Overview
No individual lives for ever, yet species continue for millions of years because organisms produce offspring like themselves. Reproduction is this process by which a living organism gives rise to young ones and ensures the continuity of its species from generation to generation. This chapter surveys the different ways in which organisms, and especially plants, reproduce. It begins with life span and the two broad modes, asexual and sexual. Asexual reproduction, in which a single parent produces genetically identical offspring (clones), is examined through binary fission, budding, spore formation, fragmentation and the many forms of natural and artificial vegetative propagation, from the rhizome of ginger and the leaf buds of Bryophyllum to cuttings, layering, grafting and tissue-culture micropropagation. Sexual reproduction, involving the fusion of male and female gametes, is then traced through its three stages: the pre-fertilisation events of gametogenesis and gamete transfer, fertilisation or syngamy, and the post-fertilisation events of zygote formation and embryogenesis. Along the way you meet the vocabulary that the next chapter depends on: juvenile and reproductive phases, monoecious and dioecious plants, haploid and diploid organisms, homogametes and heterogametes, external and internal fertilisation. These ideas underpin agriculture, horticulture and the Intermediate examination alike.
Learning Objectives
- Define reproduction and explain why it is essential for the continuity of a species.
- Distinguish between asexual and sexual reproduction on the basis of parents, gametes and genetic identity of offspring.
- Describe binary fission, budding, spore formation and fragmentation with suitable organisms.
- Explain natural vegetative propagation by roots, stems and leaves and give examples of each propagule.
- Describe the artificial methods of vegetative propagation: cutting, layering, grafting and micropropagation, and state their advantages.
- Outline the juvenile, reproductive and senescent phases of the life of a plant and relate them to life span.
- Explain gametogenesis, the terms homogametic and heterogametic, monoecious and dioecious, and the transfer of gametes including pollination.
- Describe fertilisation and the post-fertilisation events of zygote formation and embryogenesis in plants.
Topics in this chapter
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Reproduction and Life Span
Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring, so there is a cycle of birth, growth and death, and the species continues from one generation to the next. Reproduction is one of the fundamental characteristics of living organisms; a lion that does not hunt dies, but a lion that does not reproduce ends its line. It also brings variation into a population, which is the raw material of evolution.
Every organism lives only for a certain period. The period from birth to natural death is its life span. Life spans vary enormously: the fruit fly lives about a month, a dog for 15 to 20 years, a crow about 15 years, a butterfly one to two weeks, a rice plant for a single season, and a banyan or peepal tree for several hundred years. The oldest living things on earth are trees; some conifers are more than four thousand years old. It is important to notice that life span is not necessarily correlated with size: a crow lives longer than a parrot's chick or a mango tree may outlive an elephant, and a tiny tortoise may live 100 to 150 years. Whatever the life span, death of every individual is a certainty, and the survival of the species depends on reproduction.
Organisms reproduce in a great variety of ways, depending on their habitat, their internal physiology and many other factors. Broadly there are two modes. In asexual reproduction a single parent produces offspring with or without the involvement of gamete formation; the offspring are morphologically and genetically identical to the parent and to one another and are called clones. In sexual reproduction two parents, or two gametes from one parent, take part; male and female gametes fuse to form a zygote, and the offspring differ from the parents and from each other because of the recombination of genetic material.
Asexual reproduction is common among single-celled organisms and among plants and animals with relatively simple organisation. It is fast and economical, requiring no partner, but it produces no variation, so the population is vulnerable to a change in the environment. Sexual reproduction is slower and more elaborate but produces variation and is found in all higher plants and animals. Many organisms, for instance sponges, hydra, yeast and most flowering plants, use both modes at different times, switching to sexual reproduction when conditions become unfavourable.
- A rice plant completes its life in one season, a mango tree lives for over a hundred years and a banyan for many centuries; each must reproduce before it dies.
- Sugarcane is grown from stem pieces (asexual, all plants identical), whereas paddy is grown from seed produced by sexual reproduction.
- Hydra buds asexually in good conditions and produces gametes when the pond dries or food is scarce.
- Reproduction = production of offspring similar to the parent; life span = period from birth to natural death.
- Asexual: one parent, no gamete fusion, clones. Sexual: two gametes fuse, zygote, variation.
Asexual Reproduction: Fission and Budding
In asexual reproduction a single individual (parent) is capable of producing offspring, and the offspring are not only identical to one another but are also exact copies of the parent. Since gametes and their fusion are not involved, the method is quick. The simplest kinds of asexual reproduction are seen in unicellular organisms, in which cell division itself is reproduction.
Binary fission. In many single-celled organisms such as Amoeba, Paramecium and bacteria, the parent cell divides into two equal halves, each of which grows rapidly into an adult. In Amoeba the nucleus divides first by mitosis and then the cytoplasm constricts into two daughter cells (simple binary fission). In Paramecium the division is always transverse to the long axis, and in Euglena it is longitudinal. Under unfavourable conditions Amoeba withdraws its pseudopodia and secretes a three-layered, hard covering, the cyst, around itself; this is encystation. When favourable conditions return, the encysted Amoeba divides by multiple fission to produce many minute amoebae, or pseudopodiospores, and the cyst wall bursts to release them; this is sporulation. Multiple fission is also seen in the malarial parasite Plasmodium.
Budding. In yeast (Saccharomyces), a small protuberance or bud grows out from the parent cell. The nucleus divides and one daughter nucleus migrates into the bud. The bud may separate to live independently, or it may remain attached and itself bud, so that chains of cells are formed. Budding is thus an unequal division. In the multicellular animal Hydra, a bud develops as an outgrowth of the body wall, forms a mouth and tentacles, and finally detaches as a new individual; sponges too reproduce by internal buds called gemmules.
Members of the Kingdom Fungi and simple plants such as algae reproduce asexually by special structures too. The commonest of these are the zoospores of aquatic fungi and algae (for example Chlamydomonas), which are motile with flagella, and the non-motile conidia of Penicillium, formed in chains at the tips of special hyphae called conidiophores. Each such spore germinates into a new individual. The advantage of the spore method is that enormous numbers of light spores are produced and are carried by wind or water over large distances, so the organism spreads quickly.
- Amoeba divides by simple binary fission in a pond; when the pond dries it encysts and later releases many pseudopodiospores by sporulation.
- A drop of sugar solution with yeast shows chains of budding cells under the microscope within a few hours.
- Green mould (Penicillium) on a stale orange releases clouds of conidia that spread the fungus to nearby fruits.
- Binary fission: 1 parent cell → 2 equal daughter cells (Amoeba, Paramecium, bacteria).
- Multiple fission: 1 encysted cell → many daughter cells (Amoeba sporulation, Plasmodium).
- Budding: unequal division producing a small bud that grows into a new individual (yeast, Hydra).
Spore Formation and Fragmentation
Many plants and fungi reproduce asexually by producing special reproductive units called spores, and many simple plants and animals reproduce by breaking into pieces. Both methods are effective ways of multiplying quickly and colonising new places.
Zoospores. In algae such as Chlamydomonas, Ulothrix and in aquatic fungi, the protoplast of a cell divides to form a number of small, naked, motile spores, each bearing two or more flagella. These zoospores are released into the water, swim about, settle, lose their flagella and grow into new individuals. Zoospore formation depends on the presence of water and is typical of aquatic or moist habitats.
Conidia. In terrestrial fungi such as Penicillium and Aspergillus, non-motile spores called conidia are cut off externally in chains from the tips of specialised, erect hyphae called conidiophores. They are dry, light and coloured (green in Penicillium, black in Aspergillus niger), and are dispersed by air currents. On a moist substrate each conidium germinates by putting out a germ tube that grows into a new mycelium. Other spore types are the sporangiospores formed inside a sac-like sporangium in bread mould (Rhizopus) and the aplanospores of some algae.
Gemmae. The liverwort Marchantia produces small, green, multicellular, lens-shaped bodies called gemmae inside cup-like gemma cups on the upper surface of its thallus. Raindrops splash the gemmae out; each grows into a new thallus. Sponges likewise produce internal buds called gemmules that survive adverse seasons.
