Biology 12

Sexual reproduction in Flowering plants

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Are we not lucky that plants reproduce sexually? The myriads of flowers that we enjoy gazing at, the scents and the perfumes that we swoon over, the rich colours that attract us, are all there as an aid to sexual reproduction. Flowers do not exist only for us to be used for our own selfishness. All flowering plants show sexual reproduction. A look at the diversity of structures of the inflorescences, flowers and floral parts, shows an amazing range of adaptations to ensure formation of the end products of sexual reproduction, the fruits and seeds. In this chapter, let us understand the morphology, structure and the processes of sexual reproduction in flowering plants (angiosperms).
Flower : A fascinating organ of Angiosperms
Structure of a Flower

Human beings have had an intimate relationship with flowers since time immemorial. Flowers are objects of aesthetic, ornamental, social, religious and cultural value : they have always been used as symbols for conveying important human feelings such as love, affection, happiness, grief, mourning, etc. List at least five flowers of ornamental value that are commonly cultivated at homes and in gardens. Find out the names of five more flowers that are used in social and cultural celebrations in your family. Have you heard of floriculture, What does it refer to? To a biologist, flowers are morphological and embryological marvels and the sites of sexual reproduction. In class XI, you have read the various parts of a flower. Figure 2.1 will help you recall the parts of a typical flower. Can you name the two parts in a flower in which the two most important units of sexual reproduction develop?

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Pre-fertilisation: structures and events
Much before the actual flower is seen on a plant, the decision that the plant is going to flower has taken place. Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium. Inflorescences are formed which bear the floral buds and then the flowers. In the flower the male and female reproductive structures, the androecium and the gynoecium differentiate and develop. You would recollect that the androecium consists of a whorl of stamens representing the male reproductive organ and the gynoecium represents the female reproductive organ.
Anther - Structure


Stamen, Microsporangium and Pollen Grain
Figure 2.2a shows the two parts of a typical stamen − the long and slender stalk called the filament, and the terminal generally bilobed structure called the anther. The proximal end of the filament is attached to the thalamus or the petal of the flower. The number and length of stamens are variable in flowers of different species. If you were to collect a stamen each from ten flowers (each from different species) and arrange them on a slide, you would be able to appreciate the large variation in size seen in nature. Careful observation of each stamen under a dissecting microscope and making neat diagrams would elucidate the range in shape and attachment of anthers in different flowers. A typical angiosperm anther is bilobed with each lobe having two theca, i.e., they are dithecous (Figure 2.2 b). Often a longitudinal groove runs lengthwise separating the theca. Let us understand the various types of tissues and their organisation in the transverse section of an anther (Figure 2.3 a). The bilobed nature of an anther is very distinct in the transverse section of the anther. The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.
The microsporangia develop further and become pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains. Structure of microsporangium: In a transverse section, a typical microsporangium appears near circular in outline. It is generally surrounded by four wall layers (Figure 2.3 b) − the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen. The innermost wall layer is the tapetum. It nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus. Can you think of how tapetal cells could become bi-nucleate? When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the centre of each microsporangium.
Anther - Structure

Anther at the time of dehiscence and Sculpturing

Structure and Germination of a pollen grain

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Microsporogenesis : As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads. What would be the ploidy of the cells of the tetrad?
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. The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis. The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad (Figure 2.3 a). As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains (Figure 2.3 b). Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther (Figure 2.3 c).

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Pollen grain: The pollen grains represent the male gametophytes. If you touch the opened anthers of Hibiscus or any other flower you would find deposition of yellowish powdery pollen grains on your fingers. Sprinkle these grains on a drop of water taken on a glass slide and observe under a microscope. You will really be amazed at the variety of architecture − sizes, shapes, colours, designs − seen on the pollen grains from different species (Figure 2.4).
Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. 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. Pollen grains are wellpreserved as fossils because of the presence of sporopollenin. The exine exhibits a fascinating array of patterns and designs. Why do you think the exine should be hard? What is the function of germ pore? The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The cytoplasm of pollen grain is surrounded by a plasma membrane. When the pollen grain is mature it contains two cells, the vegetative cell and generative cell (Figure 2.5b). 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. In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
Pollen grains of many species cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders − asthma, bronchitis, etc. It may be mentioned that Parthenium or carrot grass that came into India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy.
Pollen grains are rich in nutrients. It has become a fashion in recent years to use pollen tablets as food supplements. In western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and race horses (Figure 2.6).

Pollination

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When once they are shed, pollen grains have to land on the stigma before they lose viability if they have to bring about fertilisation. How long do you think the pollen grains retain viability? The period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperature and humidity. In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release, and in some members of Rosaceae, Leguminoseae and Solanaceae, they maintain viability for months. You may have heard of storing semen/ sperms of many animals including humans for artificial insemination. It is possible to store pollen grains of a large number of species for years in liquid nitrogen (− 196⁰C). Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.
The Pistil, Megasporangium (ovule) and Embryo sac
The gynoecium represents the female reproductive part of the flower. The gynoecium may consist of a single pistil (monocarpellary) or may have more than one pistil (multicarpellary). When there are more than one, the pistils may be fused together (syncarpous) (Figure 2.7b) or may be free (apocarpous) (Figure 2.7c). Each pistil has three parts (Figure 2.7a), 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. Recall the definition and types of placentation that you studied in Class XI. 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).
The Megasporangium (Ovule) : Let us familiarise ourselves with the structure of a typical angiosperm ovule (Figure 2.7d). A typical ovule ( Angiosperm)

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 nucellus except at the tip where a small opening called the micropyle is organised. Opposite the micropylar end, is the chalaza, representing the basal part of the ovule.
Enclosed within the integuments is a mass of cells called the nucellus. Cells of the nucellus have abundant reserve food materials. Located in the nucellus is the embryo sac or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore.
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Megasporogenesis : 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 of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus. The MMC undergoes meiotic division. What is the importance of the MMC undergoing meiosis? Meiosis results in the production of four megaspores (Figure 2.8a).
Development of Megaspore (Angiosperm)

Female gametophyte : In a majority of flowering plants, one of the megaspores is functional while the other three degenerate. Only the functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development. What will be the ploidy of the cells of the nucellus, MMC, the functional megaspore and female gametophyte?
Let us study formation of the embryo sac in a little more detail. (Figure 2.8b). The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2− nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4− nucleate and later the 8− nucleate stages of the embryo sac. It is of interest to note that these mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation. After the 8− nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac. Observe the distribution of cells inside the embryo sac (Figure 2.8b, c).
Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. There is a characteristic distribution of the cells within the embryo sac. Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. The large central cell, as mentioned earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at maturity, though 8− nucleate is 7− celled.
Pollination
Embryo Sac (Angiosperm)

