Reproduction in Lower and Higher Plants revision notes

• Reproduction is important for continuation of species and life.
• There are two types of reproduction: asexual and sexual.
• Asexual reproduction produces genetically identical offspring without the fusion of gametes.
• Organisms can reproduce asexually through fragmentation, budding, spore formation, binary fission, conidia formation, and gemma formation.
• Fragmentation occurs when multicellular organisms break into fragments, which grow into new individuals.
• Budding is the most common method of asexual reproduction in unicellular organisms, where one or more outgrowths (buds) develop into new individuals.
• Spore formation occurs in Chlamydomonas through flagellated, motile zoospores that can grow independently into new individuals.
• Binary fission occurs in Chlorella, Diatoms, and Chlamydomonas.
• Conidia formation occurs in Penicillium.
• Gemma formation occurs in Marchantia.

Vegetative Reproduction :

• Plants can reproduce asexually through their vegetative parts, resulting in genetically identical offspring.
• Agriculture and horticulture use vegetative reproduction to propagate desired varieties of plants.
• Cutting is a method where a small piece of a vegetative part of a plant with one or more buds is used for propagation.
• Grafting involves joining parts of two plants so they grow as one, with scion joined onto a rooted plant called stock.
• Budding is a type of grafting where only one bud is joined onto the stock.
• Tissue culture is a method where a small amount of plant tissue is grown to give many plantlets, also known as micropropagation.

Sexual Reproduction :

• Sexual reproduction involves fusion of two compatible gametes.
• Plants reach maturity before they can reproduce sexually.
• Flowers are specialized reproductive structures in plants.
• The flower produces haploid gametes and ensures fertilization.
• A typical flower has four whorls: calyx, corolla, androecium, and gynoecium.
• Sexual reproduction involves meiosis, fusion of gametes, and genetically dissimilar offspring.
• Variations are useful for the survival and evolution of species.
• The process of sexual reproduction is divided into pre-fertilization, fertilization, and post-fertilization stages.
• The male reproductive whorl of a flower is called androecium.
• The individual member of androecium is called stamen, which consists of filament, connective, and anther with two anther lobes (theca).

Structure of Anther :

• Anther is tetrasporongiate, with each lobe containing two pollen sacs.
• The archesporial cell differentiates into an inner sporogenous cell and an outer primary parietal cell.
• The sporogenous cell gives rise to the microspore tetrad, while the parietal cell forms the anther wall layers.
• The mature anther wall consists of four layers: epidermis, endothecium, middle layer, and tapetum.
• Epidermis is the outermost protective layer made up of flattened cells.
• Endothecium is a sub-epidermal layer made up of radially elongated cells with fibrous thickenings.
• Middle layer is a thin-walled layer made up of 1-2 layered cells that may disintegrate in mature anther.
• Tapetum is the innermost nutritive layer that encloses the sporogenous tissue.

Microsporogenesis :

• Each microspore mother cell divides meiotically to form a tetrad of haploid microspores (pollen grains).
• The typical pollen grain is a non-motile, unicellular body with a single nucleus, surrounded by a two-layered wall called sporoderm.
• The outer layer, exine, is thick and made up of a complex, non-biodegradable substance called sporopollenin.
• Exine may be smooth or sculptured and is resistant to chemicals.
• Germ pores are thin areas in the exine meant for the growth of the emerging pollen tube during germination of the pollen grain.
• The inner wall layer, intine, consists of cellulose and pectin.

Development of male gametophyte :

• Pollen grain marks the beginning of male gametophyte
• It undergoes first mitotic division to produce bigger, naked vegetative cell and small, thin walled generative cell
• The vegetative cell is rich in food and having irregular shaped nucleus
• The generative cell floats in the cytoplasm of vegetative cell
• The second mitotic division is concerned with generative cell only and gives rise to two non-motile male gametes
• The pollen grains are shed from the anther, at this two- celled stage in most of the angiosperms
• Female reproductive whorl of flower is gynoecium (Pistil)
• Individual member of gynoecium is called carpel (megasporophyll)
• A flower with many, free carpels is called apocarpous (e.g. Michelia)
• A syncarpous flower is one that has many carpels fused together (e.g. Brinjal)
• Typical carpel has three parts viz, ovary, style and stigma
• The number of ovules in the ovary varies e.g. paddy, wheat and mango are uniovulate whereas tomato and lady’s finger are multi ovulate

