Class 12 Sexual Reproduction In Flowering Plants Notes: CBSE Biology Chapter 1

Sexual Reproduction in Flowering Plants

• Sexual reproduction is the process by which new organisms are formed from the fusion of male and female gametes from two parents.

• The flower is the primary reproductive structure. Within the flowers, the reproductive organs, or sporophylls, are produced.

• Sporophylls are classified into two types: microsporophylls (stamen) and megasporophylls (carpel). 

• A carpel is an ovary that contains an ovule, a style, and a stigma.

• There are three types of stamen: filament, anther, and connective.

• Stamen is distinguished as filament, anther and connective.

• Sexual reproduction in flowering plants can be broken down into three steps:

i) Pre-fertilization 

ii) Double fertilization 

iii) Post-fertilization 

Pre-Fertilization: Structure and Events

The following pre-fertilization events can be studied: 

i) Pollen grain formation 

ii) Embryo sac formation 

iii) Pollination

iv) Pollen pistil interaction 

Pollen Grain Formation 

Male reproductive unit (Stamen) 

• A stamen is an angiosperm's male reproductive unit. It is composed of an anther and a filament. The anther is bilobed, with each lobe containing four pollen sacs or microsporangia. 

• A number of pollen grains are contained in each pollen sac. A dithecous anther's four pollen sacs are located in the four corners.

• Dithecous anther: An anther with two lobes connected by a non-sporangious tissue called the connective. 

• The anther wall is composed of four layers of cells.

• To release pollen grains, anther dehiscence through slits.

Anther Development 

• The development of an anther begins with a mass of homogeneous meristematic cells surrounded by an epidermis.

• Four lobes are formed, as are four layers of archesporial cells.

• Archesporial cells: A primitive cell or group of primitive cells that divide to form two types of cells: a primary parietal cell and a primary sporogenous cell.

• The parietal cell divides several times to form the anther wall, whereas the sporogenous cell divides less frequently to form microspores or the pollen mother cells (PMC).

• The tapetum is the innermost layer of the cell wall that comes into contact with the PMCs. In pollen formation, the tapetum plays a crucial role.

• Tapetum: This is a tissue found within the anther that feeds the growing spores.

• The endothecium is the layer beneath the epidermis.

Wall Layers of Anther

• The epidermis is a single layer of cells that serves as a protective layer.

• Endothecium is a single-layered second wall. Cells are thickened with cellulose and a trace of pectin and lignin. It aids in the dehiscence of anthers.

• Middle layers – Ranges from 1-6. When the anther matures, the middle layer degenerates.

• Tapetum – 

a) The anther wall's innermost layer surrounding the sporogenous tissue.

b) Tapetal cells contain nutrients.

c) They have multiple nuclei and are polyploid.

d) The ubisch bodies settle in the exine of the microspore wall.

e) There are two kinds of tapetum:

(i) Secretary / glandular – The tapetal cells remain in place throughout the development of the microspore, eventually degenerating.

(ii) Amoeboid / periplasmodial – The tapetal cells rupture the radial wall, allowing the protoplast to enter the pollen chamber. These protoplasts are now joining together to form the periplasmodium. 

Microsporogenesis 

• Microsporogenesis is the formation and differentiation of microspores.

• PMCs go through meiosis. Tetrahedral tetrads are formed by each.

• Cytokinesis can be either sequential or simultaneous.

• Tetrads are classified into five types: tetrahedral, isobilateral, decussate, T-shaped, and linear. The most common shape is tetrahedral.

• The cell wall is formed after meiosis –I and meiosis –II in successive types, resulting in an isobilateral pollen tetrad. Monocots have it as a distinctive trait.

• In the simultaneous type, each nuclear division in a microspore mother cell is followed by the formation of a cell wall.

The tetrahedral arrangement is separated from the microspores. After that, they are surrounded by a two-layered wall. The outer wall is known as the exine, and the inner wall is known as the intine.

• The pollen grains are the male gametophyte's first cells. 

• The tapetum is depleted, and the anther becomes a dry structure. The pollen is liberated by the anther dehiscence.

• A tetrad's four nuclei remain functional to form four microspores.

• In some cases, all four pollen remain attached, forming compound pollen grains, as in Juncus jatropha. 