Fragmentation. In filamentous algae such as Spirogyra, the filament breaks into two or more fragments because of mechanical injury, the death of intervening cells or the dissolution of the middle lamella, and each fragment grows into a complete filament by cell division. Fragmentation is also seen in many fungi (pieces of hypha), in the liverwort and moss thalli, and among animals in Hydra, sponges and the flatworm Planaria. In Planaria the animal, cut into pieces, regenerates a whole worm from each piece; this ability of an organism to grow back lost parts, called regeneration, is closely allied to asexual reproduction.
All these methods share the features of asexual reproduction: a single parent, mitotic divisions only, offspring genetically identical to the parent, and rapid multiplication in favourable conditions.
- Chlamydomonas produces 2, 4, 8 or 16 zoospores inside the parent cell wall; they escape, swim and grow.
- Rhizopus on bread bears black sporangia; each sporangium bursts to release hundreds of sporangiospores.
- A Spirogyra filament left in a jar breaks into pieces overnight and each piece elongates into a new filament.
- Zoospores = motile, flagellate asexual spores (Chlamydomonas); conidia = non-motile, exogenous spores on conidiophores (Penicillium).
- Fragmentation: parent body breaks into fragments, each regenerating a new individual (Spirogyra, Hydra, Planaria).
Vegetative Propagation: Meaning and Propagules
In plants, the term vegetative propagation is used for asexual reproduction in which a new plant develops from a vegetative part, that is, a root, stem or leaf, of the parent, without the formation of seeds or spores. The structure that gives rise to the new plant is called a vegetative propagule. Because only mitotic divisions are involved, all the plants produced from one parent are genetically identical, a clone. Vegetative propagation may be natural, occurring on its own in nature, or artificial, carried out by gardeners and farmers.
The units of natural vegetative propagation are familiar plant organs, usually modified for storage, that carry buds capable of growing into new shoots.
- Runner: a slender, creeping stem with long internodes that roots at the nodes and forms daughter plants (Oxalis, grasses such as Cynodon).
- Rhizome: a horizontal underground stem with buds at the nodes; each piece with a bud grows into a plant (ginger, turmeric, banana, Canna).
- Sucker: an underground lateral shoot that comes up at a distance from the parent (banana, Chrysanthemum, mint, pineapple).
- Tuber: a swollen underground stem whose 'eyes' are buds; cut pieces with an eye each produce plants (potato).
- Offset: a short, thick runner of aquatic plants with a rosette of leaves and roots at its tip (water hyacinth, Pistia).
- Bulb: a condensed stem with fleshy scale leaves and axillary buds (onion, garlic, lily).
- Corm and bulbil: Colocasia and Gladiolus corms bear buds; bulbils are fleshy buds on the inflorescence of Agave or in leaf axils of Dioscorea that fall and root.
These propagules also help the plant survive the dry or cold season underground (perennation). The whole plant body may act as a propagule too: pieces of the stem of sugarcane and rose root readily. The disadvantage in nature is that many of these plants become invasive weeds, since they multiply so easily; the water hyacinth (Eichhornia), called the 'terror of Bengal', was introduced to India for its beautiful flowers and has choked lakes, ponds and canals all over the country, propagating vegetatively through offsets at a phenomenal rate and depleting water bodies of oxygen, so that fish die.
- A single potato cut into four pieces, each with an eye, planted in a field gives four plants, and each plant yields many tubers.
- A piece of ginger rhizome with even one bud sprouts in the kitchen shelf during the rainy season.
- Water hyacinth doubles its area in a pond within about two weeks by producing offsets, covering the whole surface in a season.
- Vegetative propagule = a root, stem or leaf part capable of growing into a new plant without seeds.
- Natural propagules: runner, rhizome, sucker, tuber, offset, bulb, corm, bulbil, leaf buds.
Natural Vegetative Propagation by Roots, Stems and Leaves
Natural vegetative propagation may be studied according to the organ that serves as the propagule.
By roots. Some roots bear adventitious buds that grow into aerial shoots called root suckers. In sweet potato (Ipomoea batatas), the swollen tuberous roots bear buds and are used to raise new plants. In Dahlia the fasciculated roots sprout only when a piece of the old stem with a bud is attached. Roots of guava, Murraya, Albizzia and Dalbergia produce root suckers naturally, which is why a guava tree is often surrounded by young shoots.
By stems. The greatest variety of propagules comes from stems. Underground stems, rhizome, tuber, corm and bulb, have already been described. Sub-aerial stems are specialised for spreading: the runner of grasses and Oxalis creeps along the surface and roots at nodes; the stolon of strawberry and jasmine arches through the air before touching ground; the offset of water hyacinth and Pistia floats; the sucker of mint, banana and Chrysanthemum grows underground and comes up as a shoot some distance away. Aerial stems propagate too: cut stem pieces of sugarcane, rose, Bougainvillea and Duranta root at the nodes; the bulbils of Agave, Dioscorea and garlic are fleshy buds that drop and grow.
By leaves. In Bryophyllum (sprout leaf plant, Kalanchoe), adventitious buds develop in the notches along the margin of the fleshy leaf. Each bud grows into a tiny plantlet with roots while still on the leaf; the plantlets drop off and take root in the soil. A detached leaf lying on moist soil produces a row of plantlets. In Begonia, buds arise from the cut veins of a leaf placed on wet sand; in Kalanchoe from the leaf margin, and in the walking fern (Adiantum caudatum) the leaf tip touches the ground and roots to form a new plant.
The significance of natural vegetative propagation is that it allows plants to spread and to survive when seed production fails, but it can produce dense, weedy stands. The example the syllabus stresses is Eichhornia, the water hyacinth, but Lantana, Parthenium and Cynodon grass also owe their success partly to vegetative spread. Because the offspring are clones, a disease that kills one plant can kill the whole population, and no new variation arises.
- A Bryophyllum leaf kept on a damp tray shows plantlets with roots at each marginal notch within ten days.
- Sweet potato growers plant pieces of the tuberous root, which sprout from adventitious buds.
- Guava trees put up root suckers around the trunk; gardeners detach and replant them.
- Root propagules: sweet potato, Dahlia (with stem bud), root suckers of guava. Stem propagules: rhizome, tuber, corm, bulb, runner, stolon, offset, sucker. Leaf propagules: Bryophyllum, Begonia, Kalanchoe.
Artificial Vegetative Propagation: Cutting and Layering
Gardeners, farmers and horticulturists deliberately multiply plants from vegetative parts because the method has clear advantages: it is quick, the offspring are exact copies of a desirable parent (a high-yielding, disease-resistant or beautifully flowered variety), it preserves varieties that do not breed true from seed, and it propagates seedless plants such as banana, seedless grapes and some oranges that cannot be raised from seed at all. Plants raised vegetatively also flower and fruit earlier than seedlings. The main artificial methods are cutting, layering, grafting and micropropagation.
Cutting. A cutting is a part of the plant, usually a stem but sometimes a root or a leaf, which is cut from the parent and placed in moist soil or sand where it forms adventitious roots and grows into a new plant. Stem cuttings of 20 to 30 cm with a few nodes are used for sugarcane, rose, Bougainvillea, Hibiscus, Duranta, Croton, grapes and cassava; the lower end is cut just below a node, since roots arise most readily from the node. Root cuttings are used for tamarind, lemon and Ipomoea. Leaf cuttings of Sansevieria and Begonia root from the cut edge. Dipping the cut end in a rooting hormone such as indole butyric acid (IBA) or naphthalene acetic acid (NAA) greatly improves root formation.
Layering. In layering, roots are induced on a stem while it is still attached to the parent plant, so the branch is nourished until it can support itself; only then is it cut off and planted. In simple (ground) layering, a lower, flexible branch is bent down, a small part is buried in the soil (often after a slanting cut or removal of a ring of bark at the buried part) and the tip is left above ground; roots develop at the buried portion in a few weeks, after which the layer is severed (jasmine, strawberry, grapevine, Ixora). In air layering or gootee, used for woody plants whose branches cannot be bent to the ground such as mango, litchi, guava, pomegranate, orange and Ficus, a ring of bark about 2 to 3 cm wide is removed from a healthy branch, the wound is covered with moist moss or sphagnum wrapped in polythene and tied at both ends. Food moving down the phloem accumulates above the ring, and roots emerge into the moss in six to eight weeks; the branch is then cut below the roots and planted. Mound layering and trench layering are variants used in nurseries.
