In the last article, we have studied fertilization and the formation of the endosperm. In this article, we shall study types of the endosperm.

Types of endosperm

Nuclear Endosperm:

The primary endosperm nucleus divides repeatedly to form a large number of free nuclei. No cell wall formation takes place at this stage (karyokinesis). A central vacuole appears later. It is followed by cell wall formation which is centripetal. Hence, a multicellular endosperm is formed. It is the most common type.

The process of cell plate formation may not be complete as in the case of coconut. Its peripheral portion has outer oily multicellular solid endosperm and inner free nuclear, degenerated multinucleate liquid endosperm called coconut milk.

Cellular Endosperm:

Here wall formation occurs immediately after the division of the primary endosperm nucleus. i.e. karyokinesis is followed by cytokinesis. Subsequent divisions also are accompanied by cell wall formation. As a result, the endosperm becomes cellular from the beginning. It is not common. It is mostly observed in dicots. Example- Balsam, Petunia.

Helobial Endosperm:

The first division of the primary endosperm nucleus is cellular i.e. wall formation takes place following the first division. However, inside each of these newly formed cells, free nuclear divisions occur. But finally, the endosperm becomes cellular following the pattern of development of nuclear endosperms. Hence, helobial endosperm is a combination of cellular and nuclear endosperms. It is common in monocots.

Post Fertilization changes:

Development of Embryo:

The embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after a certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo. Though the seeds differ greatly, the early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons.

The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radical or root tip. The root tip is covered with a root cap.

Formation of fruits and seeds:

As ovules mature into seeds, the ovary develops into a fruit, i.e., the transformation of ovules into seeds and ovary into fruit proceeds simultaneously. The wall of the ovary develops into the wall of fruit called the pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc.

Many fruits have evolved mechanisms for dispersal of seeds. In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off. However, in a few species such as apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits Most fruits, however, develop only from the ovary and are called true fruits.

Structure of Seed:

The ovules after fertilization, develop into seeds. A seed is made up of a seed coat and an embryo. The embryo is made up of a radicle, an embryonal axis and one (as in wheat, maize) or two cotyledons (as in gram and pea).

The outermost covering of a seed is the seed coat. The seed coat has two layers, the outer testa, and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit.

Parthenocarpy:

In most of the species, fruits are the results of fertilization, there are a few species in which fruits develop without fertilization. Such fruits are called parthenocarpic fruits. Banana is one such example.

Parthenocarpy can be induced through the application of growth hormones like gibberellins and such fruits are seedless. E.g. seedless grapes.

Apomixis:

Seeds, in general, are the products of fertilization, a few flowering plants such as some species of Asteraceae and grasses, have evolved a special mechanism, to produce seeds without fertilization, called apomixis. Thus, apomixis is a form of asexual reproduction that mimics sexual reproduction.

There are several ways of development of apomictic seeds. In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilization. E.g. Family Asteraceae, some grasses.

Practical use of Apomixis:

• Hybrid varieties of several of our food and vegetable crops are being extensively cultivated. The cultivation of hybrids has tremendously increased productivity. One of the problems of hybrids is that hybrid seeds have to be produced every year.
• If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters. Production of hybrid seeds is costly and hence the cost of hybrid seeds becomes too expensive for the farmers.
• If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crops year after year and he does not have to buy hybrid seeds every year.
• Because of the importance of apomixis in the hybrid seed industry, active research is going on in many laboratories around the world to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.

Polyembryony:

In many Citrus and Mango varieties, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. In such species, each ovule contains many embryos. The occurrence of more than one embryo in a seed is referred as polyembryony.

Significance of Fruits and Seeds:

• Dormancy: It is a temporary suspension of growth. The growth inhibitors prevent germination. During this period seeds are dispersed at different places.
• Viability: It is a functional ability of the seed to germinate after considerable dormancy period.
• Reserve Food: A fully developed embryo is nourished by the food stored in the endosperm of cotyledons.
• Protective Coat: Testa, the outer hard seed coat gives protection against mechanical shocks, fluctuations in temperature and dry condition. The testa has no effect of digestive juices on it.
• Dispersal: Some seeds produce wing, a hair-like structure suitable for dispersal.
• Edible Fruits: Many fruits are eaten by animals and seeds are thrown.
• Hence fruit and seeds are main agencies for the spread of the species.

