproduction in Angiosperms. After reading this article you will learn about: 1. Introduction to Modes of Reproduction 2. Various Modes of Reproduction in Angiosperms.
Introduction to Modes of Reproduction:
In angiosperms or flowering plants, there are several modes of reproduction. Generally, they are arranged in two large groups of reproduction, i.e., (i) asexual or vegetative and (ii) sexual types.
In asexual or vegetative reproduction, the offspring are produced from the somatic cells, while in sexual reproduction there is fusion of male and female gametes.
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In the case of vegetative reproduction, any part of the plant, i.e., stem, root or leaf, is capable of growing into a new plant, in addition to performing its sexual functions. Sometimes, in certain plants, buds and bulbils are developed, which develop into new plants.
In sexual reproduction, the gametes from male and female organs of the flower are fused to produce a zygote. In angiosperms, these organs are generally called, androecium and gynoecium, respectively.
In some plants, certain special modes of reproduction are found, which are commonly known as parthenogenesis, sporophytic budding, polyembryony, apomixis, apospory, and micro- propagation. The production of synthetic or artificial seeds is also possible through tissue culture.
Various Modes of Reproduction in Angiosperms:
Sexual Reproduction:
The Flower:
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The flower is a highly specialized reproductive shoot. Each typical flower consists of four distinct types of members arranged in four separate but closely set whorls, one above the other, on the top of a long or short stalk. The lower two whorls are called accessory whorls, and the upper two essential or reproductive whorls because only these two are directly concerned in reproduction.
The essential whorls consist of two kinds of sporophylls: microsporophylls or stamens and megasporophylls or carpels. Both kinds of sporophylls may be present in a flower (hermaphrodite flower), or only one (unisexual flower) may be seen in some types.
Androecium (Andros = male):
This is male whorl of the flower. It consists of stamens or microsporophylls which are regarded as the male organ of the flower. Each stamen consists of three parts, i.e., filament, anther and connective. The anther filament is the slender stalk of the stamen, and the anther is the expanded head borne by the filament at its tip.
Each consists usually of two lobes connected together by a sort of mid-rib, the connective. The anther bears four chambers or pollen sacs each filled with pollen grains or microspores. Pollen grains are produced in large quantities in the pollen sacs.
Gynoecium (Gyne = female):
This is female whorl, and its component parts are known as carpels or megasporophylls. The gynoecium or pistil consists of ovary, style and stigma. The swollen basal part of the gynoecium which forms one or more chambers is known as ovary. The small rounded head of the gynoecium is known as the stigma.
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The ovary contains one or more little, roundish or oval, egg-like bodies which are the rudiments of seeds and are known as ovules. Each ovule encloses a large oval cell known as the embryo sac. On maturation, the ovary gives rise to the fruit and the ovules to seeds.
Homologies:
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Carpel = Megasporophyll
Ovule = Megasporangium
Embryo sac = Female gametophyte
Egg = Female gamete
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Stamen = Microsporophyll
Anther (pollen sac) = Microsporangium
Pollen grain = Microspore
Germinating pollen grain = Male gametophyte
The reproductive process in angiosperms is as follows:
Development of the Male Gametophyte:
Microsporangium (pollen sac):
The epidermis is the outermost layer of microsporangium. The cells of epidermis are generally stretched and flattened. The layer next to the epidermis is the endothecium or fibrous layer. As a rule, by the development of the fibrous bands of thickening the endothecium becomes hygroscopic and is, therefore, mainly responsible for the dehiscence of mature anther.
The anther is generally bilobed, containing two longitudinally running chambers or pollen sacs per lobe. Each chamber contains a large number of pollen grains. The anther wall is composed of 4 to 5 layers. The innermost layer of these wall layers develops into a single layered tapetum.
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The tapetal layer is of great physiological significance as all the food material entering into the sporogenous tissue diffuses through, this layer. Ultimately the cells of tapetal layer disorganise. Thus, tapetum makes a nutritive layer for the developing microspores.
The cells of endothecium are thin walled along the line of dehiscence of each anther lobe. The opening through which the pollen grains are discharged from the pollen sac is called stomium.