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In the preceding sections you have learnt that the male and female gametes in flowering plants are produced in the pollen grain and embryo sac, respectively. As both types of gametes are non− motile, they have to be brought together for fertilisation to occur. How is this achieved? Pollination is the mechanism to achieve this objective. Transfer of pollen grains (shed from the anther) to the stigma of a pistil is termed pollination. Flowering plants have evolved an amazing array of adaptations to achieve pollination. They make use of external agents to achieve pollination. Can you list the possible external agents?
Fertilization

Pollination

Kinds of Pollination : Depending on the source of pollen, pollination can be divided into three types.
(i) Autogamy : In this type, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower (Figure 2.9a). In a normal flower which opens and exposes the anthers and the stigma, complete autogamy is rather rare. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity and also, the anthers and the stigma should lie close to each other so that self− pollination can occur. Some plants such as Viola (common pansy), Oxalis, and Commelina produce two types of flowers – chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all (Figure 2.9c). In such flowers, the anthers and stigma lie close to each other. When anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination. Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross− pollen landing on the stigma. Cleistogamous flowers produce assured seed− set even in the absence of pollinators. Do you think that cleistogamy is advantageous or disadvantageous to the plant? Why?
(ii) Geitonogamy – Transfer of pollen grains from the anther to the stigma of another flower of the same plant. Although geitonogamy is functionally cross− pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.
(iii) Xenogamy – Transfer of pollen grains from anther to the stigma of a different plant (Figure 2.9b). This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma. Agents of Pollination : Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination. Majority of plants use biotic agents for pollination. Only a small proportion of plants use abiotic agents. Pollen grains coming in contact with the stigma is a chance factor in both wind and water pollination. To compensate for this uncertainties and associated loss of pollen grains, the flowers produce enormous amount of pollen when compared to the number of ovules available for pollination. Pollination by wind is more common amongst abiotic pollinations. Wind pollination also requires that the pollen grains are light and non− sticky so that they can be transported in wind currents. They often possess well− exposed stamens (so that the pollens are easily dispersed into wind currents, Figure 2.10) and large often− feathery stigma to easily trap air− borne pollen grains. Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence; a familiar example is the corn cob − the tassels you see are nothing but the stigma and style which wave in the wind to trap pollen grains. Wind− pollination is quite common in grasses.
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Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons. As against this, you would recall that water is a regular mode of transport for the male gametes among the lower plant groups such as algae, bryophytes and pteridophytes. It is believed, particularly for some bryophytes and pteridophytes, that their distribution is limited because of the need for water for the transport of male gametes and fertilisation.
Some examples of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera. Not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind as in most of the land plants. In Vallisneria, the female flower reach the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried passively by water currents (Figure 2.11a); some of them eventually reach the female flowers and the stigma. In another group of water pollinated plants such as seagrasses, female flowers remain submerged in water and the pollen grains are released inside the water. Pollen grains in many such species are long, ribbon like and they are carried passively inside the water; some of them reach the stigma and achieve pollination. In most of the water-pollinated species, pollen grains are protected from wetting by a mucilaginous covering. Both wind and water pollinated flowers are not very colourful and do not produce nectar. What would be the reason for this? Majority of flowering plants use a range of animals as pollinating agents. Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) and bats are the common pollinating agents. (Figure 2.11b). Among the animals, insects, particularly bees are the dominant biotic pollinating agents. Even larger animals such as some primates (lemurs), arboreal (tree-dwelling) rodents, or even reptiles (gecko lizard and garden lizard) have also been reported as pollinators in some species. Often flowers of animalpollinated plants are specifically adapted for a particular species of animal.
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Majority of insect-pollinated flowers are large, colourful, fragrant and rich in nectar. When the flowers are small, a number of flowers are clustered into an inflorescence to make them conspicuous. Animals are attracted to flowers by colour and/or fragrance. The flowers pollinated by flies and beetles secrete foul odours to attract these animals. 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; an example is that of the tallest flower of Amorphophallus (the flower itself is about 6 feet in height). A similar relationship exists 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. Why do not you observe some flowers of the following plants (or any others available to you): Cucumber, Mango, Peepal, Coriander, Papaya, Onion, Lobia, Cotton, Tobacco, Rose, Lemon, Eucalyptus, Banana? Try to find out which animals visit them and whether they could be pollinators.You'll have to patiently observe the flowers over a few days and at different times of the day. You could also try to see whether there is any correlation in the characteristics of a flower to the animal that visits it. Carefully observe if any of the visitors come in contact with the anthers and the stigma as only such visitors can bring about pollination. Many insects may consume pollen or the nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers. You may or may not be able to identify the pollinators, but you will surely enjoy your efforts!
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Outbreeding Devices : Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower. Continued self-pollination result in inbreeding depression. Flowering plants have developed many devices to discourage selfpollination and to encourage cross-pollination. In some species, pollen release and stigma receptivity are not synchronised. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen. 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. Both these devices prevent 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.
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Pollen-pistil Interaction