Both cutting and layering reproduce the parent exactly, but they require the plant to form adventitious roots readily; for plants that do not, grafting is used.
- Sugarcane 'setts', pieces of cane with two or three nodes, are laid in furrows; buds at the nodes sprout and roots grow from the node ring.
- A rose gardener takes 20 cm cuttings of hardwood, dips them in IBA powder and roots them in sand.
- Mango is air-layered: a ringed branch wrapped in moist moss and polythene roots in about two months and is then cut and potted.
- Cutting: detached stem/root/leaf piece → adventitious roots in soil → new plant.
- Layering: rooting of a stem while attached to the parent; ground layering (jasmine) and air layering or gootee (mango, litchi).
- Rooting hormones: IBA and NAA promote adventitious root formation on cuttings.
Grafting and Micropropagation
Grafting is the joining of parts of two plants so that they grow as one. The rooted plant that supplies the root system is the stock (rootstock), and the piece of the desired variety that is inserted into it is the scion. The scion is selected for the quality of its fruit or flowers, the stock for its vigour, hardiness, disease resistance and adaptation to the local soil. The two are cut so that their cambium layers are in close contact; the cambium of both then produces callus, the tissues unite, vascular connections form and the scion grows on the roots of the stock. Grafting succeeds only between closely related plants (same species, or same genus, rarely the same family) and only in dicots and gymnosperms, which have a cambium; it is not possible in monocots.
Types of grafting are named after the way the scion and stock are joined: whip or tongue grafting (slanting cuts with a tongue on each, stock and scion of equal thickness), wedge (cleft) grafting (scion cut to a wedge and inserted into a split stock, used in mango and apple), crown grafting (several scions in the bark of a thick stock), side grafting, approach (inarch) grafting (two independently rooted plants joined side by side while both remain rooted, the scion's own roots being cut after union, common in mango and sapota), and bud grafting (budding), in which a single bud with a shield of bark is inserted under a T-shaped cut in the bark of the stock (rose, citrus, rubber). The union is bound with tape and protected with grafting wax.
Grafting is used to propagate plants that do not root easily, to combine the fruit quality of one variety with the vigour or dwarfness of another, to grow several varieties on one tree, and to replace an old variety with a new one.
Micropropagation. This is the modern method of raising large numbers of plants in the laboratory by tissue culture. A small piece of plant tissue, the explant (shoot tip, axillary bud, leaf disc or even a single cell), is surface-sterilised and placed on a sterile nutrient medium containing minerals, sugar, vitamins and the growth regulators auxin and cytokinin, in a culture bottle under controlled light and temperature. The cells divide to form an unorganised mass, the callus, which is induced to form shoots and roots (organogenesis) or somatic embryos, giving plantlets that are hardened and transferred to soil. Because every cell of a plant carries the full genetic information and can regenerate the whole plant, a property called totipotency, thousands of identical plants can be raised from one explant within months. Micropropagation is used for orchids, banana, potato, sugarcane, teak, eucalyptus and ornamental plants, and shoot-tip culture yields virus-free plants because the growing tip is usually free of viruses. Its drawbacks are cost, the need for sterile technique and the same absence of variation as in any cloning.
- A local hardy mango seedling as stock with a scion of the Banganapalli variety produces a tree bearing Banganapalli fruit on tough roots.
- Rose nurseries bud-graft a coloured variety onto a wild Rosa stock; the bud shoots out in three weeks.
- Banana tissue-culture labs in Telangana supply lakhs of uniform, disease-free Grand Naine plantlets to farmers every year.
- Grafting: stock (roots) + scion (desired shoot) joined at the cambium → one plant; possible only in plants with cambium (dicots, gymnosperms).
- Micropropagation: explant → callus on nutrient medium with auxin + cytokinin → plantlets; based on totipotency.
Sexual Reproduction: Meaning and Phases of Life
Sexual reproduction involves the formation of male and female gametes, either by the same individual or by different individuals of opposite sex, and their fusion to form a diploid zygote which develops into a new organism. It is an elaborate, slow and energy-demanding process compared with asexual reproduction, but because the gametes carry different combinations of genes, the offspring show variation, which is important for the survival of the species in a changing environment. Sexual reproduction is found in most plants, in all vertebrates and in most invertebrates, and it is the only mode of reproduction in flowering plants that involves seeds.
All organisms have to reach a certain stage of growth and maturity before they can reproduce sexually. The period of growth before this is called the juvenile phase; in plants it is called the vegetative phase. It varies in length: a few days in annual weeds, about two months in rice or wheat, four to five years in mango, and 10 to 15 years in some fruit trees. The end of the juvenile phase marks the beginning of the reproductive phase, seen in higher plants when they begin to flower.
Plants differ markedly in their flowering behaviour. Annuals (rice, wheat, marigold) and biennials (radish, carrot, cabbage, which grow one year and flower the next) flower once and die. Perennials (mango, coconut, rose) flower every year, or every season, for many years. Bamboo species are remarkable: they flower only once in their lifetime, generally after 50 to 100 years, produce large numbers of fruits and then die. The plant Strobilanthes kunthiana (neelakuranji) flowers once in 12 years; in September and October 2006 it turned the hills of Kerala, Karnataka and Tamil Nadu blue and then the plants died. Flowering that occurs once in a lifetime is called monocarpic, and repeated flowering polycarpic. In animals the reproductive phase shows cyclical changes: the oestrous cycle of non-primate mammals such as cows, sheep and dogs, and the menstrual cycle of primates; some animals breed only in a particular season (seasonal breeders) and others throughout the year (continuous breeders).
The reproductive phase is followed by the senescent phase (old age), marked by slowing of metabolism, loss of vigour and finally death. In plants, the reproductive and senescent phases are often controlled by hormones and by environmental factors such as day length and temperature, which is why farmers can manipulate flowering by controlling irrigation, pruning and lighting.
- Rice sown in June is in its vegetative phase until about the 60th day, flowers in September (reproductive phase) and senesces by October.
- Bamboo clumps in a forest all flower together after decades, fruit heavily and die, an event that can cause rodent population explosions.
- Neelakuranji flowered across the Nilgiri hills in 2006 and 2018, the 12-year interval being a fixed juvenile period.
- Phases of life: juvenile (vegetative) → reproductive → senescent.
- Monocarpic = flowers once and dies (annuals, bamboo); polycarpic = flowers repeatedly (mango, coconut).
Events in Sexual Reproduction and Gametogenesis
Although organisms differ enormously in structure, the events of sexual reproduction follow a regular sequence that can be grouped into three stages: pre-fertilisation events (gametogenesis and gamete transfer), fertilisation (fusion of gametes) and post-fertilisation events (zygote formation and embryogenesis).
Gametogenesis is the formation of the two types of gametes, male and female. Gametes are haploid cells. In some algae the two gametes are so similar in appearance that they cannot be called male and female; they are called homogametes or isogametes (as in Cladophora and some species of Chlamydomonas). In most organisms the two gametes are morphologically distinct, heterogametes: the male gamete is small and motile, the antherozoid or sperm, and the female gamete is large, non-motile and food-laden, the egg or ovum (Fucus, all higher plants and animals).
Sexuality in organisms. Sexual reproduction in organisms generally involves the fusion of gametes from two different individuals, but this is not always so. In plants, the terms bisexual and unisexual are applied to flowers and to the whole plant. Many fungi and plants bear both male and female reproductive structures on one individual and are bisexual, or in flowering plants monoecious (cucurbits, coconut, maize, castor, Chara among algae). Plants that bear male and female structures on separate individuals are unisexual or dioecious (papaya, date palm, mulberry, Marchantia among bryophytes). The words homothallic and monoecious denote the bisexual condition, and heterothallic and dioecious the unisexual condition, the first word of each pair being used for fungi and algae. Among animals, earthworms, sponges, tapeworms and leeches are bisexual (hermaphrodite), while cockroaches, humans and most vertebrates are unisexual.