Science > Biology > Botany > Reproduction in Plants > Types of Endosperm and Fruit Formation

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In last article, we have studied about pollination and its types. In this article, we shall study the process of fertilization.

Out Breeding Devices:

Most angiosperms produce bisexual flowers (hermaphrodite). But most of them avoid self-pollination naturally or we can say that in plants cross-pollination is preferred to self-pollination. Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.

• In some species, pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen. It prevents autogamy.
• In some other species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower. It prevents autogamy.
• The third device to prevent inbreeding is self-incompatibility. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
• Another device to prevent self-pollination is the production of unisexual flowers. If both male and female flowers are present on the same plant such as castor and maize (monoecious), it prevents autogamy but not geitonogamy. In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.

Pollen-Pistil Interaction:

All the events from the deposition of pollen grains on stigma to the entry of the pollen tube in the ovule are referred as pollen-pistil interaction. Pollination does not guarantee the transfer of the right type of pollen (compatible pollen of the same species as the stigma). Often, pollen of the wrong type, either from other species or from the same plant (if it is self-incompatible), also land on the stigma. The pistil has the ability to recognize the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible).

If the pollen is of the wrong type, the pistil rejects the pollen by preventing pollen germination on the stigma or the pollen tube growth in the style. The ability of pistil to identify pollen is due to chemical stimuli.

If it is of the right type, the pistil accepts the pollen and promotes post-pollination events that lead to fertilization. Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. The pollen tube grows through the tissues of the stigma and style and reaches the ovary.

Double Fertilization:

The process of fusion of male gamete with the female gamete to form a diploid zygote (2n) is called fertilization. The fusion of one male gamete with the egg and another male gamete with the secondary nucleus is called double fertilization. Double fertilization is a characteristic property of angiosperms. It was discovered by S. G. Navaschin in Lillium and Fritillaria species.

Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus. The filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube.

After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote.

The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) As this involves the fusion of three haploid nuclei it is termed triple fusion.

Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilization, an event unique to flowering plants. The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

Significance of Double Fertilization:

• Double fertilization involves the use of both the male gametes produced by a pollen grain.
• Thus the possibility of poly-embryology and there is increases in the chances of survival of the new plant.
• There are two fusions and hence have two products.
• In the product of first fusion, the diploidy in the life cycle is restored. The diploid zygote develops into an embryo which consequently develops into a new plant.
• The second fertilization product triploid primary endosperm nucleus (PEN) develops into nutritive tissue called endosperm, which provides nourishment to developing embryo.
• Double fertilization is a characteristic feature of angiosperms, hence seeds of angiosperms are more viable.

Post-Fertilization Changes:

Development of endosperm

Endosperm development precedes embryo development. The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo.

In the most common type of endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei. This stage of endosperm development is called free-nuclear endosperm. Subsequently, cell wall formation occurs and the endosperm becomes cellular. The number of free nuclei formed before cellularization varies greatly. The coconut water from tender coconut that you are familiar with, is nothing but free-nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.

Endosperm may either be completely consumed by the developing embryo (e.g., pea, groundnut, beans, grams) before seed maturation. Such seeds are called non-endospermic or ex-albuminous seeds.

The endosperm may persist in the mature seed (e.g. castor, sunflower, maize, wheat, and coconut) and be used up during seed germination. Such seeds are called endospermic or albuminous seeds.

Science > Biology > Botany > Reproduction in Plants > Fertilization

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In this article, we are going to study important step in sexual reproduction in plants i.e. pollination. We shall also study its types.

Transfer of pollen grains (shed from the anther) to the stigma of a pistil is termed pollination. Depending on the source of pollen, pollination can be divided into two types.

a) Self Pollination  i) Autonomy          ii) Geitonogamy

b) Cross Pollination    i) Xenogamy         ii) Hybridization

Self Pollination:

Transfer of pollen grains from the anther to the stigma of the same flower or different flower produced on the same plant is called self-pollination.