Structure of Anatropous ovule :

• Each ovule is attached to the placenta of the ovary by a funiculus.
• The most common type of ovule in angiosperms is anatropous with the micropyle directed downwards and adjacent to the funiculus.
• The ovule consists of central parenchymatous tissue called the nucellus, which is surrounded by two protective integuments.
• The narrow opening at the apex of the ovule is called micropyle and the base of the ovule opposite to the micropyle is called chalaza.
• The embryo sac, which is the female gametophyte, is an oval, multicellular structure embedded in the nucellus.

Megasporogenesis : 

• Megaspore mother cell (MMC) is a diploid cell.
• It becomes differentiated in the nucellus towards the micropylar end of the ovule.
• The process of formation of haploid megaspores from MMC is called megasporogenesis.

Development of female gametophyte :

• Megaspore mother cell undergoes meiosis to form linear tetrad of haploid cells i.e. megaspores
• The lowest megaspore towards the center of the nucellus becomes the functional megaspore, which is the first cell of female gametophyte
• The functional megaspore undergoes three successive, free nuclear mitotic divisions to form a seven-celled and eight-nucleated structure called an embryo sac
• The embryo sac consists of the large central haploid egg cell, two supporting haploid synergid cells, and three haploid antipodal cells at the chalazal end
• The two haploid polar nuclei of the large central cell fuse to form a diploid secondary nucleus or definitive nucleus just prior to fertilization
• Female gametophyte development is endosporous within the megaspore and is concealed in the ovule enclosed by the ovary.

 Pollination :

• Abiotic Pollination: In this type of pollination, pollen grains are carried from one flower to another by non-living agents like wind and water. Wind pollination is the most common type of abiotic pollination.
• Biotic Pollination: In this type of pollination, pollen grains are carried from one flower to another by living agents like birds, insects, and other animals.
• Self-Pollination: It occurs when the pollen from the anther of a flower is transferred to the stigma of the same flower or to another flower on the same plant. It results in inbreeding.
• Cross-Pollination: It occurs when the pollen from the anther of one flower is transferred to the stigma of a flower on another plant of the same species. It results in outbreeding.

Autogamy:

• Pollination of a bisexual flower by its own pollen grains
• Offsprings are genetically identical to their parents
• Examples include pea and Clitoria

Geitonogamy:

• Transfer of pollen grains to the stigma of a different flower produced on the same plant
• Functionally similar to cross-pollination, but cannot bring about genetic variations
• Only of ecological significance
• Example includes Cucurbita maxima
• Pollen grains come from the same plant

Xenogamy (cross-pollination/outbreeding):

• Pollination where pollen grains of one flower are deposited on the stigma of a flower of a different plant belonging to the same species, with the help of a pollinating agent
• Generates genetically varied offspring
• Majority of flowering plants depend on the transfer of pollen grains
• Virtually all seed plants need to be pollinated
• Most food and fiber crops grown throughout the world depend upon pollinators for reproduction.
• Pollination agents are responsible for the transfer of pollen from the anther to the stigma of a flower, which is necessary for fertilization and reproduction.
• Pollination agents can be divided into two categories: abiotic agents and biotic agents.

 Abiotic Agents

• Abiotic agents are non-living agents that include wind and water.
• Wind pollination (anemophily) is the most common form of abiotic pollination.
• Anemophilous flowers have small, inconspicuous, colorless flowers without nectar or fragrance.
• Pollen grains produced by anemophilous flowers are light in weight and produced in large numbers.
• Stigmas of anemophilous flowers are feathery to trap pollen carried by the wind.
• Stamens of anemophilous flowers are exserted with long filaments and versatile anthers.