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 recognise the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible). If it is of the right type, the pistil accepts the pollen and promotes post-pollination events that leads to fertilisation. 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 the pistil to recognise the pollen followed by its acceptance or rejection is the result of a continuous dialogue between pollen grain and the pistil. This dialogue is mediated by chemical components of the pollen interacting with those of the pistil. It is only in recent years that botanists have been able to identify some of the pollen and pistil components and the interactions leading to the recognition, followed by acceptance or rejection. As mentioned earlier, following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores (Figure 2.12a). The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style and reaches the ovary (Figure 2.12b, c). You would recall that in some plants, pollen grains are shed at two-celled condition (a vegetative cell and a generate cell). In such plants, the generative cell divides and forms the two male gametes during the growth of pollen tube in the stigma. In plants which shed pollen in the three-celled condition, pollen tubes carry the two male gametes from the beginning. Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus (Figure 2.12d, e). Many recent studies have shown that filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube. All these events − from pollen deposition on the stigma until pollen tubes enter the ovule − are together referred to as pollen-pistil interaction. As pointed out earlier, pollen-pistil interaction is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen. The knowledge gained in this area would help the plant breeder in manipulating pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids. You can easily study pollen germination by dusting some pollen from flowers such as pea, chickpea, Crotalaria, balsam and Vinca on a glass slide containing a drop of sugar solution (about 10 per cent). After about 15− 30 minutes, observe the slide under the low power lens of the microscope. You are likely to see pollen tubes coming out of the pollen grains. As you shall learn in the chapter on plant breeding (Chapter 9), a breeder is interested in crossing different species and often genera to combine desirable characters to produce commercially 'superior' varieties. Artificial hybridisation is one of the major approaches of crop improvement programme. In such crossing experiments it is important to make sure that only the desired pollen grains are used for pollination and the stigma is protected from contamination (from unwanted pollen). This is achieved by emasculation and bagging techniques. If the female parent bears bisexual flowers, removal of anthers from the flower bud before the anther dehisces using a pair of forceps is necessary. This step is referred to as emasculation. Emasculated flowers have to be covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen. This process is called bagging. When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma, and the flowers are rebagged, and the fruits allowed to develop.
If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower rebagged.
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DOUBLE FERTILISATION
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) (Figure 2.13a). 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 fertilisation, 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.
POST-FERTILISATION : STRUCTURES AND EVENTS
Following double fertilisation, events of endosperm and embryo development, maturation of ovule(s) into seed(s) and ovary into fruit, are collectively termed post-fertilisation events.
Endosperm
Endosperm development precedes embryo development. Why? 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 cellularisation 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) before seed maturation or it may persist in the mature seed (e.g. castor and coconut) and be used up during seed germination. Split open some seeds of castor, peas, beans, groundnut, fruit of coconut and look for the endosperm in each case. Find out whether the endosperm is persistent in cereals − wheat, rice and maize. 2.4.2 Embryo Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after 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. Figure 2.13 depicts the stages of embryogeny in a dicotyledonous embryo. The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo. A typical dicotyledonous embryo (Figure 2.14a), 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. Embryos of monocotyledons (Figure 2.14 b) possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile. Soak a few seeds in water (say of wheat, maize, peas, chickpeas, ground nut) overnight. Then split the seeds and observe the various parts of the embryo and the seed.
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Seed
In angiosperms, the seed is the final product of sexual reproduction. It is often described as a fertilised ovule. Seeds are formed inside fruits. A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis. The cotyledons (Figure 2.15a) of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes). Mature seeds may be non-albuminous or albuminous. Non-albuminous seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut). Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor, sunflower).
Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm. Integuments of ovules harden as tough protective seed coats (Figure 2.15a). The micropyle remains as a small pore in the seed coat. This facilitates entry of oxygen and water into the seed during germination. As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass). The general metabolic activity of the embryo slows down.
The embryo may enter a state of inactivity called dormancy, or if favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate. 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 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. Recall the classification of fruits and their dispersal mechanisms that you have studied in an earlier class. Is there any relationship between number of ovules in an ovary and the number of seeds present in a fruit? 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 (Figure 2.15b)../ Most fruits however develop only from the ovary and are called true fruits. Although in most of the species, fruits are the results of fertilisation, there are a few species in which fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. Banana is one such example. Parthenocarpy can be induced through the application of growth hormones and such fruits are seedless.
Seeds offer several advantages to angiosperms. Firstly, since reproductive processes such as pollination and fertilisation are independent of water, seed formation is more dependable. Also seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas. As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own. The hard seed coat provides protection to the young embryo. Being products of sexual reproduction, they generate new genetic combinations leading to variations.
Seed is the basis of our agriculture. Dehydration and dormancy of mature seeds are crucial for storage of seeds which can be used as food through out the year and also to raise crop in the next season. Can you imagine agriculture in the absence of seeds, or in the presence of seeds which germinate straight away soon after formation and cannot be stored? How long do the seeds remain alive after they are dispersed? This period again varies greatly. In a few species the seeds lose viability within a few months. Seeds of a large number of species live for several years. Some seeds can remain alive for hundreds of years. There are several records of very old yet viable seeds. The oldest is that of a lupine, Lupinus arcticus excavated from Arctic Tundra. The seed germinated and flowered after an estimated record of 10,000 years of dormancy.
A recent record of 2000 years old viable seed is of the date palm, Phoenix dactylifera discovered during the archeological excavation at King Herod’s palace near the Dead Sea. Extract(14)
After completing a brief account of sexual reproduction of flowering plants it would be worth attempting to comprehend the enormous reproductive capacity of some flowering plants by asking the following questions: How many eggs are present in an embryo sac? How many embryo sacs are present in an ovule? How many ovules are present in an ovary? How many ovaries are present in a typical flower? How many flowers are present on a tree? And so on... Can you think of some plants in which fruits contain very large number of seeds. Orchid fruits are one such category and each fruit contain thousands of tiny seeds. Similar is the case in fruits of some parasitic species such as Orobanche and Striga. Have you seen a tiny seed of Ficus? How large is the tree of Ficus developed from that tiny seed. How many billions of seeds does each Ficus tree produce? Can you imagine any other example in which such a tiny structure can produce such a large biomass over the years?
Apomixis and polyembryony
Although seeds, in general are the products of fertilisation, a few flowering plants such as some species of Asteraceae and grasses, have evolved a special mechanism, to produce seeds without fertilisation, called apomixis. What is fruit production without fertilisation called? 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 fertilisation. More often, as in many Citrus and Mango d is 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. Occurrence of more than one embryo in a seed is referred as polyembryony. Take out some seeds of orange and squeeze them. Observe the many embryos of different sizes and shapes from each seed. Count the number of embryos in each seed. What would be the genetic nature of apomictic embryos? Can they be called clones?
Hybrid varieties of several of our food and vegetable crops are being extensively cultivated. 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 become 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 crop year after year and he does not have to buy hybrid seeds every year. Because of the importance of apomixis in 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.
Exercises

1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place.

The male gametophyte or the pollen grain develops inside the pollen chamber of the anther, whereas the female gametophyte (also known as the embryo sac) develops inside the nucellus of the ovule from the functional megaspore.

2. Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.

1. Microsporogenesis is the process of the formation of microspore tetrads from a microspore mother cell through meiosis. Megasporogenesis is the process of the formation of the four megaspores from a megaspore mother cell in the region of the nucellus through meiosis 2. Microsporogenesis occurs inside the pollen sac of the anther. Megasporogenesis occurs inside the ovule. Both events (microsporogenesis and megasporogenesis) involve the process of meiosis or reduction division which results in the formation of haploid gametes from the microspore and megaspore mother cells. Microsporogenesis results in the formation of haploid microspores from a diploid microspore mother cell. On the other hand, megasporogenesis results in the formation of haploid megaspores from a diploid megaspore mother cell.

3. Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.

The correct development sequence is as follows: Sporogenous tissue - pollen mother cell- microspore tetrad -Pollen grain - male gamete . During the development of microsporangium, each cell of the sporogenous tissue acts as a pollen mother cell and gives rise to a microspore tetrad, containing four haploid microspores by the process of meiosis (microsporogenesis). As the anther matures, these microspores dissociate and develop into pollen grains. The pollen grains mature and give rise to male gametes.

4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.

An ovule is a female megasporangium where the formation of megaspores takes place.
The various parts of an ovule are
Funiculus - It is a stalk-like structure which represents the point of attachment of the ovule to the placenta of the ovary.
Hilum - It is the point where the body of the ovule is attached to the funiculus.
Integuments -They are the outer layers surrounding the ovule that provide protection to the developing embryo.
Micropyle - It is a narrow pore formed by the projection of integuments. It marks the point where the pollen tube enters the ovule at the time of fertilization.
Nucellus -It is a mass of the parenchymatous tissue surrounded by the integuments from the outside. The nucellus provides nutrition to the developing embryo. The embryo sac is located inside the nucellus.
Chalaza -It is the based swollen part of the nucellus from where the integuments originate.