In autogamy, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower.

In geitonogamy, there is a transfer of pollen grains from the anther of one flower to the stigma of the different flowers of the same plant.

Advantages of Self Pollination:

• It is a sure method of fertilization.
• No external agencies or medium required for the pollination.
• This is the most economical method of pollination.
• For this method, attractive flowers, fragrance, Vivid colours and nectar production is not required.
• There is less wastage of pollen grains.
• Genetic stability can be maintained in the progeny.

Disadvantages of Self Pollination:

• Progeny shows less vigour due to continued self-pollination.
• No possibility of introduction of new desirable characters
• Undesirable characters cannot be eliminated.
• It does not help in evolution.
• Disease resistant capacity becomes less.

Cross Pollination:

Transfer of pollen grains from the anther of a flower to the stigma of another flower produced on the different plant having dissimilar genetic makeup is called cross-pollination. It is also called allogamy.

In xenogamy, pollination is achieved by transfer of pollen grains from the anther of a flower to the stigma of another flower produced on the different plant but of same species.

In hybridization, pollination is achieved by the transfer of pollen grains from the anther of a flower to the stigma of another flower produced on the different plants but of different species.

Advantages of Cross Pollination:

• Progeny shows enhanced vigour.
• Offspring are more viable and resistant.
• There is a possibility to get new desirable characters.
• It involves genetic recombination and hence brings variations.
• The yield of the crop can be maintained.
• It helps in evolution.
• Undesirable characters of the plant can be eliminated.

Disadvantages of Cross Pollination:

• Pollination may fail due to distance barrier.
• Flowers have to totally depend on the external agencies for pollination.
• More wastage of pollen.
• It may introduce some undesirable characters.
• It is not economical because a lot of energy of the plant is wasted in attracting carriers.
• Genetic Purity is not maintained.

Agencies of Pollination:

As pollen grain are non-motile, they require an external agency to transfer themselves from anther to the stigma. There are two categories of agencies of pollination

• Abiotic agents: Non-living physical factors like water, wind
• Biotic agents: Living organisms like insects, birds, bats, and animals.

Anemophily:

When pollen is transported by wind, this mode of pollination is called anemophily. It is the most primitive type of pollination. Many of the world’s most important crop plants are wind-pollinated. These include wheat, rice, corn, rye, barley, and oats.

Many economically important trees are also wind-pollinated. These include pines, spruces, firs, and many hardwood trees, including several species cultivated for nut production.

Wind-pollinated plants do not invest in resources that attract pollinating organisms, such as showy flowers, nectar, and scent. Instead, they produce larger quantities of light, dry pollen from small, plain flowers that can be carried on the wind. Female structures on wind-pollinated plants are adapted to capture the passing pollen from the air, but the majority of the pollen goes to waste.

Hydrophily:

When pollen is transported by water, this mode of pollination is called hydrophily. and plants are called hydrophilous. It is a very rare type of pollination, even in aquatic plants. water is a regular mode of transport for the male gametes among the lower plant groups such as algae, bryophytes, and pteridophytes.

Some examples of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera.

Water-pollinated plants do not invest in resources that attract pollinating organisms, such as showy flowers, nectar, and scent. Instead, they produce unwettable pollen. Stigma is long and sticky. Generally, flowers are unisexual. There are two types of hydrophilic pollination Hypohydrophily and b) Epihydrophily

Hypohydrophily:

Hypohydrophily is a true hydrophily that occurs beneath the surface of the water. It occurs in completely submerged plants and their pollen grains are waterborne. Example: Zostera marina, Ceratophyllum, etc.

Ceratophyllum desnersum:

In Ceratophyllum desnersum (that is, a submerged freshwater plant), the male flower bears 30 to 45 stamens. The mature anthers break at the base, mount to the surface of the water and dehisce there. The liberated pollen germinates and sinks in water. While sinking, they come in contact with the stigma of female flowers to produce pollination.

Zostera marina:

In Zostera marina, the pollen grains are elongated (up to 2,500 mm), like a needle and without exine. They have similar specific gravity as that of water; thus float beneath the surface of the water. Whenever they reach the stigma, they coil about it and germinate.