On the maturity of the anther, a strain is exerted on the stomium due to the loss of water by the cells of endothecium, with the result the stomium ruptures and the anther dehisces. Generally, the mature anther dehisces by means of slits or apical pores.
The Pollen Grains:
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The pollen grains or microspores are the male reproductive bodies of a flower, and are contained in the pollen sac or microsporangia. They are very minute in size and are like particles of dust. Each pollen grain consists of a single microscopic cell, possessing two coats: the exine and the intine.
The exine, tough cutinized layer, which is often provided with spinous outgrowths or reticulations of different patterns and sometimes smooth. The exine is made up of a complex substance, called sporopollenin. The inline is a thin, delicate cellulose layer lying internal to the exine. The exine possesses one or more thin places known as germ pores. There are usually three germ pores in dicots and one in monocots.
The development of the male gametophyte is remarkably uniform in angiosperms. Pollen grain is the first cell of a male gametophyte. This cell undergoes only two divisions, with the result of first division two cells are formed: a large vegetative cell and a small generative cell.
The second division is concerned with generative cell only. This division may take place either in the pollen grain or in the pollen tube, and give rise to two male gametes. The life of male gametophyte is very short as compared to that of the sporophyte.
Vegetative and Generative Cells:
As already described, the first division of the pollen grain gives rise to the vegetative and generative cells. The first formed walled and peripheral cell is the generative cell, while the larger, naked, central cell, which fills the remainder of the spore-wall cavity, is the vegetative or tube cell.
The nuclei of generative and vegetative cells differ in size, structure and in staining qualities. The nucleus of vegetative cell possesses a prominent nucleus, while the nucleus of generative cell contains a small nucleolus. The cytoplasm of generative cell is hyaline and is almost without RNA, whereas that of vegetative cell is rich in RNA.
The DNA contents of both the nuclei are same in the beginning but later on they increase in the generative nucleus. Eventually the generative cell loses contact with the microspore (pollen grain) wall, and is being shifted into the vegetative cell, where it may lie in any part of it.
Thus, the pollen grain becomes two-celled. Generally, the pollen grains are being shed from the microsporangium (pollen sac) in two-celled stage for pollination.
Development of the Female Gametophyte:
The megasporangium or ovule:
An ovule or megasporangium develops from the base or the inner surface of the ovary. It is a small generally oval structure and consists chiefly of a central body of tissue, the nucellus and one or two integuments. Each ovule is attached in the placenta by a small stalk called the funiculus.
The place of attachment of the stalk with the main body of the ovule is called the hilum. In an inverted ovule, the funicle fuses with the main body of the ovule, forming a sort of ridge, known as the raphe. The upper end of the raphe which is the .unction of the integuments and the nucellus is called the chalaza.
The nucellus makes the main body of the ovule, which is made up of parenchyma tissue. Nucellus is the megasporangium proper and is surrounded by two coats, the integuments. A small opening is left at the apex of the integuments; this is called the micropyle.
When there are two integuments then the inner integument is formed first and followed by the formation of the outer integument. A large oval cell lying embedded in the nucellus towards the micropylar end is the embryo sac. This makes the most important part of the mature ovule. It is the embryo sac, which bears the embryo later on.
Placentation:
The placenta is an outgrowth of a parenchymatous tissue in the inner wall of the ovary to which the ovule or ovules (megasporangia) remain attached. The placentae usually develop on the margins of carpels, either along their whole line of union, called the suture or at their base or apex. The manner in which the placentae are distributed in the cavity of the ovary is known as placentation.
In the simple ovary (i.e., of one carpel), there is one common type of placentation, known as marginal, and in the compound ovary (i.e., of two or more carpels united together) placentation may be axile, central and free-central, basal, parietal and superficial as shown in fig. 46.14.
P. Maheshwari, F.R.S. (1904-1966):
He worked on embryological aspects, especially the embryo sac of several plants belonging to more than 100 angiospermous families.
His famous books of international fame are:
1. Introduction to the Embryology of Angiosperms
2. Recent advances in Embryology of Angiosperms (1963, edited by P. Maheswari).
He devoted his life for plant embryology, and very often referred to as ‘Father of Indian Plant Embryology.’ He was honoured with fellowship of Royal Society.