Cell division during gamete formation. Gametes are always haploid, but the parent body may be haploid or diploid. In haploid organisms such as monerans, fungi, algae and bryophytes, gametes are produced by mitosis. In diploid organisms such as pteridophytes, gymnosperms, angiosperms and most animals, specialised diploid cells called meiocytes (gamete mother cells) undergo meiosis, and one set of chromosomes is passed into each gamete. In flowering plants the microspore mother cells in the anther and the megaspore mother cell in the ovule are the meiocytes, and the pollen grain and the embryo sac represent the reduced, gamete-producing generation. The chromosome number of a meiocyte is therefore twice that of the gamete: 24 and 12 in rice, 42 and 21 in wheat, 20 and 10 in maize, 46 and 23 in humans.
- Cladophora produces isogametes: two identical flagellated cells fuse, so neither can be called male or female.
- In Fucus a large non-motile egg is fertilised by a small motile antherozoid – heterogamy.
- Rice: meiocyte 24 chromosomes, gamete 12; wheat: meiocyte 42, gamete 21; maize: 20 and 10; onion: 32 and 16.
- Sequence: pre-fertilisation (gametogenesis + gamete transfer) → fertilisation (syngamy) → post-fertilisation (zygote + embryogenesis).
- Homogametes (isogametes) = similar gametes; heterogametes = dissimilar antherozoid and egg.
- Haploid parent → gametes by mitosis; diploid parent → meiocyte → gametes by meiosis (chromosome number halved).
Gamete Transfer and Pollination
After gametes are formed, the male and female gametes must be brought together to make fusion possible. This is gamete transfer. In a majority of organisms the male gamete is motile and the female gamete is stationary, so the male gamete has to travel; the exceptions are a few fungi and algae in which both gametes are motile. A medium through which the male gametes move is therefore needed.
Transfer through water. In algae, bryophytes (mosses and liverworts) and pteridophytes (ferns), the flagellated antherozoids swim through a film of water to reach the egg in the female sex organ (oogonium or archegonium), attracted by chemicals secreted by the egg. Because a large proportion of the male gametes fail to reach the egg and are lost, these organisms produce male gametes in several thousand times the number of female gametes. Water is essential; a moss cannot complete fertilisation on a dry day.
Transfer in seed plants. In gymnosperms and angiosperms the male gametes are non-motile (except in a few gymnosperms such as Cycas) and the plants are terrestrial, so water cannot be the medium. The male gametes are carried within the pollen grain, which is transferred from the anther to the stigma. This transfer is pollination. Pollen grains are produced in enormous numbers, are dry, light and often have thick, sculptured walls (the exine) that resist drying, and they are carried by wind, water, insects, birds, bats and other animals. When a compatible pollen grain lands on the stigma it germinates, forming a pollen tube that grows through the style carrying the two male gametes and delivers them to the ovule. The pollen tube thus replaces water as the vehicle of the male gametes, which is one reason seed plants conquered the land.
Pollination may be self-pollination (autogamy), when pollen from an anther is transferred to the stigma of the same flower, as in pea, wheat and rice, or cross-pollination, when pollen goes to the stigma of another flower of the same plant (geitonogamy) or of a different plant of the same species (xenogamy). Xenogamy is the only type that brings genetically different gametes together. In bisexual flowers and in monoecious plants both self- and cross-pollination are possible; in dioecious plants only cross-pollination can occur. The detailed agencies and adaptations of pollination are studied in the next chapter.
In animals, gamete transfer occurs by the male depositing sperm in water near the eggs (frogs, most fishes) or inside the female's body through copulation (insects, reptiles, birds, mammals).
- A moss cushion after rain: antherozoids from the antheridia swim in the water film to the archegonia on the same or neighbouring plants.
- Maize sheds clouds of pollen from the tassel, and the silks (styles) of the cob catch grains carried by wind.
- A bee visiting a mustard field carries pollen from one plant to the stigma of another, achieving xenogamy.
- Gamete transfer: motile male gamete travels to stationary female gamete; medium = water (algae, bryophytes, pteridophytes) or pollen tube after pollination (seed plants).
- Pollination = transfer of pollen from anther to stigma; autogamy (same flower), geitonogamy (same plant), xenogamy (different plant).
Fertilisation (Syngamy)
The most vital event of sexual reproduction is the fusion of the two haploid gametes to form a diploid cell, the zygote. This process is called syngamy or fertilisation. In heterogametic organisms it involves two steps: plasmogamy, the fusion of the cytoplasm of the two gametes, followed by karyogamy, the fusion of their nuclei. The zygote is the first cell of the new generation; it restores the diploid chromosome number that meiosis had halved and combines genes from both parents, which is the ultimate source of variation.
Fertilisation does not take place in all organisms that form gametes. In some, such as rotifers, honeybees, some lizards and birds (turkey), the female gamete develops into a new organism without fusing with a male gamete. This is parthenogenesis. In plants, the development of a seed without fertilisation, called apomixis, is seen in some grasses and in Asteraceae, and a fruit formed without fertilisation is parthenocarpic (banana).
Where fertilisation occurs is a basis for classifying organisms. In external fertilisation the gametes fuse outside the body of the organism, in the surrounding water. This is seen in most aquatic organisms, including the majority of algae, fishes and amphibians. Because the medium is open and the chance of any one sperm meeting an egg is small, such organisms release very large numbers of gametes into the water (synchronised release in many species), and even so a large proportion of the eggs and young are eaten by predators; the offspring are exposed with no parental protection. In internal fertilisation the gametes fuse inside the body of the female organism. The egg is retained inside, the male gamete is motile and is delivered to it, and the number of eggs produced is small, though the number of sperm is still very large. Internal fertilisation is the rule in terrestrial organisms: all fungi that reproduce sexually, bryophytes, pteridophytes, gymnosperms, angiosperms, reptiles, birds and mammals.
In flowering plants, fertilisation is internal and is preceded by pollination. The pollen tube discharges two male gametes into the embryo sac; one fuses with the egg to form the zygote (syngamy), and the other fuses with the two polar nuclei to form the triploid primary endosperm nucleus. Because two fusions occur, the process is called double fertilisation and it is unique to angiosperms; it is described in full in the next chapter. Fertilisation in plants normally takes place several hours to days after pollination, and it is the trigger for all the post-fertilisation changes in the flower.
- A frog lays hundreds of eggs in a pond and the male sheds sperm over them – external fertilisation with heavy losses to predators.
- In a pea flower the pollen tube delivers the male gametes to the egg inside the ovule – internal fertilisation with only a few ovules per flower.
- Honeybee drones develop from unfertilised eggs by parthenogenesis; workers and queens develop from fertilised eggs.
- Syngamy: haploid male gamete (n) + haploid female gamete (n) → diploid zygote (2n); plasmogamy then karyogamy.
- External fertilisation: fusion outside the body in water (algae, fishes, frogs); internal fertilisation: fusion inside the female body (land plants, reptiles, birds, mammals).
- Parthenogenesis = development of the female gamete without fertilisation (rotifers, honeybees, some lizards).
Post-fertilisation Events: The Zygote
The events of sexual reproduction that occur after the formation of the zygote are the post-fertilisation events. They include the development of the zygote into an embryo, and in plants the conversion of the ovule into the seed and the ovary into the fruit.
The zygote. The zygote is the diploid, single-celled beginning of every sexually reproducing organism, the vital link that ensures continuity of the species between one generation and the next. In organisms with external fertilisation it is formed in the external medium (usually water); in those with internal fertilisation it is formed inside the body of the organism, and in plants inside the ovule. Its further development depends on the kind of life cycle and the environment.