Epihydrophily:

Hypohydrophily is a pseudo hydrophily that occurs on the surface of the water. In Vallisneria male and female plants are separate. At maturity, male flowers are detached from male inflorescence and begin to float on the water surface. The coiled female plant undergoes uncoiling at maturity and reaches the water surface. The male flowers surround the female flower and undergo anthesis i.e. formation of a mature pollen grain.

Entomophily:

When pollen is transported by insects, this mode of pollination is called entomophily and plants are called entomophilous. Insect-pollinated plants have large and attractive flowers with vivid and bright colours. They have fragrance and nectar. If flowers are small they are grouped and called an inflorescence. e.g. sunflower. If flowers blossom at night they are white and have a very pleasant fragrance.

To sustain animal visits, the flowers have to provide rewards to the animals. Nectar and pollen grains are the usual floral rewards. For harvesting the reward(s) from the flower the animal visitor comes in contact with the anthers and the stigma. The body of the animal gets a coating of pollen grains, which are generally sticky in animal-pollinated flowers.

When the animal carrying pollen on its body comes in contact with the stigma, it brings about pollination. In some species, floral rewards are in providing safe places to lay eggs.

There is a relationship between a species of moth and the plant Yucca where both species – moth and the plant cannot complete their life cycles without each other. The 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.

Not all insects are causing pollination. They visit flowers for nectar only. Such insects are called pollen robbers or nectar robbers.

In salvia, the stamen is bifurcated into two connective branches. The upper branch of connective bears fertile anther lobe while the lower one is sterile. When insects enter flower for nectar, it pushes the lower sterile anther backward, which results in bending of the upper fertile anther. Now, upper fertile anther comes in contact with the insect body and pollens are dusted on the insect’s body. When such a dusted insect visits another flower with matured gynoecium, the pollens are received by stigma. This mechanism is called the lever mechanism.

Ornithophily:

When pollen is transported by birds, this mode of pollination is called ornithophily and plants are called ornithophilous. Bird-pollinated plants have large and attractive flowers with vivid and bright colours. They have thick and fleshy flower parts. Corolla is tubular and funnel-shaped. They don’t have fragrance. They produce a large amount of sugary nectar. Pollen grains are sticky. The common pollinating birds are sunbirds, hummingbirds, crow, Bulbul.

E.g. Callistemon (bottlebrush), Bignonia, Butea, Bombax ( Silk Cotton).

Chiropterophily:

When pollen is transported by bats, this mode of pollination is called chiropterophily and plants are called chiropterophilous. Bat-pollinated plants have large and stout flowers to hold the weight of a bat. These flowers are open at night only and produce the fermented fragrance of rotten fruit to which bats are attracted. Flowers have a large number of stamens and produce pollens in large numbers.

E.g. Anthocephallus (kadamb), Kigelia pinata, Adansonia (Baobab tree), Bauhinia.

Science > Biology > Botany > Reproduction in Plants > Pollination and its Types

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The flower is the reproductive unit in the angiosperms. It is meant for sexual reproduction. A typical flower has four different kinds of whorls arranged successively on the stalk or pedicel, called thalamus or receptacle. These whorls are calyx, corolla, androecium and gynoecium. Calyx and corolla are accessory organs, Androecium and gynoecium are reproductive organs. When a flower has both androecium and gynoecium, it is bisexual. A flower having either only stamens or only carpels is unisexual.

Parts of Flower involved in sexual reproduction:

Androecium:

The androecium is composed of stamens. Each stamen which represents the male reproductive organ consists of a stalk or a filament and an anther. Each anther is usually bilobed and each lobe has two chambers, the pollen-sacs. The microspores (pollen grains) are produced in pollen-sacs. A sterile stamen is called staminode.

Gynoecium:

The gynoecium is the female reproductive part of the flower and is made up of one or more carpels. A carpel consists of three parts namely stigma, style and ovary. The ovary is the enlarged basal part, on which lies the elongated tube, the style. The style connects the ovary to the stigma. The stigma is usually at the tip of the style and is the receptive surface for pollen grains. Each ovary bears one or more ovules attached to a flattened, cushion-like placenta. Ovules develop into seeds and the ovary matures into a fruit.