5. What is meant by monosporic development of female gametophyte?

The female gametophyte or the embryo sac develops from a single functional megaspore. This is known as monosporic development of the female gametophyte. In most flowering plants, a single megaspore mother cell present at the micropylar pole of the nucellus region of the ovule undergoes meiosis to produce four haploid megaspores. Later, out of these four megaspores, only one functional megaspore develops into the female gametophyte, while the remaining three degenerate.

6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.

The female gametophyte (embryo sac) develops from a single functional megaspore. This megaspore undergoes three successive mitotic divisions to form eight nucleate embryo sacs. The first mitotic division in the megaspore forms two nuclei. One nucleus moves towards the micropylar end while the other nucleus moves towards the chalazal end. Then, these nuclei divide at their respective ends and redivide to form eight nucleate stages. As a result, there are four nuclei each at both the ends i.e., at the micropylar and the chalazal end in the embryo sac. At the micropylar end, out of the four nuclei only three differentiate into two synergids and one egg cell. Together they are known as the egg apparatus. Similarly, at the chalazal end, three out of four nuclei differentiates as antipodal cells. The remaining two cells (of the micropylar and the chalazal end) move towards the centre and are known as the polar nuclei, which are situated in a large central cell. Hence, at maturity, the female gametophyte appears as a 7-celled structure, though it has 8 nucleate.

7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

There are two types of flowers present in plants namely Oxalis and Viola − chasmogamous and cleistogamous flowers. Chasmogamous flowers have exposed anthers and stigmata similar to the flowers of other species. Cross-pollination cannot occur in cleistogamous flowers. This is because cleistogamous flowers never open at all. Also, the anther and the stigma lie close to each other in these flowers. Hence, only self-pollination is possible in these flowers.

8. Mention two strategies evolved to prevent self-pollination in flowers.

Self-pollination involves the transfer of pollen from the stamen to the pistil of the same flower.
Two strategies that have evolved to prevent self pollination in flowers are as follows: In certain plants, the stigma of the flower has the capability to prevent the germination of pollen grains and hence, prevent the growth of the pollen tube. It is a genetic mechanism to prevent self-pollination called self- incompatibility. Incompatibility may be between individuals of the same species or between individuals of different species. Thus, incompatibility prevents breeding. In some plants, the gynoecium matures before the androecium or vice-versa. This phenomenon is known as protogyny or protandry respectively. This prevents the pollen from coming in contact with the stigma of the same flower.

9. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?

Self-incompatibility is a genetic mechanism in angiosperms that prevents self-pollination. It develops genetic incompatibility between individuals of the same species or between individuals of different species. The plants which exhibit this phenomenon have the ability to prevent germination of pollen grains and thus, prevent the growth of the pollen tube on the stigma of the flower. This prevents the fusion of the gametes along with the development of the embryo. As a result, no seed formation takes place.

10. What is bagging technique? How is it useful in a plant breeding programme?

Various artificial hybridization techniques (under various crop improvement programmes) involve the removal of the anther from bisexual flowers without affecting the female reproductive part (pistil) through the process of emasculation. Then, these emasculated flowers are wrapped in bags to prevent pollination by unwanted pollen grains. This process is called bagging. This technique is an important part of the plant breeding programme as it ensures that pollen grains of only desirable plants are used for fertilization of the stigma to develop the desired plant variety.

11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.

Triple fusion is the fusion of the male gamete with two polar nuclei inside the embryo sac of the angiosperm. This process of fusion takes place inside the embryo sac. When pollen grains fall on the stigma, they germinate and give rise to the pollen tube that passes through the style and enters into the ovule. After this, the pollen tube enters one of synergids and releases two male gametes there. Out of the two male gametes, one gamete fuses with the nucleus of the egg cell and forms the zygote (syngamy). The other male gamete fuses with the two polar nuclei present in the central cell to form a triploid primary endosperm nucleus. Since this process involves the fusion of three haploid nuclei, it is known as triple fusion. It results in the formation of the endosperm. One male gamete nucleus and two polar nuclei are involved in this process.

12. Why do you think the zygote is dormant for sometime in a fertilised ovule?

The zygote is formed by the fusion of the male gamete with the nucleus of the egg cell. The zygote remains dormant for some time and waits for the endosperm to form, which develops from the primary endosperm cell resulting from triple fusion. The endosperm provides food for the growing embryo and after the formation of the endosperm, further development of the embryo from the zygote starts.

13. Differentiate between:
(a) hypocotyl and epicotyl; (b) coleoptile and coleorrhiza; (c) integument and testa; (d) perisperm and pericarp.

(a) Hypocotyl Epicotyl 1. The portion of the embryonal axis which lies below the cotyledon in a dicot embryo is known as the hypocotyl. The portion of the embryonal axis which lies above the cotyledon in a dicot embryo is known as the epicotyl. 2. It terminates with the radicle. It terminates with the plumule.
(b) Coleoptile Coleorrhiza It is a conical protective sheath that encloses the plumule in a monocot seed. It is an undifferentiated sheath that encloses the radicle and the root cap in a monocot seed.
(c) Integument Testa It is the outermost covering of an ovule. It provides protection to it. It is the outermost covering of a seed.
(d) Perisperm Pericarp It is the residual nucellus which persists. It is present in some seeds such as beet and black pepper. It is the ripened wall of a fruit, which develops from the wall of an ovary.
14. Why is apple called a false fruit? Which part(s) of the flower forms the fruit? 15. What is meant by emasculation? When and why does a plant breeder employ this technique? 16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why? 17. Explain the role of tapetum in the formation pollen-grain wall. 18. What is apomixis and what is its importance? Top

Sexual Reproduction in plants

Notes

2. Megasporogenesis occurs inside ovules of carpels. It produces haploid megaspores. One haploid megaspore survives. It gives rise to female gametophyte or embryo sac having one female gamete or oosphere.
3. Part of sexual reproduction involving sporogenesis, fertilisation and embryogeny is called embryology.

Anther (Microsporogenesis)

4. Anther is bilobed and tetrasporangiate with two long cylindrical microsporangia or pollen sacs (sacs having pollen grains or microspores) in each of the two lobes.
5. A common epidermis occurs on the outside. The pollen sacs develop hypodermally at the four corners of the anther from a strip of archesporium.
a. Archesporial cells divide periclinally into outer primary parietal cells and inner primary sporogenous cells. Parietal cells form wall of microsporangium. It consists of a layer of endothecium, 1-3 middle layers and a layer of tapetum.
b. Endothecial cells are large. They often develop fibrous thickenings of cellulose on inner and radial walls before becoming dead. Endothecium is, therefore, also called fibrous layer.
c. The epidermal as well as hypodermal cells lying between two microsporangia of an anther lobe remain thin-walled and form stomium (line of dehiscence). Middle layers usually degenerate and provide nourishment to sporogenous cells.
d. Tapetum is the major nourishing layer. Its cells become large, multinucleate and polyploid through endomitosis and endopolyploidy.
In amoeboid or invasive type, the tapetal cells fuse to form a plasmodium that masses in between the spore mother cells, e.g., Lily, Alisma, Typha, Tradescantia.
In secretory or glandular type, the tapetal cells remain in situ or parietal. They secrete nourishment that passes into sporogenous cells, e.g., Symphoricarpus. Ultimately both types degenerate.
e. Besides nourishment nutritional functions like producing enzymes, IAA and food materials), the tapetum provides Ubisch granules (after the scientist who discovered the role of tapetum) for forming exine of pollen grains, pollenkitt on entomophilous pollen grains and proteins for compatibility.
f. Ubisch granules contain sporopollenin and other materials required for formation of a part of pollen grain exine.