Megasporogenesis:
The archesporium is hypodermal in origin. At some early stage in the development of the ovule, usually at the time of the initiation of the integumentary primordia, single hypodermal cell known as primary archesporial cell, becomes differentiated at the apex of the nucellus beneath the epidermis.
It can be distinguished from other neighbouring cells owing to its large size, conspicuous deeply staining nucleus, and dense cytoplasm. Usually this primary archesporial cell divides periclinically forming an outer primary parietal cell and an inner primary sporogenous cell.
The primary parietal cell may divide further several times both by anticlinal and periclinical divisions forming a variable amount of parietal tissue, or sometimes it remains undivided. The primary sporogenous cell usually does not divide further and functions directly as the megaspore mother cell.
Usually the megaspore mother cell divides meiotically forming a tetrad of four megaspores. This usual process of meiotic division is termed megasporogenesis. Here the first division (i.e., meiosis I) is always transverse and gives rise to two cells.
The second is also transverse (i.e., meiosis II), and thus in total four cells are being formed. The four megaspores thus formed in an axial row within the nucellus forming a linear tetrad.
Of the four megaspores, so formed, each with half (n) the usual number (2n) of chromosomes, the three upper ones degenerate and appear as dark caps, while the lowest one functions, and gives rise to the embryo sac. The developing megaspore encroaches upon and absorbs the other three degenerating megaspores of tetrad and the neighbouring cells of the nucellus.
Female gametophyte:
The megaspore (n) makes the beginning of the female gametophyte. The nucleus of the functional megaspore divides and develops into the female gametophyte or the embryo sac. The female gametophyte of angiosperms is very much reduced and totally dependent for its nutrition upon the tissue of the sporophyte.
Depending on the number of megaspores taking part in the development, the embryo sacs (female gametophytes) of angiosperms may be classified into three main categories; monosporic, bisporic and tetrasporic (Panchanan Maheshwari, 1950).
In monosporic type, only one of the four megaspores takes part in the development of the female gametophyte (embryo sac). In bisporic type, two megaspore nuclei take part in the development of the female gametophyte. However, in tetrasporic type, all the four megaspore nuclei take part in the development of female gametophyte.
The functional megaspore (n) is the first cell of the female gametophyte. It divides by three successive divisions to form an eight-nucleate female gametophyte or embryo sac. Here, the nucleus of the functional megaspore divides to form two nuclei; the primary micropylar and the primary chalazal nuclei.
These nuclei again divide, so that the number is increased to four. Each of these nuclei divides, so that altogether eight nuclei are formed in the embryo sac, four at each end. The female gametophyte increases in size. Now, one nucleus from each end or pole passes inwards, and the two polar nuclei fuse together somewhere in the middle of the embryo-sac, forming the secondary nucleus (2n).
The remaining three nuclei at the micropylar end, each surrounded by a very thin wall, form the egg apparatus. The egg apparatus consists of two synergids and an egg cell. The other three nuclei at the opposite or chalazal end, lying in a group, often surrounded by very thin walls, form the antipodal cells. This type of embryo-sac is the most common and generally known as the normal type.
Pollination:
In angiosperms, the pollen grains are being transferred from the anther to the stigma, and is termed pollination. This phenomenon was first discovered by Camerarius (1694) in the end of seventeenth century. According to him, pollination is essential for the production of the seed.
The pollination may be of two types: self pollination (autogamy) and cross pollination (allogamy). The transfer of the pollen-from the anther of a flower to the stigma of the same flower is known as self pollination or autogamy, whereas cross pollination or allogamy is the transference of the pollen from one flower to another flower.
The cross pollination is of three types:
1. Xenogamy:
In this type, the pollination takes place between flowers borne on two different plants of the same species.
2. Geitonogamy:
This type of pollination takes place between the flowers developed on the same plant.
3. Hybridism:
Such pollination takes place between two flowers of two different plants of the allied species or sometimes even allied genera.
In the condition in which the pollen are discharged from the anther, they show considerable resistance to environmental changes. Sometimes, they remain viable for several weeks.