Structure of Anther (Microsporangium):

In a transverse section, a typical microsporangium appears near circular in outline. It is generally surrounded by four wall layers the epidermis, endothecium, middle layers and the tapetum.

The epidermis consists of flattened cells and is protective in the function. It is outermost common wall layer of the anther. Endothecium is internal to the epidermis. Cells of endothecium help for dehiscence of anther at maturity. Middle layers are internal to endothelium. They are of three layers of parenchyma cells. The cells of these layers degenerate on maturity and the two pollen sacs of each lobe merge to form a single chamber. Tapetum surrounds sporanenous cells and provide nutrition to developing pollen grains.

As each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad. Each one is 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, 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 or pollen grains are formed that are released with the dehiscence of the anther

Structure of pollen grains:

The pollen grains represent the male gametophytes. It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which 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 grain exine has prominent apertures called germ pores where sporopollenin is absent. 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 a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.

Development of Male gametophyte:

Before pollination in pollen sack:

The protoplast of pollen grain divides by mitosis to form to unequal cells. The smaller cell is called the generative cell. It has a large nucleus, thin cytoplasm and it lacks reserve food and vacuole. The large Cell is called vegetative cell or tube cell. It has large vacuole, cytoplasm, nucleus and reserve food.

After pollination on the stigma:

When the two-celled pollen grain comes in contact with sugary stigmatic secretion it absorbs it. Due to absorption of secretion, the pressure of cytoplasm on the intine increases and the intine of pollen grain comes out of germ pore to form a pollen tube. The pollen tube starts growing towards the ovule trough style due to chemical stimulus in the ovary.

The tube nucleus, cytoplasm and generative cell starts migrating into the pollen tube. The generative cell divides by mitosis to form two haploid non-motile male gametes.

Gynoecium:

The gynoecium represents the female reproductive part of the flower. The gynoecium may consist of a single pistil (monocarpellary) or may have more than one pistil (multicarpellary). Each pistil has three parts the stigma, style and ovary. 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 (locule). The placenta is located inside the ovarian cavity. Arising from the placenta are the megasporangia, commonly called ovules. The number of ovules in an ovary may be one (wheat, paddy, mango) to many (papaya, water melon, orchids).

Structure of Ovule:

The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle. Each ovule has one or two protective envelopes called integuments. Integuments encircle the ovule 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. It consists of many diploid parenchyma cells. 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 through reduction division.

Functions of Parts of Ovule:

• Funicle: To support, projection and conduction
• Nucellus: Development of female gametophyte takes place in it.
• Integuments: Protection to nucellus and embryo sac
• Micropyle: It forms a passage for pollen tube to enter in the ovule.
• Antipodals: They are accessory cells and degenerate after fertilization.

The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.

Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region. The diploid MMC (2n) undergoes meiosis to form a tetrad of haploid megaspores (n)

Development of Female Gamete:

The chalazal megaspore remains functional, while the other three degenerate gradually.  The functional megaspore undergoes enlargement and develops into the female gametophyte. The nucleus of the functional haploid 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.

There is cellular organization in which 3 celled egg apparatus is formed at mycropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. Three cells are at the chalazal end and are called the antipodals. The large central cell, has two polar nuclei. Thus, a typical angiosperm contains embryo sac, at maturity, though 8-nucleate is 7-celled.

Science > Biology > Botany > Reproduction in Plants > Androecium and Gynoecium

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Sexual reproduction is also called amphimixis (Gk. amphi – both, mixis – union) or syngenesis (Gk. syn – together, genesis – origin) or amphigony (Gk. amphi – both, gony – marriage). In this article, we shall study sexual reproduction in plants.

When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction. Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism.

It is an elaborate, complex and slow process as compared to asexual reproduction. Because of the fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parents or amongst themselves.

Preparation for Sexual Reproduction:

All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants. This phase is of variable durations in different organisms. The end of juvenile/vegetative phase which marks the beginning of the reproductive phase can be seen easily in the higher plants when they come to flower.