In organisms belonging to fungi and algae, the zygote develops a thick wall that is resistant to desiccation and damage, and it undergoes a period of rest before germinating. Such a resting zygote is called a zygospore in Rhizopus and Spirogyra, and an oospore in Fucus and Vaucheria. In these haploid organisms (haplontic life cycle) the zygote is the only diploid stage; it divides by meiosis at germination to form haploid spores which grow into haploid individuals. In organisms with a diploid body (diplontic life cycle, such as Fucus, gymnosperms, angiosperms and animals) the zygote divides by mitosis to develop into an embryo and then the adult. In organisms with a haplo-diplontic life cycle such as bryophytes and pteridophytes, the zygote divides mitotically to form the diploid sporophyte, which later produces haploid spores by meiosis.
The zygote is thus a landmark from which the chromosome number and the genetic constitution of the new individual are fixed. Whether it will develop immediately or rest, and whether its first division is meiotic or mitotic, are the two questions that define the life cycle of the organism. In flowering plants the zygote lies at the micropylar end of the embryo sac, rests for a short time until the endosperm has begun to form, and then divides mitotically to begin embryogenesis.
Notice the economy of nature here: the zygote of a moss, a fern, a pine and a mango is essentially the same kind of cell, a fertilised egg, but the fate of the cell, and the whole design of the organism's life, depend on which division comes first and on how well the zygote is protected. The seed, with its protective coat and stored food, is the culmination of this protection in seed plants.
- Spirogyra zygospore: thick-walled, survives the dry season at the bottom of the pond and germinates by meiosis to give four haploid nuclei, one of which forms a new filament.
- Fucus oospore develops directly by mitosis into a diploid thallus.
- In a wheat ovule the zygote rests for a few hours while the primary endosperm nucleus divides, then begins mitotic divisions to form the embryo.
- Zygote = first diploid cell of the new generation; zygospore (Rhizopus, Spirogyra) and oospore (Fucus) are resting zygotes.
- Haplontic cycle: zygote divides by meiosis; diplontic and haplo-diplontic cycles: zygote divides by mitosis.
Embryogenesis, Seed and Fruit Formation
Embryogenesis is the process of development of the embryo from the zygote. During embryogenesis the zygote undergoes repeated cell divisions (mitosis) and cell differentiation. Cell divisions increase the number of cells in the developing embryo, while cell differentiation helps groups of cells to undergo the modifications that produce specialised tissues and organs, so that a single cell becomes an organised body.
Animals are classified by where the zygote develops. In oviparous animals (reptiles, birds, most fishes and amphibians) the development of the zygote takes place outside the body of the female; fertilised eggs covered with a hard calcareous shell are laid in a safe place and hatch after incubation. In viviparous animals (most mammals, including humans) the zygote develops into a young one inside the body of the female, which is delivered by the mother. Because of proper embryonic care and protection, the chances of survival of the young are greater in viviparous organisms.
In flowering plants the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off; the calyx may persist in some plants (brinjal, chilli, tomato). The pistil, however, remains attached to the plant. The zygote develops into the embryo (with one or two cotyledons), and the triploid primary endosperm nucleus develops into the nutritive endosperm. The ovule develops into the seed, its integuments becoming the seed coat, and the ovary develops into the fruit, the ovary wall becoming the pericarp, which is thick and fleshy in some fruits and thin and dry in others. The fruit protects the seeds and aids their dispersal. On germination the seed's embryo grows into a new plant, completing the cycle.
The changes can be tabulated: ovary → fruit; ovary wall → pericarp; ovule → seed; integuments → seed coat; micropyle → micropyle of seed; funicle → stalk of seed; hilum → hilum; nucellus → usually consumed, sometimes perisperm (black pepper, beet); zygote → embryo; primary endosperm nucleus → endosperm; synergids and antipodals → degenerate.
The seed is a remarkable achievement of sexual reproduction: it packages the embryo with food and a protective coat, allows a resting period (dormancy) through which the plant survives drought or cold, and provides a unit for dispersal to new places and for storage by humans. Seeds also carry the new gene combinations produced by syngamy, so a field of rice raised from seed has the variation that a clone of sugarcane lacks; plant breeders depend on this to develop improved varieties.
- A hen's egg is a zygote developing outside the mother; a calf develops inside the cow and is born alive.
- After pollination of a pea flower the petals fall within days, the ovary elongates into a pod (fruit) and each ovule becomes a pea (seed).
- The green star on a brinjal fruit is the persistent calyx; the fruit itself is the swollen ovary.
- Embryogenesis = zygote → embryo by mitotic cell division + cell differentiation.
- Oviparous: zygote develops outside the female body (birds, reptiles); viviparous: inside the female body (mammals).
- Post-fertilisation in angiosperms: ovary → fruit, ovary wall → pericarp, ovule → seed, integuments → seed coat, zygote → embryo, PEN → endosperm.
Asexual versus Sexual Reproduction: a Comparison
Having studied both modes, it is useful to set them side by side, since 'distinguish between asexual and sexual reproduction' is a standard examination question, and since the advantages of each explain why so many plants keep both.
| Feature | Asexual reproduction | Sexual reproduction |
| Number of parents | One | Usually two (or one bisexual individual) |
| Gametes | Not formed; no fusion | Male and female gametes formed and fuse |
| Cell division | Mitosis only | Meiosis in gamete formation, then mitosis |
| Reproductive units | Whole cell, spores, buds, vegetative propagules | Gametes; zygote; seeds in seed plants |
| Offspring | Genetically identical to parent (clones) | Genetically different from parents and one another |
| Variation | None except by mutation | Present, through recombination |
| Speed and cost | Rapid, simple, economical | Slow, elaborate, energy-demanding |
| Occurrence | Unicellular organisms, simple plants and animals, vegetative propagation in higher plants | Most plants and animals, all seed plants |
| Evolutionary role | Multiplies a successful genotype in a stable environment | Provides variation for adaptation and evolution |
Advantages of asexual reproduction and vegetative propagation. A single individual can multiply; a good variety is preserved exactly; there is no dependence on pollinators or on seed viability; seedless plants can be propagated; new plants are established faster and flower sooner. These are the reasons why farmers grow potato, sugarcane, banana, ginger, turmeric and many ornamentals vegetatively.
Disadvantages. No variation, so no capacity to adapt; a disease or pest that affects one plant affects all; viruses are transmitted through propagules; the stock may degenerate over many generations; and vegetative propagules are bulky and short-lived compared with seeds.
Advantages of sexual reproduction. Variation produced by crossing over during meiosis, by independent assortment and by the union of gametes from different parents gives the population the raw material for natural selection and for plant breeding; seeds are compact, dormant and easily dispersed and stored; harmful mutations can be eliminated. The cost is the elaborate machinery of flowers, pollinators, fertilisation and seed development, and the uncertainty that a desirable parent will not breed true.
Nature's solution, in most plants, is to combine both: reproduce vegetatively to occupy a favourable site quickly, and reproduce sexually to disperse and to generate variation for the future. Hydra, yeast, Spirogyra, strawberry and potato all illustrate this dual strategy.
- All Cavendish bananas in the world are clones; a single fungus (Panama disease) threatens the entire crop because no plant differs from another.
- Wheat varieties raised from seed differ from one another, so a breeder can select a rust-resistant plant among thousands.
- Strawberry spreads by runners in a bed but produces seeds on its fruit for dispersal by birds.
- Asexual: one parent, mitosis, clones, no variation, fast. Sexual: two gametes, meiosis + syngamy, variation, slow.
Key Concepts
- Reproduction
- The biological process by which an organism gives rise to young ones similar to itself, ensuring continuity of the species.
- Life span
- The period from birth to natural death of an organism, which is not correlated with body size.
- Asexual reproduction
- Reproduction by a single parent without gamete fusion, producing genetically identical offspring called clones.
- Clone
- A group of morphologically and genetically identical individuals derived from one parent by asexual means.
- Binary fission
- Division of a parent cell into two equal daughter cells, as in Amoeba, Paramecium and bacteria.
- Budding
- Asexual reproduction by an outgrowth (bud) that separates from the parent, as in yeast and Hydra.
- Zoospore
- A motile, flagellated asexual spore produced by aquatic algae and fungi such as Chlamydomonas.
- Conidium
- A non-motile asexual spore formed externally in chains on a conidiophore in fungi such as Penicillium.
- Vegetative propagation
- Asexual reproduction in plants in which a new plant arises from a root, stem or leaf of the parent.