In certain cases even in hermaphrodite flowers, self-pollination does not take place. This happens because of heterostyly, e.g., in Primula vulgaris; dichogamy, where the maturity of male and female sex organs of the flowers is attained at different times, e.g., in Impatiens- herkogamy, in which the structure of male and female sex organs in the flowers acts as barrier for self-pollination, and self-sterility, as found in Petunia axillaris.
As mentioned, pollination is of two kinds:
(1) Self pollination or autogamy (auto = self; gamos = marriage) and
(2) Cross-pollination or allogamy (alios = different).
1. Self-Pollination (Autogamy):
This kind of pollination is the transference of pollen grains from the anther of a flower to the stigma of the same flower or from a flower (male or bisexual) to a flower (female or bisexual), both found on the same individual plant.
Here only one parent plant is concerned to give rise to the offspring. The self-pollination is, however, presented in unisexual flowers borne by two separate plants, and also in many bisexual flowers. The under mentioned adaptations are commonly found in flowers to achieve self-pollination.
Homogamy (homos = same):
In this condition, the anthers and the stigmas of a bisexual flower mature at the same time:
(i) Here some of the pollen grains may reach the stigma of the same flower through the agency of wind or insects, thus effecting self-pollination.
(ii) The filaments of the anthers may recoil and bring the mature anther close to the stigma (e.g., in Mirabilis jalapa). The anthers then burst and discharge their pollen right on the surface of the stigma. In some cases, the stigmas move back and touch the anthers to achieve self-pollination when cross-pollination fails (e.g., in members of Asteraceae and Malvaceae families).
(iii) In some drooping flowers the style is longer than the filaments, whereas in certain erect flowers the reverse may be the case.
(iv) Sessile or sub-sessile anthers may lie at the mouth of the narrow corolla tube and the stigma, while pushing out through the tube brushes against anthers (e.g.. in Ixora, Gardenia and Vinca).
Cleistogamy (kleistos = closed):
Some of the bisexual flowers do not open and are known as cleistogamous or closed flowers. In such flowers, the pollen grains are distributed on the stigma of the same flowers. Such cleistogamous flowers are very small and inconspicuous.
They are not coloured, and do not secrete any nectar. These flowers are not even scented. This type of pollination is found in the underground flowers of Commelina benghalensis, Viola, Drosera, Oxalis, etc.
2. Cross-Pollination (Allogamy):
The cross-pollination is induced by external agents which carry the pollen grains of one flower and deposit them on the stigma of another flower, the two being borne by two separate plants of the same or closely allied species. These agencies may be insects (e.g., bees, flies, moths, etc.), animals (e.g., birds, snails, etc.), wind and water.
The allogamy (cross-pollination) is the rule in unisexual flowers borne by two separate plants, while in bisexual flowers, it also occurs generally. Nature favours cross-pollination and there are so many adaptations in flowers to achieve this type of pollination.
Entomophily (entomon = insect, phileo = to love):
This type of pollination takes place through the agency of insects. It is of general occurrence among plants. The insect-loving flower possesses various adaptations by which they attract insects and use them as carriers of pollen grains for the purpose of cross-pollination.
The main such adaptations are colour, nectar and scent. The flowers of Asteraceae and Lamiaceae families are generally pollinated by the bees and butterflies.
Generally, the pollen grains of entomophilous flowers are sticky. The stigma is also sticky. Pollen grains and nectar are very often used as food materials by the insects. Flowers generally attract insects by their colour, nectar or scent, or they visit the flowers in search of food, or shelter from sun and rain.
Thus, as the insects visit the flowers, their body gets dusted with pollen grains, and when they fly and visit other flowers, they brush against the stigma which being sticky at once receives the pollen grains from their body. Thus, cross-pollination is achieved.
In several species of Ficus, the insects enter the chamber of the inflorescence (hypanthodium) through the apical pore, and as they move over the unisexual flowers inside the chamber, the pollination is achieved. Female flowers lie at the base of the cavity and open earlier, whereas male flowers lie near the apical opening and open later so that pollen grains have to be brought over from another inflorescence.
Flowers are generally adapted for pollination by some specific insects. For example, in snapdragon and other such flowers with saccate corolla, only the insects of particular size and weight can open the mouth of the corolla. On the other hand, long-tongued insects can pollinate the flowers with long corolla tubes.