In animals, the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour. The reproductive phase is also of variable duration in different organisms. In non-primate mammals like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction are called oestrus cycle. Whereas in primates (monkeys, apes, and humans) it is called menstrual cycle.

Characteristics of Sexual Reproduction:

• It takes place with the help of gametes.
• It is biparental in origin i.e. two parents are involved (both male and female).
• A male gamete and a female gamete fuse together and fertilization takes place.
• It involves both meiosis and mitosis,
• It is a slower and lengthy method of reproduction.
• Variations occur in sexual reproduction. Offspring are different from parents, genetically and physically. It is important for evolution.
• It occurs more frequently in higher organisms than in prokaryotes. 

Advantages of Sexual Reproduction:

• Sexual reproduction is biparental in origin i.e. two parents are involved (both male and a female). Thus during sexual reproduction fusion of gametes from different parents occur. It results in genetic recombination causing variations. Offspring produces in this type is different from parents, genetically and physically. Thus, it ensures the survival of species in a population and helps in evolution.
• Since both the parents are involved, the newly formed individual (offspring) has characteristics of both the parents.
• The genetic variations produced during sexual reproduction help offspring to adapt to different environments. It also provides vigour and vitality to the offspring. Offspring can adapt better to changing environmental conditions. Hence, the species produced by sexual reproduction survive more than those produced by asexual reproduction.

Disadvantages of Sexual Reproduction:

• Sexual reproduction is usually bi-parental. Hence in sexual reproduction, the two genders must find each other to be able to reproduce. It takes time and energy to locate a suitable mate with the preferred traits that are desired so that the offspring produced by the union can thrive. 
• Fertilization has a chance factor. Sexual reproduction is not a 100% successful method of creating offspring. Some chosen mates may be infertile. Others may not have the gametes come together, despite numerous attempts at creating offspring.
• Sexual reproduction involves the first mating of suitable mates. The fusion of gametes and fertilization. It involves both meiosis and mitosis. Hence it is a slow and lengthy process and requires a lot of time.
• Sexual reproduction can produce numerous offspring at one time. Humans may typically have one child through reproduction, but twins, triplets, and larger multiples are possible. Horses may typically have one foal, but cats and dogs may have more than a dozen in a litter. Compared to asexual reproduction, however, there are usually fewer offspring produced over time. 

Sexual Reproduction in Plants:

Plants can reproduce asexually or sexually. But the majority of the flowering plants reproduce sexually. The flower is the reproductive part of a plant i.e., both male and female gametes are produced by flowers. Sexual reproduction in plants takes place in flowers.  During sexual reproduction, they produce seeds. Plants in their lifetime produce seeds that germinate, grow and develop into new plants. These new plants flower, scatter their seeds and die.

Sexual Reproduction in lower plants:

Bryophytes e.g. Moss:

Moss plants grow in damp places because they depend on moisture to sexually reproduce.  They need water for the sperm to swim towards the eggs for fertilization. Mosses reproduce by spores, which are analogous to the seeds of flowering plants. The spores are single celled.  Spores are housed in the brown capsule that sits on the seta. The spores ripen and are dispersed from the capsule, and some land in areas where there is enough moisture for them to grow.

Some mosses have cups on their tops that produce sperm, these are male plants. The female plants has eggs between overlapping leaves. As the sperm become mature they swim through water to the eggs and fertilize them. The fertilized egg then produces the stalked brown capsule.

Algae e.g. Spirogyra:

In spirogyra, sexual reproduction takes place by conjugation. Two filaments lie side by and produce short tubes that grow and connect the opposite cells. During this time the protoplasts of conjugating cells (gametangia) recede, round up and function as gametes. At the point of contact, the tips of these processes dissolve (bycytase) to form conjugation tube between opposite cells. This structure looks like ladder. Hence, the conjugation is called scalariform or H-shape conjugation. The contents then move from male cell (male gametangia) to female cell (female gametangia) and their nuclei combine, forming a zygote. This process is known as conjugation. The zygote secretes a thick wall to become zygospore. Therefore, in the late stage of conjugation, male gametangia remain empty and the female gametangia contain zygospores.

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