- Vegetative propagule
- A vegetative unit such as a rhizome, tuber, runner, bulb or leaf bud that grows into a new plant.
- Layering
- An artificial method of propagation in which a stem is induced to root while still attached to the parent plant.
- Grafting
- Joining the scion of one plant to the rooted stock of another so that their cambia unite and they grow as one plant.
- Micropropagation
- Raising large numbers of identical plantlets from a small explant on sterile nutrient medium, based on totipotency.
- Juvenile phase
- The period of growth before an organism becomes capable of sexual reproduction, called the vegetative phase in plants.
- Gametogenesis
- The formation of haploid male and female gametes, by mitosis in haploid parents and by meiosis in diploid parents.
- Heterogametes
- Morphologically distinct male (antherozoid) and female (egg) gametes, as opposed to identical isogametes.
- Monoecious
- A plant that bears both male and female flowers or reproductive structures on the same individual, such as maize.
- Pollination
- The transfer of pollen grains from the anther to the stigma, the means of gamete transfer in seed plants.
- Syngamy
- The fusion of two haploid gametes to form a diploid zygote, also called fertilisation.
- Embryogenesis
- The development of the zygote into an embryo by repeated mitotic divisions and cell differentiation.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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Define life span. Why is it said that life span is not correlated with the size of an organism? / जीवन काल को परिभाषित कीजिए। ऐसा क्यों कहा जाता है कि जीवन काल जीव के आकार से संबंधित नहीं है?
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Life span is the period from birth to natural death of an organism. It varies greatly among species: a butterfly lives one to two weeks, a fruit fly about a month, a rice plant one season, a dog 15 to 20 years, a crow 15 years, a tortoise 100 to 150 years, and a banyan tree several centuries. Life span is not correlated with size because small organisms may live long and large ones may live briefly: a crow is much smaller than a dog but has a comparable life span, a mango tree far smaller than a whale may live longer, and a tortoise, smaller than an elephant, outlives it. The length of life is decided by the species' genetic programme and physiology, not by body size. / जीवन काल किसी जीव के जन्म से प्राकृतिक मृत्यु तक की अवधि है। यह प्रजातियों में बहुत भिन्न होता है: तितली एक-दो सप्ताह, फल मक्खी लगभग एक माह, धान का पौधा एक ऋतु, कुत्ता 15-20 वर्ष, कौआ 15 वर्ष, कछुआ 100-150 वर्ष और बरगद का पेड़ कई शताब्दियाँ जीता है। जीवन काल आकार से संबंधित नहीं है क्योंकि छोटे जीव लंबा और बड़े जीव छोटा जीवन जी सकते हैं: कौआ कुत्ते से बहुत छोटा है पर उसका जीवन काल तुलनीय है, आम का पेड़ व्हेल से बहुत छोटा होकर भी अधिक जी सकता है, और हाथी से छोटा कछुआ उससे अधिक जीता है। जीवन की लंबाई प्रजाति के आनुवंशिक कार्यक्रम और शरीरक्रिया से तय होती है, शरीर के आकार से नहीं।
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Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered a type of asexual reproduction? / अलैंगिक और लैंगिक जनन में अंतर बताइए। कायिक जनन को भी अलैंगिक जनन का एक प्रकार क्यों माना जाता है?
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In asexual reproduction a single parent produces offspring without the formation or fusion of gametes, only mitotic divisions are involved, and the offspring are genetically identical to the parent and to one another (clones), so there is no variation; it is fast and simple and occurs in unicellular organisms, simple plants and animals, and in vegetative propagation of higher plants. In sexual reproduction two gametes, male and female, usually from two parents, are formed by meiosis and fuse in syngamy to form a zygote; the offspring differ genetically from the parents and from one another, so variation is produced; the process is slow and elaborate and occurs in most plants and animals. Vegetative reproduction is considered asexual because a new plant develops from a vegetative part such as a rhizome, tuber, runner or leaf bud by mitotic divisions alone, with a single parent and without any gametes, so the offspring are clones of the parent, which are exactly the features of asexual reproduction. / अलैंगिक जनन में एक ही जनक युग्मकों के निर्माण या संलयन के बिना संतति उत्पन्न करता है, केवल समसूत्री विभाजन होते हैं, और संतति जनक तथा आपस में आनुवंशिक रूप से समान (क्लोन) होती है, अतः विविधता नहीं आती; यह तेज़ और सरल है और एककोशिकीय जीवों, सरल पौधों-जंतुओं तथा उच्च पौधों के कायिक प्रवर्धन में होता है। लैंगिक जनन में प्रायः दो जनकों से नर और मादा युग्मक अर्धसूत्री विभाजन से बनते हैं और युग्मकसंलयन में मिलकर युग्मनज बनाते हैं; संतति जनकों और आपस में आनुवंशिक रूप से भिन्न होती है, अतः विविधता उत्पन्न होती है; यह धीमी और जटिल प्रक्रिया है और अधिकांश पौधों-जंतुओं में होती है। कायिक जनन को अलैंगिक इसलिए माना जाता है क्योंकि नया पौधा प्रकंद, कंद, उपरिभूस्तारी या पत्ती की कलिका जैसे कायिक भाग से केवल समसूत्री विभाजनों द्वारा, एक ही जनक से और बिना किसी युग्मक के बनता है, अतः संतति जनक की क्लोन होती है, जो ठीक अलैंगिक जनन के लक्षण हैं।
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Describe the process of binary fission in Amoeba and budding in yeast. / अमीबा में द्विखंडन और यीस्ट में मुकुलन की प्रक्रिया का वर्णन कीजिए।
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In binary fission the parent Amoeba stops moving and becomes rounded; its nucleus divides by mitosis into two daughter nuclei, and then the cytoplasm constricts in the middle and divides so that two equal daughter amoebae are formed, each with one nucleus, which grow to full size. Under unfavourable conditions Amoeba encysts and later divides by multiple fission to release many small pseudopodiospores. In budding in yeast, a small outgrowth or bud appears on one side of the parent cell; the nucleus divides and one daughter nucleus moves into the bud along with some cytoplasm; the bud enlarges, a wall forms between it and the parent, and it may separate to live independently or remain attached and bud again, forming chains of cells. Budding is thus an unequal division, unlike binary fission. / द्विखंडन में जनक अमीबा चलना बंद कर गोल हो जाता है; उसका केंद्रक समसूत्री विभाजन से दो संतति केंद्रकों में बँटता है, फिर कोशिकाद्रव्य बीच में सिकुड़कर विभाजित होता है जिससे एक-एक केंद्रक वाले दो समान संतति अमीबा बनते हैं, जो पूरे आकार तक बढ़ते हैं। प्रतिकूल परिस्थितियों में अमीबा पुटी बना लेता है और बाद में बहुखंडन से अनेक छोटे कूटपाद बीजाणु मुक्त करता है। यीस्ट में मुकुलन में जनक कोशिका के एक ओर छोटा उभार या मुकुल निकलता है; केंद्रक विभाजित होता है और एक संतति केंद्रक कुछ कोशिकाद्रव्य के साथ मुकुल में चला जाता है; मुकुल बड़ा होता है, उसके और जनक के बीच भित्ति बनती है, और वह अलग होकर स्वतंत्र रह सकता है या जुड़ा रहकर फिर मुकुलन कर कोशिकाओं की शृंखला बनाता है। इस प्रकार मुकुलन द्विखंडन के विपरीत असमान विभाजन है।
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What is vegetative propagation? Describe natural vegetative propagation by stems with examples. / कायिक प्रवर्धन क्या है? तने द्वारा प्राकृतिक कायिक प्रवर्धन का उदाहरण सहित वर्णन कीजिए।
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Vegetative propagation is asexual reproduction in plants in which a new plant develops from a vegetative part of the parent, a root, stem or leaf, without the formation of seeds or spores, the unit being called a vegetative propagule. Stems are the commonest natural propagules. Underground stems store food and bear buds: the rhizome of ginger, turmeric and banana sprouts from nodal buds; the tuber of potato sprouts from its eyes; the corm of Colocasia and Gladiolus from buds on the condensed stem; and the bulb of onion and garlic from axillary buds among its scale leaves. Sub-aerial stems spread the plant: the runner of grasses and Oxalis roots at nodes, the stolon of strawberry and jasmine arches to the ground, the offset of water hyacinth and Pistia floats away with a rosette of leaves and roots, and the sucker of mint, banana and Chrysanthemum comes up from below at a distance. Aerial bulbils of Agave and Dioscorea drop and root, and cut pieces of sugarcane stem root at the nodes. / कायिक प्रवर्धन पौधों में अलैंगिक जनन है जिसमें बीज या बीजाणु बने बिना जनक के कायिक भाग, जड़, तना या पत्ती, से नया पौधा बनता है और उस इकाई को कायिक प्रवर्ध कहते हैं। तने सबसे सामान्य प्राकृतिक प्रवर्ध हैं। भूमिगत तने भोजन संचित करते हैं और कलिकाएँ धारण करते हैं: अदरक, हल्दी और केले का प्रकंद पर्वसंधि की कलिकाओं से अंकुरित होता है; आलू का कंद अपनी आँखों से; अरबी और ग्लैडियोलस का घनकंद संघनित तने की कलिकाओं से; और प्याज-लहसुन का शल्ककंद शल्क पत्तियों के बीच की कक्षस्थ कलिकाओं से। अर्ध-वायवीय तने पौधे को फैलाते हैं: घास और ऑक्सैलिस का उपरिभूस्तारी पर्वसंधियों पर जड़ें देता है, स्ट्रॉबेरी और चमेली का भूस्तारी झुककर ज़मीन छूता है, जलकुंभी और पिस्टिया का अंतःभूस्तारी पत्तियों और जड़ों के गुच्छे के साथ तैरकर दूर जाता है, और पुदीना, केला और गुलदाउदी का अंतःभूस्तारी प्ररोह नीचे से दूर निकलता है। एगेव और डायोस्कोरिया की वायवीय पत्रकंदिकाएँ गिरकर जड़ पकड़ती हैं, और गन्ने के कटे तने के टुकड़े पर्वसंधियों पर जड़ें देते हैं।
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Explain layering and grafting as methods of artificial vegetative propagation. What are their advantages? / कृत्रिम कायिक प्रवर्धन की विधियों के रूप में दाब लगाना और कलम बाँधना समझाइए। इनके क्या लाभ हैं?