• Microspores are present as pollinium in the Asclepiadaceae and Orchidaceae families.

• Pollinium: A mass of pollen grains found on each anther lobe. The entire mass of pollen grains is transferred as a unit when the pollinium is attached to pollinating agents such as insects.

Pollen Grain 

• Pollen grains come in all forms and sizes.

• It is generally round and has a diameter of 25 – 30m.

• The pollen grain has a haploid, unicellular body with a single nucleus. It has a two-layered exterior wall.

• The wall, or sporoderm, is made up of two layers.

• The outer layer is quite thick. It is known as the exine. It is composed of sporopollenin. • The inner wall is thin and is referred to as the intine. It is composed of pecto-cellulose.

• Exine can be thick and sculptured or smooth. It is cuticularised, and the cutin is sporopollenin, which is resistant to chemical and biological decomposition. This keeps the pollen wall intact for a long time. It also contains proteins that are involved in enzymatic and compatibility reactions.

• Exine is classified as inner endexine and outer ektexine. The ektexine is further subdivided into three layers: the inner continuous foot layer, the middle discontinuous baculate layer, and the outermost discontinuous tectum.

• Tectum aids in pollen grain identification and classification by family, genus, or species.

• Pollen grain has pores or furrows in it. The exine is not present in these areas. Germ pores are formed when the areas are circular. Germ furrows are formed when the areas are elongated.

• Intine is a thin and pliable material. It is composed of cellulose and pectin. The intine extends out to form the pollen tube during pollen germination. 

• The pollen grains’ cytoplasm is high in starch and unsaturated oils. They begin to uninucleate and eventually become 2-3 celled.

• In Calotropis and orchids, the pollen of each anther lobe formed a characteristic mass known as pollinium. 

• Pollen grains are classified as monoclopate (with one germ pore), biclopate (with two germ pores), or triclopate (with three germ pores).

• The study of pollen is termed palynology.

Development of Male Gametophyte 

• Inside the pollen grain, the nucleus grows in size. It divides mitotically to give rise to two unequal daughter cells: a larger vegetative cell or tube cell and a smaller generative cell.

• Pollination can take place when the pollen grain is two-celled (tube + generative) or three celled (tube + two male gametes).

• In plants, however, such as cereals, male gametes form while the pollen is still within the anther. When pollen is shed at the two-celled stage, the generative cell divides after the pollen has landed on the stigma.

• The generative cell's cytoplasm contains little conserved food material, while the vegetative cell's cytoplasm includes fat, carbohydrate, and protein granules.

Pollen Products 

1. Pollen supplements: Pollen grain is high in carbohydrates and unsaturated fat. They are taken in the form of tablets and syrups and are used to improve vital body functions. Pollen consumption boosts performance and is used by athletes as well as racehorses.

2. Pollen creams: Pollen grains provide UV protection. As a result, they are used in creams and emulsions to provide skin smoothness and protection.

Pollen Viability 

• Pollen viability refers to the amount of time that pollen grains remain viable or functional. 

• It is estimated by temperature and humidity.

• For 30 minutes, pollen grains are viable.

Pollen allergy

• Pollen grains cause severe allergies. It causes fever as well as common respiratory disorders such as asthma and bronchitis.

• Carrot grass (Parthenium hysterophorus) is a major allergen source. It also harms the internal organs of the body. It arrived in India alongside imported wheat. 

Female Reproductive Unit (Pistil)

• The female reproductive unit of a flower is the pistil or gynoecium.

• A carpel or pistil is made up of three parts: the stigma, the style, and the ovary.

• Stigma: The part of the body that receives pollen grains.

• The stalk that connects the stigma to the ovary is known as the style.

• Ovary: A swollen region at the base of the ovary. One to several ovules is found in the ovary.

• The ovule is a megasporangium surrounded by integuments. The ovule matures into a seed after fertilization. It is oval and whitish in colour.

• Funiculus: The stalk that joins the ovule and the placenta together.

• Hilum: The point at which the funicle attaches to the ovule.

• The fusion of the funiculus with the body of the ovule results in the formation of a raphe (ridge). 

• The nucellus is a parenchymatous tissue that is equivalent to the megasporangium. The nucellus can be thin or thick.

• Chalaza refers to the origin of integuments.