Pollination in Calotropis:
This is member of Asclepiadaceae and is pollinated by bees. In this flower, the filaments of stamens form a tube around gynoecium. The anthers are fused with the stigma to form a 5-angled disc called gynostegium.
The staminal tube gives out distinct lobes called the corona. It is fused with the petals. The anther lobes, that are fused with the stigmatic disc, have straight and parallel sides and are separated only by long narrow clefts.
Within these clefts there are inter-staminal chambers. The pollen grains in the anthers are grouped in the form of mass called pollinium. The pollinium develops a rider mechanism or the translator. It consists of two arms called the caudicles. The caudicles carry the two pollinia on one side and are fused to form a black and sticky dot on the other side. This dot-like structure is called the corpusculuni.
There are five corpuscula at the angles of gynostegium from two adjacent anthers. An insect crawling about over the flowers is sooner or later trapped, through one of its legs becoming caught in one of the clefts between adjacent anthers.
The insect can release itself only by drawing the leg upwards through the clefts and this it does, but as the leg becomes free at the top of the cleft, it catches in the notch of the corpusculum so that further movements pull this together with its attached pollinia, away from its anchorage on the gynostegium.
The released insect in due course visits another flower and again becomes caught by the leg in the similar way. While drawing the leg, this time, through the anther cleft the pollinia brought from the previous flower are torn away from corpusculum and are deposited in the inter-staminal chamber. The new translators are carried away and there is repetition of the whole process.
Pollination in Salvia:
An interesting type of cross-pollination takes place in Salvia by insects. In this flower, there are two stamens. The two anther lobes of each stamen are widely separated by the elongated curved connective which plays freely on the filament. The upper lobe is fertile and the lower one sterile. In the natural position, the connective remains upright.
When the insect enters the tube of the corolla it pushes the lower sterile anther lobe of each stamen; the connective swings round with the result that the upper fertile lobe comes down and strikes the back of the insect with force and dusts it with pollen grains.
The flower is protandrous, and on the maturity of the stigma it bends down and touches the back of the insect and receives the pollen grains from it. Thus, pollination is effected.
Anemophily (anemos = wind):
In many cases, pollination is achieved by wind. The wind pollinated flowers are small and inconspicuous. They are neither coloured nor showy. They do not have any smell and they do not secrete any nectar. The anthers produce an immense quantity of pollen grains. A large quantity of pollen grains is being wasted during transit from one flower to another.
The pollen grains are quite light and dry, and sometimes provided with wings (e.g., in Pinus, a gymnosperm) to facilitate distribution by wind. In the wind loving flowers, the stigmas are comparatively large and protruding, some-times branched and often feathery (e.g., grasses, bamboos, palms, cereals, millets, sedges, sugarcane, etc.).
The maize plant makes a good example of this type. The plant bears a large number of male flowers in a terminal panicle, and in the lower part of the plant one or two female spadices, each in the axil of a leaf, surrounded by spathes. A cluster of fine, silky and long styles is seen.
On the maturity, the anthers burst and a cloud of dust-like pollen grains is seen floating in the air near the plant. Some of these pollen grains are entangled by the protruding stigmas and thus pollination is effected.
Hydrophily (hydor = water):
Water also acts as an agent of pollination. This is commonly found in water plants, specially submerged ones, such as Vallisneria, Ceratophyllum, Hydrilla and Zostera. The mode of pollination in Vallisneria (submerged aquatic plant) is as follows:
The plant is dioecious. The male plant bears a large number of minute male flowers in a small spadix surrounded by a spathe and borne on a short stalk, whereas the female plant bears solitary female flowers each on a long slender pedicel.
This stalk of the flower elongates and takes the female flower to the surface of the water. The spathe bursts releasing the male flowers from the spadix, while still closed, and float on the surface of the water. The anthers burst and the sticky pollen grains adhere to the surface of trifid stigmas which thereafter close.
As soon the pollination is over, the stalk of the female flower becomes spirally coiled and pulls the female flower down into the water. The fruit develops and matures under water a little above the bottom.