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Layering is the induction of roots on a stem while it is still attached to the parent plant. In ground layering a flexible lower branch is bent, a portion is wounded and buried in moist soil with the tip exposed; roots form at the buried part in a few weeks and the rooted layer is cut off and planted (jasmine, grapevine, strawberry). In air layering or gootee, used for woody plants like mango, litchi and guava, a ring of bark is removed from a branch, the wound is wrapped in moist moss and polythene, roots grow into the moss in six to eight weeks, and the branch is cut below the roots. Grafting is the joining of a scion (a shoot piece of the desired variety) to a stock (a rooted plant chosen for hardiness) so that their cambium layers are in contact and unite; types include wedge, whip, approach and bud grafting. Advantages: the offspring are exact copies of a superior parent; plants that root poorly or are seedless can be multiplied; the fruit quality of the scion is combined with the vigour or disease resistance of the stock; the new plants flower and fruit much earlier than seedlings; and several varieties can be grown on one tree. / दाब लगाना जनक पौधे से जुड़े रहते हुए तने पर जड़ें उत्पन्न करना है। भूमि दाब में लचीली निचली शाखा को झुकाकर, एक भाग को घायल कर नम मिट्टी में दबाया जाता है और सिरा बाहर रहता है; दबे भाग पर कुछ सप्ताह में जड़ें बनती हैं और जड़युक्त दाब को काटकर रोपा जाता है (चमेली, अंगूर, स्ट्रॉबेरी)। वायु दाब या गूटी में, जो आम, लीची और अमरूद जैसे काष्ठीय पौधों के लिए प्रयुक्त होती है, शाखा से छाल का छल्ला हटाकर घाव को नम मॉस और पॉलीथीन में लपेटा जाता है, छह-आठ सप्ताह में मॉस में जड़ें बनती हैं और शाखा को जड़ों के नीचे से काट लिया जाता है। कलम बाँधना सांकुर (इच्छित किस्म का प्ररोह टुकड़ा) को मूलवृंत (सहनशीलता के लिए चुना गया जड़युक्त पौधा) से इस प्रकार जोड़ना है कि दोनों की एधा परतें संपर्क में आकर जुड़ जाएँ; इसके प्रकार हैं फन्नी, व्हिप, उपागम और कलिका कलम। लाभ: संतति श्रेष्ठ जनक की सटीक प्रतिलिपि होती है; कम जड़ें बनाने वाले या बीजरहित पौधे बढ़ाए जा सकते हैं; सांकुर की फल गुणवत्ता मूलवृंत की ओजस्विता या रोग प्रतिरोध से जुड़ जाती है; नए पौधे बीजू पौधों से बहुत पहले फूलते-फलते हैं; और एक पेड़ पर कई किस्में उगाई जा सकती हैं।
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What is micropropagation? Explain the principle on which it is based and mention its uses. / सूक्ष्मप्रवर्धन क्या है? यह जिस सिद्धांत पर आधारित है उसे समझाइए और इसके उपयोग बताइए।
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Micropropagation is the production of a large number of genetically identical plants from a small piece of plant tissue, the explant, by tissue culture under sterile laboratory conditions. The explant, such as a shoot tip or axillary bud, is surface-sterilised and placed on a nutrient medium containing minerals, sucrose, vitamins and growth regulators; with the right balance of auxin and cytokinin the cells divide into a callus, which is then induced to form shoots and roots or somatic embryos, and the plantlets are hardened and transferred to soil. The principle is cellular totipotency: every living plant cell carries the complete genetic information of the plant and, under suitable conditions, can divide and differentiate to regenerate a whole plant. Uses: rapid multiplication of orchids, banana, potato, sugarcane, teak and ornamentals; production of virus-free plants by meristem or shoot-tip culture; propagation of rare and endangered plants; year-round supply of uniform planting material; and conservation of germplasm. / सूक्ष्मप्रवर्धन निर्जर्म प्रयोगशाला परिस्थितियों में ऊतक संवर्धन द्वारा पौधे के ऊतक के एक छोटे टुकड़े, कर्तोतक, से बड़ी संख्या में आनुवंशिक रूप से समान पौधे उत्पन्न करना है। प्ररोह शीर्ष या कक्षस्थ कलिका जैसे कर्तोतक को सतह-निर्जर्मित कर खनिजों, सुक्रोज़, विटामिनों और वृद्धि नियामकों वाले पोषक माध्यम पर रखा जाता है; ऑक्सिन और साइटोकाइनिन के उचित अनुपात से कोशिकाएँ विभाजित होकर कैलस बनाती हैं, जिसे फिर प्ररोह और जड़ें या कायिक भ्रूण बनाने को प्रेरित किया जाता है, और पादपकों को कठोरीकृत कर मिट्टी में स्थानांतरित किया जाता है। सिद्धांत कोशिकीय पूर्णशक्तता है: पौधे की हर जीवित कोशिका में पौधे की पूरी आनुवंशिक जानकारी होती है और उपयुक्त परिस्थितियों में वह विभाजित और विभेदित होकर पूरा पौधा पुनर्जनित कर सकती है। उपयोग: ऑर्किड, केला, आलू, गन्ना, सागौन और सजावटी पौधों का तीव्र गुणन; विभज्योतक या प्ररोह-शीर्ष संवर्धन से विषाणु-मुक्त पौधों का उत्पादन; दुर्लभ और संकटग्रस्त पौधों का प्रवर्धन; समान रोपण सामग्री की वर्ष भर आपूर्ति; और जननद्रव्य का संरक्षण।
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Describe the three phases in the life of a plant. What is unusual about the flowering of bamboo and Strobilanthes kunthiana? / पौधे के जीवन की तीन अवस्थाओं का वर्णन कीजिए। बाँस और स्ट्रोबिलैन्थस कुन्थियाना के पुष्पन में क्या असामान्य है?