• At the opposite end of the integuments from the chalazal end, there is a pore. Micropyle is the term for it.

• The developing embryo sac may be nourished by the inner region of the integument. It is known as endothelium.

• Cuticle covers the outer region of each integument and the nucellus.

• The integumentary cells of the castor bean (Ricinus) proliferate at the microplylar region. This results in a structure known as the caruncle. It serves two purposes

i. It absorbs water and promotes seed germination.

ii. Because it is made of a sugary substance, ants disperse the seeds.

Megasporogenesis

• Megasporogenesis refers to the process of producing megaspores from megaspore mother cells.

• Ovules commonly form a single megaspore mother cell (MMC) in the nucellar micropylar region. It's a big cell with a lot of cytoplasms and a big nucleus.

• The MMC goes through meiosis to produce four megaspores.

• In most flowering plants, only one of the megaspores is active. The other three have devolved.

• Only the functional megaspore can mature into a female gametophyte.

• Monosporic development refers to the formation of an embryo sac from a single megaspore.

Formation of Embryo SAC 

• Mitosis occurs in the nucleus of the functional megaspore, resulting in the formation of two nuclei. They shift to opposing poles. As a result, a two-nucleate embryo sac is formed.

• There are two more sequential mitotic nuclear divisions. This results in the formation of four nucleate and then eight nucleate embryo sac stages. 

• Cell wall development does not occur immediately after nuclear division.

• Cell walls are formed after the eighth nucleate stage. This results in the formation of a typical female gametophyte or embryo sac. Six of the eight nuclei are encased in cell walls and organized into cells. The remaining two nuclei are known as polar nuclei. 

• They are found in the large central cell, just beneath the egg apparatus. • Three cells are found together at the micropylar end. They make up the egg apparatus.

• The egg apparatus comprises two synergids and one egg cell. Filiform apparatus are special cellular thickenings at the micropylar tip found in synergids. They are crucial in directing pollen tubes into the synergid. Three cells are placed at the chalazal end. They are known as antipodals. At maturity, a typical angiosperm embryo sac has seven cells and eight nuclei.

Pollination 

• Pollination is the transfer of pollen grains from the anther to the flower's stigma.

• Pollination can be categorised into two different types, namely: self-pollination and cross-pollination.

• Self-pollination refers to the transfer of pollen grains from anthers to stigmas of the same or different flowers on the same plant. Flowers in self-pollination are genetically similar.

Self-pollination is of two kinds- autogamy and geitonogamy.

1. Autogamy: The movement of pollen grains from the anther to the stigma of the same flower. It is preferred because of the following adaptations:

a) Chasmogamous apparatuses 

• When the mature anther and stigma of the flower are exposed to pollinating agents. The stigma in Lilac is directly beneath the anthers.

b) Cleistogamy 

• Because the flowers remain closed, self-pollination is the only option. Pisum, Lathyrus, and Commelina benghalensis are a few examples.

• Bisexual flowers mature their anthers and stigma well before the bud opens. Thus, self-pollination occurs during the bud stage of plants such as peas and wheat.

2. Geitonogamy is the transfer of pollen grain from one flower's anther to the stigma of another flower of the same or genetically similar plant.

The benefits of self Pollination are shown below.

• It preserves the race's purity.

• There is no need for the plant to produce a large number of pollen grains.

• It ensures seed production.

• Self-pollination eliminates undesirable recessive traits.

Self-pollinated plants have a number of disadvantages as shown below.

• Variable, which reduces adaptability to changing environments.

• Vitality declines, eventually leading to degeneration.

Cross-Pollination

• It is defined as the transfer of pollen grains from an anther of one plant to the stigma of another plant of the same or different species. It is also referred to as allogamy.

• Pollination occurs in Xenogamy between two flowers of plants that are genetically and ecologically distinct.

Cross-Pollination Devices

1. Dicliny: Flowers are classified into two types: male and female. Plants could be either monoecious or dioecious.

2. Dichogamy: It occurs when the anther and stigma mature at different times.

(i) Protandry: Anthers mature at a faster rate. For example, Salvia, Clerodendron, Sunflower, and Rose.

(ii) Protogyny: Stigmas mature at a younger age. Plantago, Magnolia, and Mirabilis are a few examples.