6. Sporogenous cells divide to form pollen grain or microspore mother cells (P.M.C.). They are diploid and connected by plasmodesmata. The latter are broken by production of callose layer inner to cell wall. The mother cells then undergo meiosis and form tetrads of microspores or pollen grains. The arrangement in the tetrads can be tetrahedral (more common in dicots), isobilateral (more common in monocots), linear, T-shaped and decussate. The wall of the mother degenerates and the pollen grains in microspores separate.
However, in some cases they do not separate but remain united in tetrads called compound pollen grains, e.g., Typha, Cryptostegia. In orchids and milkweeds (e.g., Calotropis, Asclepias) all the pollen grains of an anther lobe remain united in a sac called pollinium. In milkweeds two adjacent pollinia are connected to a sticky centre or corpusculum by means of small stalks called caudicles. The whole structure is termed as translator.

a. In the mature anther the sterile tissue between the two pollen sacs of an anther lobe disintegrates. Except endothecium, other wall layers also degenerate. A single cavity covered by endothecium and epidermis and containing pollen grains is present in each anther lobe.
b. On drying, the endothecial cells contract and rupture the thin-walled cells in the area of stomium to expose the pollen grains. It is longitudinal dehiscence. In other types of dehiscence, the endothecium does not develope fibrous thickening but undergoes local degeneration.
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7. Pollen Grain (Microspore)

a. It is generally rounded in outline with a wall and 2-3 celled interior. Wall or sporoderm is made of two coverings, outer thick exine of sporopollenin and inner thin intine of pecto-cellulose.
Sporopollenin is highly resistant fatty substance related to cutin.
b. Exine is differentiated into inner endexine and outer ektexine. Ektexine is formed of three layers - inner continuous foot layer, middle discontinuous baculate layer and outer discontinuous tectum. Tectum and baculate layer produce designs over the surface of pollen grain. Exine is absent in certain areas called germ pores (if rounded) or germinal furrows (if elongated). c. Depending upon their number, the pollen grains are monocolpate, bicolpate, tricolpate, etc. In mono cots the pollen grains generally possess one germinal furrow (monocolpate) while in dicots the pollen grains have generally three germ pores (tricolpate). d. The sculpturing, number and position of germ pores are characteristic of each species.

The branch dealing with the study of pollen grain characteristics is known as palynology.
Insect disseminated pollen grains have a covering of yellow sticky substance called pollenkitt.
e. Internally the pollen grain has
(i) tube or vegetative cell with degenerating nucleus, vacuolate cytoplasm rich in starch and unsaturated oils and
(ii) one generative cell or two male gametes derived from it.

8. Male Gametophyte

It is formed from pollen grain (pollen grain is an immature male gametophyte). Development of male gametophyte is precocious. It begins in the microsporangium soon after tetrad formation. The protoplast divides unequally into a large tube or vegetative cell and a small generative cell. The separating callose wall dissolves and the naked generative cell comes into the cytoplasm of tube cell. The generative cell may divide into two male gametes.
a. Pollination can occur in 2-celled (tube + generative) or 3- celled (tube + two male gametes) stage. Further growth occurs on stigma. Tube cell absorbs water and nourishment, swells up and comes out of one of the germ pores or germinal furrows as pollen tube (discovered by Amici). b. Pollen tube is covered by intine. Generative cell divides into two male gametes if it has not divided already. Tube cell nucleus and two male gametes descend to the tip of pollen tube. Tube cell nucleus starts degenerating. Male gametes are lenticular or spherical, with a large nucleus and thin layer of cytoplasm. Only the tip of pollen tube has dense cytoplasm. The remaining part is vacuolate and separated by callose plugs. c. Usually there is a single pollen tube and the pollen grains are called monosiphonous. In members of family malvaceae (e.g., Malva, Althaea) and cucurbitaceae, the condition is polysiphonous with as many as 14 pollen tubes.
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9. Ovule

A carpel or pistil has a stigma or receptive region for pollen grains, a stalk or style and basal swollen region or ovary. Ovary contains one to several ovules.
a. Ovule is integumented indehiscent megasporangium of phanerogams which on fertilization ripens into a seed. It is oval and whitish. The ovule is attached to placenta by means of a stalk called funiculus or funicle. The point of attachment of funiculus to the ovule is known as hilum.
b. A raphe (ridge) is formed by the fusion of funiculus with the body of ovule. The actual megasporangium equivalent is a parenchymatous tissue called nucellus. It may be thin (tenuinucellate, e.g., compositae) or massive (crassinucellate, e.g., casuarinaceae).

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c. Nucellus is covered by one (unitegmic, higher dicots e.g., compositae, also gymnosperms) or
two integuments (bitegmic, mono cots and primitive dicots like cruciferae and malvaceae).
Tritegmic condition (e.g.,Asphodelus) is rare.
Ategmic ovule (without integument) occurs in Santalum, Loranthus, Liriosoma and Olax.
d. Place of origin of integuments is called chalaza.
A pore is present in the integuments at one end. It is known as micropyle.
The inner region of integument may provide nourishment to developing embryo sac when it is called endothelium. Outer side of each integument as well as nucellus possesses cuticle.
e. Ovule develops as a meristematic primordium over the placenta.
It first gives rise to nucellus. Integuments develop from its base. They grow and surround the nucellus on all sides except the micropyle. In the hypodermal region of nucellus towards the micropyler end differentiates as an archesporial cell.
f. The archesporial cell divides once to form an outer primary wall or parietal cell and inner sporogenous cell. Sporogenous cell commonly functions directly as megaspore mother cell (M.M.C). Megaspore mother cell undergoes meiosis to form a tetrad of haploid megaspores. The arrangement is commonly linear.
Generally, the chalazal megaspore (micropylar in Balanophora and anyone in Casuarina) is functional while the others degenerate. The functional megaspore produces the female gametophyte called embryo sac. The common type of embryo sac is monosporic polygonum type.