Zoophily (zoo = animals):
There are so many animals, such as birds, squirrels, bats, snails, etc., which are involved in cross-pollination. The pollination by birds, generally called ornithophily, is common in coral tree (Erythrina), bottle brush (Callistemon), Butea monosperma and silk cotton tree (Bombax ceiba). The pollination in Adansonia, Kigelia and Anthocephalus are carried out by bats.
This type is called chiropterophily. Snails are involved in pollination of several aroids (members of family Araceae). In aroids, the inflorescence is a spadix; the female flowers remain situated at the base of the spadix and the male flowers towards top. The stigmas mature first and the pollen grains are brought from another spadix.
Significance of Pollination:
1. Pollination is an important process which leads to fertilisation and production of seeds and fruits, which are responsible for continuity of plant life.
2. The seeds and fruits are also used as food both for animal and humans. They make source of vitamins and minerals.
3. The pollination, especially cross pollination is important for production of plants with a combination of characters from two plants.
4. Pollination is also important in the production of hybrid seeds.
Contrivances for cross-pollination:
Certain structural devices in the flowers favour cross-pollination.
These are as follows:
1. Unisexuality:
The stamens and carpels lie in separate flowers—male and female, either borne by the same plant or by two separate plants.
There are two kinds of unisexuality:
(i) Where the male and female flowers lie on the same plan’ and the plant is said to be monoecious (e.g., members of Cucurbitaceae, castor, maize, etc.),
(ii) Where the male and the female flowers are borne by one plant and the female flowers lie on another plant, it is known as dioecious (e.g., palmyra palm, Carica papaya, Morus alba, etc.). In monoecious plants, there may be self-or cross-pollination, while in dioecious plants, cross-pollination is a basic necessity.
2. Self-Sterility:
In certain flowers, the pollen grains are unable to germinate on its own stigma. It is noted in some orchids that the pollen has an injurious effect on the stigma of the same flower. In this case on the application of pollen to stigma, the stigma dries up and falls off, Abutilon, Passiflom, Malva, Prunus and Pyrus are self-sterile.
To effect the successful cross- pollination in these cases the pollen must be from two such parents which differ genetically. Cross-pollination is the only method in such cases for the setting of seeds.
3. Dichogamy (dicha = in two):
In many bisexual flowers, the anthers and stigmas often mature at different times. This condition is known as dichogamy. As the anther and the stigma mature at different times, dichogamy often checks the self-pollination.
There are two types of dichogamy:
(i) Protogyny (protos = first; gyne = female) where the gynoecium matures earlier than the anthers of the same flower, and in such cases, the stigma receives the pollen grains brought from another flower and thus cross- pollination becomes indispensable (e.g., Ficus, Mirabilis, Magnolia, Annona, etc.) and
(ii) Protandry (protos = first; andros = male) where the anthers mature earlier than the stigma of the same flower and hence the pollen grains, are carried over to the stigma of another flower (e.g., Clerodendron, Hibiscus rosa-sinensis, Abelmoschus esculentus, Helianthus annuus, Coriandrum sativum, etc.)
4. Heterostyly (heteros = different):
Some plants bear flowers of two different forms. One form possesses long stamens and a short style, while the other form possesses short stamens and a long style. This kind of bearing of stamens and styles is known as dimorphic heterostyly. In such cases, the chances of self-pollination decrease whereas chances of cross-pollination increase.
In the flowers of this type, cross-pollination readily takes place between stamens and styles of the same length borne by different flowers. Dimorphic heterostyly is seen in Oxalis, Linum, Polygonum fagopyrum, Woodfordia, etc.
5. Herkogamy (hercos = barrier):
In some homogamous flowers, there are certain structural peculiarities of the floral parts which act as a barrier to self-pollination and thus favour cross- pollination by insects.
Here are cited some important examples:
For example, in Calotropis and orchids, the pollinia are located at places where they can never come in contact with the stigma by themselves and can only be carried away by insects. The lever mechanism in Salvia also promotes cross-pollination and avoids self-pollination.
In Viola tricolor the stigma is protected by a flap or a lid that prevents contact between the stigma and anther. This flap is pushed aside by the insect and thus cross-pollination is effected.
Some of more important features are given here:
Advantages of self-pollination:
This type of pollination is almost certain in a bisexual flower, if the stamens and carpels of the flower mature at the same time.