3. Self-sterility: Tobacco and some crucifers, for example, have pollen grains that are incapable of growing over the stigma of the same flower. Prepotency refers to a pollen grain's ability to grow faster on the stigma of another plant than on the stigma of the same plant ( e.g. apple)

4. Heterostyly: The styles and stamens are at different heights within the flowers. Primula and Jasminum have two types of flowers (dimorphic heterostyly), pin-eye (long style and short stamen) and thrum-eye (long style and short stamen) (short style and long stamens). Some plants, such as Lathyrum and Oxalis, have trimorphic (3) heterostyly.

5. Herkogamy: Self-pollination is prevented by the existence of a natural or physical barrier between androecium and gynoecium.

Advantages of Cross-Pollination 

• Cross-pollination causes genetic recombination and, as a result, variation in offspring.

• Cross-pollination improves the ability of offspring to adapt to environmental changes.

• The race's defective character is eliminated and replaced by a better character. 

Disadvantages of Cross-Pollination 

• Plants must generate a large number of pollen grains.

• The very good character will most likely be spoiled.

• Because an external agency is involved, the chance factor is always present.

Significance of Pollination 

• Pollination is required for fertilization and, as a result, seed and fruit production. 

• It promotes ovarian growth.

• It leads to the creation of hybrid seeds.

• The seeds and fruits are also nutritious. 

Post Pollination Events 

• The pollen grain's nucleus divides to produce vegetative and generative cells, and a small protrusion known as a germ tube emerges from the pollen. The germ tube secretes enzymes that digest the stigma tissues. The germ tube then develops into a pollen tube. 

• The generative nucleus divides to produce two male nuclei. They are surrounded by cytoplasmic masses and appear as distinct male gametes. The pollen tube develops into the style tissues after passing through the stigma.

• The region of entry of the pollen tube into the ovule determines the type of entry.

They are as follows:

i) Porogamy: The entry of a pollen tube into an ovule via a micropyle, as in Ottelia.

ii) Chalazogamy: The entry of a pollen tube into an ovule via a chalaza, such as Casuarina.

iii) Mesogamy: Pollen tube entry into the ovule via the funicle or integuments, as in Cucurbita.

• The pollen tube usually enters the ovule via the micropyle. It then enters the synergids via the filiform apparatus. 

• Filiform apparatus directs pollen tube entry. 

Pollen – Pistil Interaction 

• Only compatible pollen from the same species can germinate. 

• Germination is linked to the action of proteins found on pollen grains and stigma that determine compatibility.

• By manipulating pollination, plant breeders can create hybrids between different species.

• Female parents with bisexual flowers use forceps to remove anthers from the flower bud before the anther dehisces. 

• The stigma of the emasculated flowers must be protected from contamination by unwanted pollen during this step, which is known as emasculation. As a result, they are covered with a suitable-sized bag to prevent the deposition of unwanted pollen. The bag is generally made up of butter paper. This process is called bagging. 

Double Fertilization 

• Fertilization is defined as the process by which male and female gametes fuse to form the zygote. 

• The zygote will eventually mature into an embryo. Two male gametes are released into the embryo sac by the pollen tube. The diploid zygote is formed when one of the male gametes fuses with the egg. This is known as syngamy or generative fertilization. The second male gamete joins the two polar nuclei. This leads to the formation of a triploid primary endosperm nucleus. This is known as triple fusion, and it is also referred to as vegetative fertilization.

• Two sexual fusions occur in an embryo sac, one in syngamy and the other in triple fusion. This is known as double fertilization.

Post Fertilization: Structure and Events 

Endosperms 

• Endosperm is a nutritive tissue that develops as a result of vegetative fertilization. The endosperm is intended to nourish the embryo. It is typically triploid.

• The effects of genes from the male gamete may be seen in the endosperm. The condition is known as xenia. This happens because the endosperm in a mature ovule is fully developed.

• The direct or indirect effect of pollen on embryo sac structure is limited to the endosperm and is not observed in the embryo. Focke (1881) described this effect. It is only found in Zea mays (maize). 

Metaxenia is the action of pollen on the seed coat or pericarp that is outside the embryo sac.

The endosperm is classified into three types based on how it develops.

1. Nuclear endosperm 

• Through repeated mitosis, the primary endosperm nucleus generates a large number of free nuclei.