For its formation, the megaspore nucleus undergoes three divisions to produce 8 nuclei. They get organised into three groups.
Cytoplasm becomes vacuolate. The embryo sac is oval, 7-celled and 8-nucleate structure which is covered by a thin membrane formed of megaspore wall../ There are three micropylar, three chalazal and one central cell. The three micropylar cells are collectively called egg apparatus. The cells are pyriform, connected by plasmodesmata and arranged in the form of a triangle with an oosphere (female gamete) and two synergids (help cells = cooperative cells).
The synergids have lateral hooks and terminal filiform apparatus for guiding pollen tubes. The three chalazal cells are known as antipodal cells. They may take part in nourishing the embryo. For this, the antipodal cells often develop haustoria.
The central cell is the largest and bounded by membrane of embryo sac. It has a vacuolated cytoplasm and two polar nuclei. The two polar nuclei often fuse in the centre to form a single diploid secondary or fusion nucleus (= definitive nucleus).
Secondary nucleus is the only diploid structure in the embryo sac.

Forms of Ovules

1. Orthotropus (Atropous, Erect). The body of ovule lies straight and upright over the funicle. Hilum, chalaza and micropyle occur on the same line with hilum and chalaza nearby, e.g., piperaccae,urticaceae, polygonaceae.
2. Anatropous (Resupinate). The body of ovule is inverted and gets fused with funiculus along its whole length on one side. The fused funiculus forms ridge called raphe. Hilum and micropyle are nearby with chalaza on the opposite side. It is the most common, in about 82% of angiosperms.
3. Hemitropous (Hemi-anatropous). It is intermediate between ortho- and anatropous types of ovules with the body of ovule lying at right angles to funiculus, fusing with the latter for less than half length. Hilum, chalaza and micropyle are separate with chalaza and micropyle being on the same straight line, e.g., members of Ranunculaceae, Primulaceae and some Cruciferae.
4. Campylotropous. The body is curved but the embryo sac is straight. Hilum, chalaza and micropyle come nearby, e.g., Capsella, Capparis, some legumes, Chenopodiaceae, caryophylaceae.
5. Amphitropous. Both body of ovule and embryo sac are curved, e.g., some crucifers.
6. Circinotropous. The funicle is large and coiled around the ovule, e.g., Opuntia.
Pollination
Pollen grains are immobile. They require an external agent or force for reaching the stigma. Pollination is the transfer of pollen grains from anthers (microsporophylls) to stigmas (receptive region of megasporophylls). It is of indirect type as compared to direct pollination in gymnosperms. It of two types –
self pollination and cross pollination,
Self Pollination
Self pollination is the transfer of pollen grains from anthers to the stigma of same or genetically similar flower. It is possible only when anthers and stigmas mature simultaneously. The phenomenon is called homogamy. Self pollination is of two types-autogamy and geitonogamy.
1. Autogamy. It is self pollination which occurs between anther and stigma of the same flower. Autogamy takes place in three circumstances:
(a) Chasmogamous Devices. The flowers are open. In Convolvulus, Catharanthus ( = Vinca) and Gardenia, the anthers occur near the mouth of corolla-tube. Stigma comes in contact with them due to growth of style. In Lilac the stigma lies exactly below the anthers. As the anthers dehisce, their pollen grains fall over the stigma. In some plants self pollination is accomplished when cross pollination fails (fail-safe device). In Mirabilis or Four O'Clock the anthers bend over the stigma while in Potato the stigma bends over the anthers. In Sunflower, the stigma can curl back to receive pollen present on the brushing hair.
(b) Cleistogamy. The flowers remain closed so that only self pollination occurs. In Commelina bengalensis and Peanut (= Groundnut Arachis hypogaea), cleistogamy is accompanied by geocarpy or formation of fruits inside the soil. In others, cleistogamous flowers remain above ground, e.g., Balsam, Viola, Oxalis. In most of these cases (exception groundnut) the flowers are chasmogamous in the beginning of flowering season but become cleistogamous later in order to ensure fruit formation.
(c) Bud Pollination. In several cultivated plants, self pollination occurs in the bud condition before the flowers open, e.g., Wheat, Tobacco, Tomato, Linseed, Jute, Rice, etc.

2. Geitonogamy. It is the transfer of pollen grains from anthers of one flower to stigma of another flower of either the same (endogamy) or genetically similar plant. Geitonogamy resembles cross pollination in the requirement of pollen transfer or pollinating agency. Importance. Self pollination maintains purity of race and superiority of variety once developed. It, however, ultimately leads to degeneration.

Cross Pollination
It is the transfer of pollen grains from the anthers of one flower to the stigma of a genetically dissimilar flower. Cross pollination is also called xenogamy. Both xenogamy and geitonogamy are included under allogamy though this term is more commonly used for cross pollination. Cross pollination requires external agents. They are of two types, abiotic (water and air) and biotic (insects, birds, bats, snails, ants and other animals).
1. Anemophily 2. Hydrophily 3. Entomophily 4. Ornithophily 5.Chiropterophily or Cheiropterophily 6. Malacophily.
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1. Anemophily (Wind Pollination). In anemophily, air currents pick up pollen from dehiscing anthers. The receptive stigmas pick up the pollen grains from air currents.
(i) A very large number of pollen grains are produced, e.g., 500,000 per flower in Cannabis, 25 million by a tassel of Maize, and 135 million by a plant of Mercurialis. Hay fever is allergic reaction to the presence of pollen in the air. Plants commonly causing hay fever are Amararnthus, Chenopodium, Castor, Sorghum and Prosopis.
(ii) Pollens are very light. They may have air sacs or wings. Winged pollen grains of Pinus are found hundreds of kilometers away from the plants.
(iii) Pollen grains are dry and unwettable.
(iv) Male flowers of anthers are more abundant. The stamens have long filaments and versatile anthers. (v) Stigmas are exserted, sticky, hairy or feathery. (vi) Flowers are small, without nectar, scent or colour. (vii) Flowers may occur in hanging spikes or catkins. Example. Date Palm, Coconut Palm, Poplar, Mulberry, Willow, Grass, Maize, Jowar, Chenopodium, Amaranthus, Cannabis.

2. Hydrophily. It is pollination brought about through the agency of water. Water pollination is of two types - epihydrophily (on surface of water, e.g., Lemna, Vallisneria) and hypohydrophily (inside water, e.g., Zostera, Ceratophyllum). Vallisneria (Tape Grass = Eel Grass) is dioecious. Male plant produces a large number of male flowers, which after breaking, rise upwards in closed state and open on the surface of water. The female plant produces female flowers singly at the tips of long pedicels that bring the flowers on the surface of water. Stigma is trifid. Male flowers are drawn in the depression round a female flower to effect pollination. After pollination the female flower is brought down into water.