Biotic Agents

• Biotic agents are living agents that include insects, birds, mammals, and reptiles.
• Entomophily is pollination by insects and is the most common form of biotic pollination.
• Flowers pollinated by insects have colorful petals and produce nectar and fragrance to attract pollinators.
• Pollen grains of insect-pollinated flowers are sticky or spiny to adhere to the bodies of pollinators.
• Birds, bats, and some insects also pollinate flowers that open at night (nocturnal pollination).
• Mammals and reptiles (e.g., bats, mice, and lizards) are important pollinators for some plant species in certain regions.

Pollination by wind (Anemophily) :

• Most important crop plants are wind pollinated, such as wheat, rice, corn, rye, barley, oats, and palms.
• Anemophilous flowers are small, colorless, and lack nectar and fragrance.
• Pollen grains are light, dry, and produced in large numbers.
• Stigma is feathery to trap pollens carried by wind currents.
• Stamens are exserted with long filaments and versatile anthers.

Pollination by water (Hydrophily) :

• Found only in some 30 genera of aquatic monocots, such as Vallisneria, Zostera, and Ceratophyllum.
• Hydrophilous flowers are small and inconspicuous.
• Perianth and other floral parts are unwettable.
• Pollen grains are long and unwettable due to the presence of mucilage.
• Nectar and fragrance are lacking in flowers.
• Hydrophily is a type of pollination that occurs in aquatic plants.
• Hydrophily can be divided into two types: hypohydrophily and epihydrophily.
• In hypohydrophily, pollination occurs below the surface of water where the pollen grains are heavier than water and sink down to be caught by stigmas of female flowers.
• In epihydrophily, the pollen grains float on the water surface and reach the stigma of female flower.
• Specific gravity of pollen grains is equal to that of water, which is why they float on the surface of water.
• Some aquatic plants are anemophilous and some are entomophilous.

Biotic Agents:

• About 80% of plants require the help of living creatures such as insects, birds, bats, and snails to transfer their pollens from one flower to another.
• Biotic agents sustain ecosystems and produce natural resources by helping plants to reproduce.

Pollination by insects (Entomophily):

• Occurs in plants such as Rose, Jasmine, Cestrum, etc.
• Adaptations in entomophilous flowers:
• They are large, showy, and often brightly colored.
• The flowers produce sweet odors and have nectar glands.
• The stigma is rough due to the presence of hair or is sticky due to mucilaginous secretion.
• The pollen grains are spiny and surrounded by a yellow sticky substance called pollen-kit.
• Some plants have special adaptations, such as the lever mechanism or turn-pipe mechanism in Salvia, to help in cross-pollination.

Pollination by birds (Ornithophily):

• Only a few types of birds are specialized for pollination.
• Flowers are usually brightly colored, large and showy.
• They secrete profuse, dilute nectar.
• Pollen grains are sticky and spiny.
• Flowers are generally without fragrance, as birds have poor sense of smell.

Pollination by Bats (Chiropterophily):

• Bats can transport pollens over long distance.
• Flowers are dull colored with strong fragrance.
• They secrete abundant nectar.
• Flowers produce large amounts of edible pollen grains.

Pollination by insects (Entomophily):

• Occurs in plants such as rose, jasmine, and cestrum.
• Flowers are large, showy and brightly colored.
• Flowers produce sweet odor and have nectar glands.
• Pollen grains are spiny and surrounded by a yellow sticky substance called pollen-kit.
• Some plants have special adaptations for the insect visitor to help in cross-pollination, e.g. lever mechanism or turn-pipe mechanism in Salvia.

Outbreeding devices (contrivances)

• Many plants prevent self-pollination to increase genetic diversity.
• Plants have evolved various sexual strategies to encourage cross-pollination.
• Examples of outbreeding devices include unisexuality (dioecism), dichogamy (protandry and protogyny), prepotency, heterostyly (heteromorphy), herkogamy, and self-incompatibility.
• Unisexuality involves plants bearing either male or female flowers, preventing self-pollination.
• Dichogamy involves anthers and stigmas maturing at different times in bisexual flowers to prevent self-pollination.
• Prepotency refers to pollen grains from other flowers germinating more rapidly on the stigma than pollen from the same flower.
• Heterostyly involves plants having different forms of flowers with stigmas and anthers at different levels to prevent pollination by the same flower.
• Herkogamy is a mechanical device that physically separates the sex organs in a flower to prevent self-pollination.
• Self-incompatibility is a genetic mechanism that inhibits pollen germination on the stigma of the same flower.