• The large multinucleate cytoplasm is then pushed to the periphery by the formation of a central vacuole.

• Walls form later, and the central vacuole vanishes. As an example, consider maize, wheat, and rice.

• In the outer portion of the coconut, there is a multicellular solid endosperm, and in the interior, there is a free nuclear liquid endosperm.

2. Cellular endosperm 

• A wall forms after each division of the primary endosperm nucleus. As a result, the endosperm is cellular from the start, as in Datura, balsam, and Petunia.

3. Helobial endosperm 

• The first division results in the formation of two cells. Within these cells, the free nuclear division may occur. They may eventually become cellular as well. For example, Eremurus and Asphodelus. 

Functions of Endosperms

(i) Endosperm nutrients aid in early seedling growth in plants with albuminous seeds.

(ii) Endosperm nourishes the developing embryo.

(iii) Coconut liquid endosperm includes cytokinins, auxins, and GA, and stimulates cytokinesis when given to a basic nutritional medium. Coconut milk can also be used to induce embryo and plantlet differentiation from various plant tissues (iv) Zeatin is a highly effective cytokinin. It is derived from maize's young endosperm. 

Embyrogeny (embryo formation) 

• It is the formation of a mature embryo from a zygote or an oospore.

• Early development results in an axially symmetric pro-embryo.

• The embryo goes through the globular stage.

• Because of the presence of a suspensor, embryo development occurs on the inner side. As a result, embryo development is endoscopic.

• Dicot embryogenesis (crucifer / onagrad):

• The zygote is categorised into two unequal cells: a larger suspensor cell directing towards the micropyle and a smaller embryo cell facing towards the antipodal region.

• The suspensor cell divides transversely, resulting in a 6-10 celled suspensor.

• The first cell of the suspensor is known as the haustorium, and the last cell (towards the embryo cell) is known as the hypophysis. It produces radicles. 

• A single embryo cell divides twice. Vertically and once transversely to produce an embryo, which is a two-tiered eight-cell. Two cotyledons and a plumule are formed by the epibasal (terminal) tier. Only hypocotyls are produced by the hypobasal (near the suspensor) tier.

• It is globular at first. Later, it takes on a heart shape before resuming its original shape.

• A dicotyledonous embryo is made up of an embryonal axis and two cotyledons.

• The epicotyl is the part of the embryonal axis that is above the level of the cotyledons. It comes to an end with the plumule, which is the tip of the stem. The plumule is the source of the future shoot.

• Hypocotyls are the parts of the plant that are below the level of the cotyledons. It comes to an end at the root tip known as radical. The radicle is the source of the future root. The root cap is responsible for protecting the root tip.

The curving of the ovule causes the cotyledons to curve as they emerge and elongate in Caspella bursa pastoris. In orchids such as Orboanche and Utricularis, the embryo does not differentiate into plumule, cotyledon, and radical.

• Monocot embryogeny (Sagittaria type) 

• The zygote divides transversely to form a vesicular suspensor cell facing the micropylar end and an embryo cell facing the chalazal end, then divides again to form a terminal and middle cell.

• The terminal cell divides vertically and transversely to form a globular embryo. It also produces a plumule and a large cotyledon. The plumule is pushed to one side as the cotyledon grows. The remnants of the second cotyledon are generally found in certain grasses. It is referred to as epiblast. The scutellum is the single cotyledon of monocots. It has the shape of a shield and appears as a terminal. 

The hypocotyls and radicle are produced by the middle cell. It may result in an increase in suspensor cell numbers. Both the radicle and the plumule are protected by sheaths. They are referred to as coleorhizae and coleoptiles, respectively. 

They could be scutellum extensions.

Formation of Seed and Fruit

Fruit refers to a ripened or fertilized ovary.

The pericarp is a fleshy or dry fruit wall formed by the ovary wall.

Epicarp, mesocarp, and endocarp are the three layers of fleshy fruit or pericarp.

It is the fruit covering that develops from the ovary wall.

• It is a dry or fleshy fruit part that serves as a protective covering and provides nutrition to the seed.

• Ripened ovules are referred to as seeds.

• The ovule's integuments form the seed coat. The testa is formed by the outer integuments, and the tegmen is formed by the inner integuments.