Disadvantages of self-pollination:
Continued self-pollination generation after generation definitely results in weaker progeny.
Advantages of cross-pollination:
(i) It always gives rise to healthier offspring in subsequent generations which are better adapted in the struggle for existence;
(ii) More abundant and viable seeds are produced;
(iii) New varieties can be developed by this method;
(iv) The adaptability of the plants to their environment is definitely better by this method.
Disadvantages of cross-pollination:
(i) The plants have to depend upon external agencies for pollination (such as wind, water, insects, and animals);
(ii) Various devices are needed to fulfil the needs of outer agencies;
(iii) There is always a considerable waste of pollen where wind is the pollinating agent in cross-pollination.
Pollen Germination and Fertilisation:
The fusion of two dissimilar sexual reproductive units or gametes is termed as fertilization. In gymnosperms, the pollen grains usually land directly on the nucellus, while in angiosperms, they fall on the stigma.
In angiosperms, the fertilization is being completed as follows:
Germination of pollen grain:
After being deposited on the stigma, the pollen grain absorbs liquid from the moist surface of the stigma, expands in size, and the intine protrudes out through a germ pore. The small tubular structure also known as pollen tube continues to elongate, and makes its way down the tissues of the stigma and style. Only the distal part of the pollen tube possesses living cytoplasm.
The stigma plays an important role in the germination of pollen grain. The stigma secretes fluid containing liquids, gums, sugar and resins. The chief function of the stigmatic secretion is to protect the pollen as well as the stigma from desiccation.
After arriving to the wall of the ovary, the pollen tube enters the ovule either through the micropyle or by some other route. The entrance of the pollen tube through the micropyle is the normal condition and is known as porogamy. In some cases the pollen tube enters the ovule through the chalaza. This condition is known as chalagogamy. The chalagogamy was first reported by Treub (1891), in Casuarina.
The period between pollination and fertilisation varies from plant to plant. Usually this period varies from 12 to 48 hours. According to P. Maheshwari (1949), the temperature is responsible for controlling the growth of pollen tube.
Gametic fusion:
In normal case, one male gamete unites with the egg to form the zygote and the second travels a little farther and unites with the secondary nucleus. This process is known as double fertilisation (Navaschin, 1898; Guignard, 1899).
As the second male gamete fuses with the secondary diploid (2n) nucleus, producing a triploid (3n) primary endosperm nucleus, this is called triple fusion. Thus in an embryo sac there occur two sexual fusions; one in syngamy (i.e., fusion of male gamete with egg), and the other in triple fusion, and therefore, the phenomenon is called double fertilisation.
Significance of Double Fertilization:
The embryo, during its growth and development receives its nourishment from endosperm, which is a product of double fertilization. This process also gives required energy to the polar nuclei, which fail to divide further.
Since the endosperm nuclei are the resultants of double fertilization, they are characterized by maternal and paternal chromosomes and thus endosperm represents the physiological aggressiveness due to hybrid vigour.
The fusion of male and female gametes as well as double fertilization are equally responsible for the production of the viable seeds because absence of any one of these two may cause lethal effect, and the viable seeds are not produced.
Pollination and Fertilization in Vitro:
In many crosses, sometimes plant breeders face the problems such as the failure of the germination of the pollen, short length of pollen tube, or a slow growth of the pollen tube. The direct injection of pollen grains into the ovary may be helpful to overcome such problems. Besides the technique of direct injection of pollen suspension into the ovary, in vitro pollination of pistils has also been accomplished.
Un-pollinated ovaries of Nicotiana rustica were grown on Nitsch’s medium containing 4 per cent sucrose and pollinated the next day with pollen collected from dehiscing anthers. The process of fertilization and the development of endosperm and embryo were normal and mature seeds were obtained (P.S. Rao, 1965).
Kanta and Maheshwari (1963) also tried to bring about fertilization of ovules in vitro. The un-pollinated ovules of Papaver somniferum were sown on an agar medium in a test tube. The pollen grains were dusted over the implanted ovules. The pollen grains germinated, and the pollen tubes grew rapidly and covered the ovules. Successful fertilization occurred in many ovules.