Pollen - Pistil Interaction 

• Pollen-pistil interaction is the process of interaction between pollen grains and the stigma, which begins with pollination and ends with fertilization.
• The interaction is essential for sexual reproduction and seed formation.
• The pistil has the ability to recognize and accept the right or compatible pollen of the same species while discarding the wrong or incompatible pollen.
• Compatibility and incompatibility of pollen-pistil is determined by special proteins.
• The compatible pollen absorbs water and nutrients from the surface of stigma, germinates, and produces a pollen tube that grows through the style, determined by specific chemicals.
• The pollen tube finally reaches the ovule and ruptures to release the contents in one of the synergids, leading to fertilization.
• Pollen-pistil interaction involves intense competition even in compatible pollen grains.
• Pollen grain can be induced to germinate in a synthetic medium using sucrose, and boric acid facilitates and accelerates pollen germination.

Artificial hybridization :

• Hand pollination is a major approach used in crop improvement.
• It involves selecting and using only the desired pollen grains for fertilization.
• This is achieved through emasculation and bagging procedure, where unwanted male reproductive organs are removed and the female reproductive organs are protected by a bag.
• Hand pollination ensures that only desirable traits are passed on to the next generation, leading to improved crop yield and quality.

Double Fertilization 

• Double fertilization is a complex fertilization mechanism in flowering plants.
• After a pollen grain has reached the surface of the stigma, it forms a pollen tube that penetrates the ovary chamber and enters the ovule.
• The growth of the pollen tube is guided by the chemicals secreted by the synergids, and it usually enters the ovule through the micropyle.
• The pollen tube carrying male gametes penetrates one of the synergids and absorbs the watery contents.
• The pollen tube ruptures, releasing the two non-motile male gametes, and siphonogamy ensures fertilization takes place.
• Syngamy is the fusion of haploid male gamete with haploid female gamete (egg) to produce a diploid zygote, while triple fusion is the fusion of the second haploid male gamete with the diploid secondary nucleus producing primary endosperm nucleus (PEN) that develops into triploid endosperm.
• Both the male gametes participate in fertilization, making it a double fertilization process.
• Double fertilization plays an important role in sexual reproduction and seed formation in angiospermic flowering plants.

Significance of Double Fertilization :

• Double fertilization is a unique feature of angiosperms where two fertilizations occur after the pollen tube enters the ovule.
• The first fertilization is the fusion of the haploid male gamete with the haploid female gamete to form a diploid zygote.
• The second fertilization is the fusion of the other haploid male gamete with the diploid secondary nucleus to form the triploid primary endosperm nucleus (PEN).
• The zygote develops into an embryo, which grows into a new plant.
• The PEN develops into the nutritive endosperm tissue.
• Double fertilization helps to ensure that the parent plant invests in the development of a seed with a food store only if the egg is fertilized.
• Double fertilization also helps to avoid polyembryony

Development of Endosperm 

• The primary endosperm nucleus is triploid and divides mitotically to form endosperm tissue.
• The endosperm develops after post-fertilization changes in the ovule.
• The embryo and endosperm develop simultaneously.
• Other cells in the embryo sac disorganize over time.
• The triploid endosperm nucleus triggers cell division and formation of endosperm.

Nuclear Type

• This type of endosperm is the most common in 161 angiospermic families.
• The primary endosperm nucleus divides mitotically without wall formation, producing free nuclei.
• A central vacuole appears, pushing the nuclei to the periphery.
• Walls develop between nuclei to form multicellular endosperm, but in some cases, cell wall formation remains incomplete.
• Wheat, sunflower, and coconut are examples of plants where cell wall formation is incomplete.
• Coconut has multicellular endosperm in the outer part and free nuclear as well as vacuolated endosperm in the center.