• In some cases, a type of third integument or aril is present, such as litchi, ingadulce (Pithecolobium), Asphodelus, and Trianthema. It adds a layer of seed protection.

• A spongy outgrowth near the micropyle is present in certain seeds, such as castor (Ricinus communis). It's referred to as caruncle. It aids seed germination by absorbing water. 

• Funiculus forms the seed stalk. 

• The stalk eventually withers and leaves a minute scar called the hilum. 

• Orchids have the smallest seeds. Because they are the lightest in the plant kingdom, they are known as dust seeds. Each orchid seed weighs approximately 20.33g when it is fresh.

• Seeds are classified as follows based on the presence or absence of endosperm.

(i) Non-endospermic or ex-albuminous: The developing embryo consumes all of the food stored in the endosperm. Gram, pea, bean, and orchid are some examples.

(ii) Endospermic or albuminous: Endosperm grows rapidly and is not completely consumed by the developing embryo. The cotyledons are thin in this case. Wheat seed, barley seed, castor seed, poppy seed, and so on are examples.

Importance of Seeds

• Evolutionary success: Seed is an evolutionary success. It shields the embryo from harm.

• Seeds contain a sufficient food reserve to feed the germinating embryo.

• Because of dispersal, seeds can colonize and populate new areas, as well as spread and propagate their species.

• Because seeds are the result of sexual reproduction, they have a wide range of variations, which aids in adaptation to a variety of environments.

• Human seed germination and sowing gave rise to agriculture, which aided in the advancement of civilization, science, and technology.

Seed Viability 

• It is the amount of time that the seeds retain their ability to germinate.

• Both genetic and environmental factors influence seed viability.

• Humidity and temperature are two environmental factors that can affect viability.

• Seed viability varies genetically from a few days (e.g., Oxalis), one season (e.g., Birch), and 2-5 years (most crop plants) to 100 years (e.g. Trifolium).

• Lotus seed viability has been found to be more than 1000 years. Phoenix dactylifera seeds discovered in King Herod's palace near the Dead Sea 2000 years ago have been found to be viable. 

• Similarly, 10,000 year old Lupins arcticus (Lupine) seeds excavated from Arctic Tundra not only germinated but also produced flowering plants.

• The viability of the seed is determined by (a) respiration and (b) germination.

• Respiring seed converts the colourless triphenyl tetrazolium chloride to the pink tripheyl formazan. 

i) Apomixis (apo - without, mixis - marriage)

• It is the formation of new individuals through asexual methods that mimic sexual reproduction, including seed formation, but do not involve gamete or sex cell fusion. • Amphimixis is a normal type of sexual reproduction with two regular features, namely meiosis and fertilization.

• Apomicts are organisms that reproduce through apomixes.

• Apomixis is controlled by genes, and individuals are genetically similar to the parent that produced them, i.e. they are clones, and members of a clone are known as ramets.

Polyembryony 

• Polyembryony refers to the process of having more than one embryo.

• Polyembryony caused by the fertilization of more than one egg cell is referred to as simple polyembryony.

• Additional embryos can be formed from various parts of the ovule, such as synergids, antipodal, nucellus, integuments, and so on.

• Citrus, groundnut, onion, Opuntia, Mangifera are some examples.

• In 1719, Leeuwenhoek discovered polyembryony. Schnarf confirmed the same in 1929.

• Polyembryony occurs more frequently in gymnosperms than in angiosperms.

• Polyembryony can be either true or false embryony.

• In false embryony, multiple embryos form in different embryo sacs in the ovule, whereas multiple embryos form in the same embryo sac in true embryony.

• Polyembryony may be caused by:

-Proembryo cleavage, for example, in the orchidaceae family.

-Development of many embryos from cells other than the egg in the embryo sac. E.g. Argemone

-The formation of numerous embryos as a result of the presence of more than one embryo sac in the same ovule, e.g. Citrus

-The formation of many embryos from structures outside the embryo sac, such as mango and Opuntia. 

• Polyembryony is important for practical reasons because nucellar embryos can produce genetically uniform parental type seedlings.

• Nucellar embryos are qualitatively superior to those obtained through vegetative propagation because nucellar embryo seedlings are disease-free and retain their superiority for an extended period of time.

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