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3. Entomophily. It is pollination brought about through the agency of insects. Entomophily is the most common and specialised type of pollination. The important traits of entomophilous flowers are
(i) Flowers are coloured. One sepal is enlarged in Mussaenda, bracts are coloured in Bougainvillea while involucre (bracts) is conspicuous in Euphorbia and Poinsettia. Commonly, petals are coloured. Bluish-purplish-violet-yellow flowers attract bees while reddish flowers attract butterflies and wasps. Night blossoming flowers are generally whitish.
(ii) Flowers commonly possess an aroma or scent.
(iii) Visiting insects are fed by either nectar (e.g., Jasmine, Buttercup, Larkspur, Adhatoda) or edible pollen (e.g., Magnolia, Papaver).
(iv) Pollen grains have a sticky surface due to pollenkitt.
(v) Stigmas are sticky.
(vi) Flowers are strong enough to bear the weight of visiting insects. They may also provide insects with shelter.
(a) Blastophaga is completely dependent upon Ficus cairica (Fig) for survival. The Fig is dependent upon this wasp for pollination.
(b) Yucca is pollinated by Pronuba (= Tegeticula) yuccasella which passes its larval stage inside the ripening ova).
The yuccas comprise the genus Yucca of 40-50 species of perennials, shrubs, and trees in the agave family Agavaceae, notable for their rosettes of evergreen, tough, sword-shaped leaves and large terminal clusters of white or whitish flowers. They are native to the hot and dry (arid) parts of North America, Central America, South America, and the West Indies.
Yuccas have a very specialized pollination system, being pollinated by the yucca moth; the insect purposefully transfers the pollen from the stamens of one plant to the stigma of another, and at the same time lays an egg in the flower; the moth larva then feeds on some of the developing seeds, but far from all.
Yuccas are widely grown as ornamental plants in gardens. Many yuccas also bear edible parts, including fruits, seeds, flowers, flowering stems, and more rarely roots, but use of these is sufficiently limited that references to yucca as food more often than not stem from confusion with the similarly spelled but botanically unrelated yuca.
Dried yucca has the lowest ignition temperature of any wood, making it desirable for fire-starting.
The "yucca flower" is the state flower of New Mexico.
(c) The flowers 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).

4. Ornithophily. It is allogamy performed by birds. Two types of long-beaked small birds take part in pollination-sun birds and humming birds (hover over the flower). Other birds performing pollination are Crow, Bulbul, Parrot, Meynah, etc. Ornithophilous flowers are large and strong with abundant nectar or edible part, e.g., Erythrina, Bombax, Agave, Bigrtonia, Grevillea, Butea, Callistemon.

5. Chiropterophily or Cheiropterophily. It is allogamous pollination performed by bats, e.g., Anthocephalus (Kadam), Bauhinia megalandra, Kigelia pinnata (Sausage Tree), Adansonia (Boabab Tree). The flowers produce strong aroma, abundant nectar and a large number of stamens (e.g.,1500 -2000 stamens per flower) in Adansonia.

6. Malacophily. Malacophily is cross pollination brought about by the agency of snails, e.g., Arisaema and some other aroids.

Devices for Cross Pollination
1. Dicliny (= Unisexuality). There are two types of flowers, male and female. The plants may be monoecious or dioecious.
2. Dichogamy. Anthers and stigmas mature at different times.
(i) Protandry. Anthers mature earlier, e.g., Salvia , Clerodendron, Sunflower, Rose.
(ii) Protogyny. Stigmas mature earlier than anthers, e.g., Plantago, Gloriosa, Magnolia, Mirabilis.

3. Self Sterility (= Self Incompatibility). Pollen grains are incapable of growing over the stigma of the same flower, e.g., Tobacco, some crucifers. Quicker growth of pollen of another plant than pollen of the same plant is called prepotency (e.g., Apple).
4. Heterostyly. Flowers have two or three heights of styles and stamens. Primula and Jasmine Jasminum have two types of flowers (dimorphic heterostyly or diheterostyly)-pin eyed (long style and short stamens) and thrum-eyed (long stamens and short style). Some plants have trimorphic heterostyly (triheterostyly), e.g., Lythrum, Oxalis species. Pollination occurs between anthers and stigmas of the same height.

5. Herkogamy.
They are mechanical devices to prevent self pollination and favour cross pollination even in homogamous bisexual flowers. In Gloriosa and Clerodendron, the mature stigmas and anthers occur in different positions. In Kalmia the stigma is exposed but anthers occur inside corolla pockets.
In Salvia there is lever mechanism or turn-pipe mechanism to shed pollen grains at the back of visiting insect. In Calotropis and Orchids pollen grains occur in sacs called pollinia.
Two adjacent pollinia are attached to a common sticky corpusculum to form a translator. Translator can be lifted by insects only. Aristolochia has brightly coloured but foul smelling pit fall protogynous flowers where f1ies once trapped come out only when deflexed hair present in the corolla tube whither and the anthers mature for providing pollen grains to them.

Pollination in Salvia. Salvia has bilipped corol1a where lower lip functions as a platform for visiting insects. Upper lip encloses essential organs. Stamens bear long connective with fertile anther lobe at one end and flat sterile anther lobes at the other end. It forms a lever or turn-pipe mechanism. Flowers possess nectar at the base of ovary. When a bee visits young flower, the plate like sterile anther lobe is pushed inwardly while the fertile anther lobe strikes the back of insect to throw pollen there. In old flower the stamens wither while the style elongates and bends the mature stigma to pick pollen grains from the back of the insect.

Importance. Cross pollination is useful in increasing yield and adaptability. It eliminates defective traits and is helpful in production of new varieties. Cross pollination may bring in hybrid vigour. It is, however, wasteful and may dilute very good characters of the race.

Fertilization
It is the fusion of male and female gametes or compatible gametes (= syngamy). In seed plants fertilization is called siphonogamy (discovered by Strasburger, 1884) because it occurs with the help of pollen tube. The pollen tube (tip with two male gametes, some cytoplasm and degenerating tube nucleus) passes through the style along its canal and transmitting or conducting tissue. It obtains its nourishment along the pathway. In the ovary the pollen tube is guided and nourished by another tissue called obturator.
Pollen tube can enter the ovule by three methods:
(a) Porogamy (most common). Through micropyle e.g., Lily
(b) Chalazogamy. Through chalaza, e.g., Juglans (Walnut), Casuarina.
(c)Mesogamy. Pollen tube penetrates laterally through integuments, e.g, Cucurbita, Populus.
After enteting ovule, pollen tube is attracted by synergids through filiform apparatus. It strikes one of the synergids and bursts open to release the two male gametes.
Generative Fertilization. It is the fusion of one male gamete with the oosphere (egg) to produce diploid zygote or oospore. The latter grows to form the embryo.
Vegetative Fertilization. It is the fusion of nucleus of a male gamete with the diploid secondary (= fusion) nucleus. Vegetative fertilization is also called triple fusion because it involves fusion of one male nucleus and two polar nuclei. Triple fusion converts central cell into triploid primary endosperm cell (the triploid nucleus is called primary endosperm nucleus). It forms the endosperm.
Double Fertilization. It is the fusion of two male gametes with two different structures in the same female gametophyte to produce two different structures for future development of plant. Double fertilization was discovered by Nawaschin (1898). It is restricted to angiosperms but is absent in some primitive families, Double fertilization is important as it provides for formation of nutritive tissue only when embryo formation is ensured.
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Endosperm
Endosperm is a nutritive tissue formed from vegetative fertilization in angiosperms. (It is female gametophyte in gymnosperms). Endosperm is meant for nourishing the embryo. It is generally triploid. Since endosperm develops fully in the fertilized ovule, it may show the effect of genes present in the male gamete. The phenomenon is called xenia. Endosperm formation is accompanied by degeneration of nucellus. Some food is also got from the plant as well as endothelium. Depending upon its mode of development, endosperm is of three types:

1. Nuclear Endosperm. 2. Cellular Endosperm 3. Helobial Endosperm.
1. Nuclear Endosperm. It is the most common type of endosperm. Primary endosperm nucleus divides and redivides to form a large number of free nuclei. A central vacuole appears and a massive peripheral multinucleate cytoplasm is formed. Wall formation occurs and a multicellular endosperm is formed. The central vacuole disappears, e.g., Capsella bursa- pastoris, Maize, Wheat, Rice, Sunflower. In Coconut there is an outer multicellular solid endosperm and inner free nuclear liquid endosperm in the centre.
2. Cellular Endosperm. Wall formation occurs after every division of primary endosperm nucleus, so that endosperm is cellular from the beginning, e.g., Datura, Petunia, Balsam.
3. Helobial Endosperm. First division produces two cells within each of which free nuclear divisions occur but ultimately they may also become cellular, e.g., Asphodelus.
During its growth endosperm crushes the nucellus. It is inturn eaten by growing embryo. Endosperm may persist in the seed as food laden tissue. Such a seed is called endospermic or albuminous seed, e.g., Castor, Coconut, Cereals. In others the embryo consumes the endosperm. The food is then stored in cotyledons. Such seeds are called non-endospermic or exalbuminous, e.g., Pea, Bean, Sunflower. Ruminate or convoluted endosperm occurs in Areca (Betel Nut) and Passiflora. Hard endosperm is found in Date and Areca.
Progressed Embryogeny (Embryo Formation) It is the development of mature embryo from zygote or oospore. Early development produces a pro-embryo which has an axial symmetry. Embryo passes through a globular stage in both mono cots and dicots. Differentiation occurs later. Development of embryo is endoscopic or on inner side because of the presence of suspensor.
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Dicot Embryogeny (Crucifer /Onagrad Type). Zygote or oospore divides into two unequal cells, larger suspensor cell towards micropyle and a smaller embryo cell (= terminal cell) towards antipodal region. The suspensor undergoes transverse divisions forming 6-10 celled suspensor. The first cell of the suspensor (towards micropyle) is large and called haustorium. The last cell of suspensor (towards embryo cell) is known as hypophysis. It forms radicle. Embryo cell divides twice vertically and once transversely to produce a two-tiered eight-celled embryo. The epibasal tier forms two cotyledons and a plumule while the hypobasal tier produces only hypocotyl. For this the octant embryo undergoes periclinal divisions producing protoderm, procambium and ground meristem. It is initially globular but with the growth of cotyledons it becomes heart-shaped and then assumes the typical shape. In Capsella bursa pastoris, the elongating cotyledons curve due to curving of the ovule itself. In/orchids, Orobanche and Utricularia, the embryo does not show distinction of plumule, cotyledons and radicle.
Monocot Embryogeny (Sagittaria Type). The zygote or oospore divides transversely producing a vesicular suspensor cell towards micropylar end and embryo cell towards the chalazal end. The embryo cell divides transversely again into a terminal and a middle cell. The terminal cell divides vertically and transversely into globular embryo. It forms a massive cotyledon and a plumule. Growth of cotyledon pushes the plumule to one side. Remains of second cotyledon occur in some grasses. It is called epiblast. The single cotyledon of mono cots is called scutellum. It is shield shaped and appears terminal. The middle cell gives rise to hypocotyl and radicle. It may add a few cells to the suspensor. Both radicle and plumule develop covering sheaths called coleorhiza and coleoptile respectively. They appear to be extensions of scutellum.

Seed and Fruit Formation
Zygote grows to form embryo while primary endosperm cell produces endosperm for its development. Endosperm corrodes the nucellus while embryo eats part or whole of endosperm. Towards maturity of embryo, growth inhibitors are formed. They stop further growth of embryo. The latter becomes dormant. Integuments are transformed into seed coats, outer testa and inner tegmen.
Along with development of seeds, the ovary wall is stimulated to grow and form fruit wall or pericarp. Thalamus and other floral parts may also proliferate along with ovary wall to form pseudocarp, false or accessory fruit.
(i) Stimulus from Pollination. Pollination provides ovary with a stimulus to produce fruit. It is possibly due to growth hormones contained in germinating pollen that also stimulate synthesis of auxin in the pistil. Many seedless fruits require the stimulus of pollination due to this effect. Normally pollination is required for fertilization and seed development.
(ii) Stimulus from developing Seeds. Cutting a deformed apple shows that deformity occurs where seed development is deficient. Nitsch (1952) devised experiment to study the effect of seed formation on development of strawberry fruits. The scientist found that an interval of nine days is required after pollination for development of fruit. Selective removal of carpels after this period, resulted in development of fruit only around the remaining carpels. The same effect is achieved by selective pollination of carpels showing that seed formation is essential for fruit development. However, the effect of both pollination and seed formation could be replaced by growth hormones like auxin.

Parthenocarpy (Noll, 1902)
It is the development of fruit from an unfertilised flower so that fruit does not contain seeds. Seedless fruits are naturally formed in Bananas and Pineapples. Certain varieties of cultivated plants also develop seedless fruits, e.g., Grape, Apple, Pea, Tomato. Pathenocarpy is of great importance to food processing industry as well as human taste but it has no biological importance to the plant.

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Sexual Incompatibility It is the inability of certain otherwise viable gametes from the genetically specific individuals to fuse with each other and produce fertile offspring. Sexual incompatibility may be interspecific or intraspecific. Interspecific incompatibilty is important as it prevents free cross pollination. It maintains genetic individuality of the species. Intraspecific incompatibility, self sterility or self incompatibility is the inability of a plant producing functional male and female gametes to produce offspring when self pollinated. It is the mechanism to prevent inbreeding. Self sterility promotes outbreeding. Self incompatibility has been reported in 66 families of angiosperms. It can be morphological or physiological.
1. Morphological Self Incompatibility. There are two or three mating types, viz., distyly (e.g., Primula), tristyly (e.g., Lyhrum).
2. Physiological Self Incompatibility. Incompatibility is due to physiology. Morphological differences are absent. (i) Gametophytic Self-incompatibility (GSI). Incompatibility resides in genotype of pollen, e.g., Liliaceae, Solanaceae, Poaceae.
(ii) Sporophytic Self-incompatibility (SSI). Incompatibility resides in genotype of sporophytic/stigmatic tissue, e.g., Asteraceae, Brassicaceae. It prevents pollen germination, retards pollen growth, deorientate pollen tube or does not allow nuclear fusion. The incompatibility is due to a single multiallelic gene. Pollen grain has a single allele (e.g., St). If it is similar to one of the two alleles of female parent (e.g., St, Sz), incompatibility will occur.

MCQs12100
1. Syngamy means-