Cellular Type :

• In some plants, the triploid primary endospermic nucleus immediately forms cell walls after division.
• This results in cellular endosperm from the beginning.
• This type of endosperm is mostly observed in 72 families of dicots.
• Examples of dicot plants with cellular endosperm include Balsam, Petunia, and Adoxa.
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Helobial Type :

• This type of endosperm occurs in the order Helobiales of monocotyledons.
• The first division of the primary endosperm nucleus is followed by a transverse wall, dividing the cell unequally.
• The smaller cell is called the chalazal cell, and the larger cell is the micropylar cell.
• The nuclei in each cell divide by free nuclear divisions, and then walls develop between nuclei in the micropylar chamber.
• This type of endosperm is intermediate between cellular and nuclear type endosperm.
• Asphodelus is an example of a plant with this type of endosperm.

Development of Embryo 

• Embryogenesis is the process of developing a zygote into an embryo.
• The embryo is developed at the micropylar end of the embryo sac.
• The growth of the embryo begins after a certain amount of endosperm is formed.
• After fertilization, the zygote divides to form a two-celled proembryo.
• The suspensor cell divides transversely to produce a filamentous suspensor of 6-10 cells.
• The lowermost cell of the suspensor is known as hypophysis.
• The embryonal initial undergoes three successive mitotic divisions to form octant.
• The lower tier of four cells of octant gives rise to hypocotyl and radicle, whereas four cells of the upper tier form the plumule and the one or two cotyledons.
• The hypophysis gives rise to the part of the radicle and root cap.
• The cells in the upper tier of octant divide in several planes to become heart-shaped, which then forms two lateral cotyledons and a terminal plumule.
• The development of the embryo is similar in both dicots and monocots up to the octant stage, after which differences appear.
• In monocot embryos, a single cotyledon occupies the terminal position, and the plumule is lateral. The single shield-shaped cotyledon is called the scutellum, and the protective sheaths of plumule and radicle are called coleoptile and coleorhiza, respectively.
• Finally, the ovule is transformed into the seed, and the ovary into the fruit.

 Seed and Fruit Development :

• Reproduction in plants involves creating offspring for the next generation through seed formation.
• Pollination is necessary for seed and fruit production.
• Seed development is initiated by fertilization, and the integuments of the ovule transform into the seed coat.
• Seeds can be endospermic or ex-albuminous, depending on whether the embryo absorbs all the food reserves from the endosperm during its developmental stages.
• Fruits provide nourishment and protection to developing seeds, and seeds serve as important propagating organs of the plant.
• Dormancy is a temporary state of metabolic arrest that facilitates survival during adverse environmental conditions, and viable seeds will not germinate until after completion of the dormancy period.

Apomixis 

• Apomixis is a type of reproduction where embryos are formed without the formation of gametes or the act of fertilization.
• Embryos develop in the ovule and eventually form seeds.
• The two main categories of apomixis are recurrent and non-recurrent apomixis, with a third category called adventive embryony.
• Recurrent apomixis involves the embryo sac arising from an archesporial cell or nucellus cells, while in non-recurrent apomixis, a haploid embryo sac is formed from the megaspore mother cell and the embryo arises from the egg through parthenogenesis or other haploid cells through apogamy.
• Adventive embryony involves the development of embryos from somatic nucellus or integuments and can result in polyembryony.
• Apomixis allows for the production of genetically identical plants quickly and effectively.

Parthenocarpy

• Parthenocarpy is the development of fruit without fertilization.
• It occurs naturally in some varieties of Pineapple, Banana, Papaya, etc.
• In these plants, the placental tissue in the unfertilized ovary produces auxin IAA which is responsible for the enlargement of ovary into fruit.
• The fruit resembles the normally produced fruit but it is seedless.

Polyembryony 

• Polyembryony is the development of more than one embryo inside a seed.
• It was first noticed in Citrus seeds by Leeuwenhoek in 1719.
• Additional embryos result from the differentiation and development of various maternal and zygotic tissues associated with the ovule of the seed.
• Polyembryony may be true or false depending on whether many embryos arise in the same embryo sac or in different embryo sacs in the same ovule.
• In adventive polyembryony, an embryo develops directly from the diploid cell of nucellus and integuments.
• In cleavage polyembryony, the zygote proembryo sometimes divides into many parts or units, each of which develops into an embryo.
• Polyembryony increases the chances of survival of new plants.
• Nucellar adventive polyembryony is significant in horticulture.