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The life of a plant passes through three phases. The juvenile or vegetative phase is the period of growth from germination until the plant is mature enough to reproduce; it lasts days in weeds, about two months in rice and several years in fruit trees. The reproductive phase begins with flowering: annuals and biennials flower once and die (monocarpic), while perennials such as mango and coconut flower every year for many years (polycarpic). The senescent phase is old age, in which metabolism slows, vigour is lost and the plant finally dies. Bamboo is unusual because most species flower only once in their lifetime, after 50 to 100 years of vegetative growth, produce enormous numbers of fruits and then die, all the plants of a clump or region flowering together. Strobilanthes kunthiana (neelakuranji) is unusual because it flowers only once in 12 years; in September and October 2006 it flowered in mass, turning large tracts of the hills of Kerala, Karnataka and Tamil Nadu blue, after which the plants died. / पौधे का जीवन तीन अवस्थाओं से गुज़रता है। किशोर या कायिक अवस्था अंकुरण से लेकर पौधे के जनन योग्य परिपक्व होने तक की वृद्धि की अवधि है; यह खरपतवारों में कुछ दिन, धान में लगभग दो माह और फलदार वृक्षों में कई वर्ष चलती है। जनन अवस्था पुष्पन से शुरू होती है: एकवर्षी और द्विवर्षी पौधे एक बार फूलकर मर जाते हैं (एकफली), जबकि आम और नारियल जैसे बहुवर्षी पौधे कई वर्षों तक हर वर्ष फूलते हैं (बहुफली)। जीर्णता अवस्था वृद्धावस्था है, जिसमें उपापचय धीमा होता है, ओज घटता है और पौधा अंततः मर जाता है। बाँस असामान्य है क्योंकि अधिकांश प्रजातियाँ 50-100 वर्ष की कायिक वृद्धि के बाद जीवन में केवल एक बार फूलती हैं, बहुत बड़ी संख्या में फल देती हैं और फिर मर जाती हैं, और एक झुरमुट या क्षेत्र के सभी पौधे एक साथ फूलते हैं। स्ट्रोबिलैन्थस कुन्थियाना (नीलकुरिंजी) असामान्य है क्योंकि यह केवल 12 वर्ष में एक बार फूलता है; सितंबर-अक्टूबर 2006 में यह सामूहिक रूप से फूला और केरल, कर्नाटक तथा तमिलनाडु की पहाड़ियों के बड़े हिस्से नीले हो गए, जिसके बाद पौधे मर गए।
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Explain the terms homogametes and heterogametes, and monoecious and dioecious, with examples. / समयुग्मक और विषमयुग्मक, तथा उभयलिंगाश्रयी और एकलिंगाश्रयी शब्दों को उदाहरण सहित समझाइए।
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Gametes are the haploid sex cells formed in gametogenesis. When the two fusing gametes are so similar in size and form that they cannot be distinguished as male and female, they are called homogametes or isogametes, as in the alga Cladophora and some species of Chlamydomonas. When the two gametes are morphologically distinct, a small motile male gamete or antherozoid and a large non-motile food-storing female gamete or egg, they are called heterogametes, as in Fucus and in all higher plants and animals. A monoecious plant bears both male and female reproductive structures, or both male and female flowers, on the same individual, so it is bisexual: examples are maize, cucurbits, coconut and castor, and the alga Chara; in fungi and algae the same condition is called homothallic. A dioecious plant bears male and female structures on separate individuals, so each plant is unisexual: examples are papaya, date palm and mulberry, and the liverwort Marchantia; in fungi and algae the condition is called heterothallic. / युग्मक युग्मकजनन में बनने वाली अगुणित लिंग कोशिकाएँ हैं। जब संलयित होने वाले दोनों युग्मक आकार और रूप में इतने समान हों कि उन्हें नर-मादा में पहचाना न जा सके, तो उन्हें समयुग्मक या समयुग्मक कहते हैं, जैसे शैवाल क्लैडोफोरा और क्लैमाइडोमोनास की कुछ प्रजातियाँ। जब दोनों युग्मक आकारिक रूप से भिन्न हों, छोटा गतिशील नर युग्मक या पुमणु और बड़ा अचल भोजन-संचित मादा युग्मक या अंड, तो उन्हें विषमयुग्मक कहते हैं, जैसे फ्यूकस और सभी उच्च पौधों-जंतुओं में। उभयलिंगाश्रयी पौधा एक ही व्यक्ति पर नर और मादा दोनों जनन संरचनाएँ या दोनों प्रकार के पुष्प धारण करता है, अतः वह द्विलिंगी है: उदाहरण मक्का, कद्दूवर्गीय, नारियल और अरंडी, तथा शैवाल कारा; कवकों और शैवालों में इसी स्थिति को समजालिक कहते हैं। एकलिंगाश्रयी पौधा नर और मादा संरचनाएँ अलग-अलग व्यक्तियों पर धारण करता है, अतः प्रत्येक पौधा एकलिंगी है: उदाहरण पपीता, खजूर और शहतूत, तथा लिवरवर्ट मार्केंशिया; कवकों और शैवालों में इसे विषमजालिक कहते हैं।
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How is gamete transfer achieved in algae and bryophytes on the one hand and in seed plants on the other? / एक ओर शैवालों और ब्रायोफाइटों में तथा दूसरी ओर बीजी पौधों में युग्मक स्थानांतरण कैसे होता है?
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In algae, bryophytes and pteridophytes the male gametes are flagellated antherozoids and the egg is stationary inside the female organ, so a film of water is essential; the antherozoids swim through water, attracted by chemicals from the egg, to reach and fertilise it. Because most male gametes are lost in the water, these plants produce male gametes in several thousand times the number of female gametes, and fertilisation cannot occur without moisture. In seed plants, gymnosperms and angiosperms, the male gametes are non-motile and water is not the medium; instead they are carried inside the pollen grain, which is transferred from the anther to the stigma (or to the ovule in gymnosperms) by wind, water or animals, the process called pollination. On the stigma the pollen grain germinates into a pollen tube which grows through the style and carries the two male gametes to the embryo sac inside the ovule. The pollen tube therefore replaces water as the vehicle of gamete transfer, which allowed seed plants to reproduce on dry land. / शैवालों, ब्रायोफाइटों और टेरिडोफाइटों में नर युग्मक कशाभिकायुक्त पुमणु होते हैं और अंड मादा अंग के भीतर स्थिर रहता है, अतः जल की परत आवश्यक है; पुमणु अंड से निकले रसायनों से आकर्षित होकर जल में तैरकर उस तक पहुँचते और निषेचित करते हैं। क्योंकि अधिकांश नर युग्मक जल में नष्ट हो जाते हैं, ये पौधे मादा युग्मकों से कई हज़ार गुना नर युग्मक बनाते हैं, और नमी के बिना निषेचन नहीं हो सकता। बीजी पौधों, अनावृतबीजी और आवृतबीजी, में नर युग्मक अचल होते हैं और जल माध्यम नहीं है; इसके बजाय वे परागकण के भीतर ले जाए जाते हैं, जो वायु, जल या जंतुओं द्वारा परागकोश से वर्तिकाग्र (अनावृतबीजियों में बीजांड) तक स्थानांतरित होता है, इस प्रक्रिया को परागण कहते हैं। वर्तिकाग्र पर परागकण अंकुरित होकर परागनली बनाता है जो वर्तिका से होकर बढ़ती है और दोनों नर युग्मकों को बीजांड के भीतर भ्रूणकोष तक ले जाती है। इस प्रकार परागनली युग्मक स्थानांतरण के वाहन के रूप में जल का स्थान लेती है, जिससे बीजी पौधे सूखी भूमि पर जनन कर सके।
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Differentiate between external and internal fertilisation. Which is more advantageous and why? / बाह्य और आंतरिक निषेचन में अंतर बताइए। कौन-सा अधिक लाभदायक है और क्यों?
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In external fertilisation the male and female gametes fuse outside the body of the organism, in the surrounding water; it occurs in most aquatic organisms such as algae, bony fishes and frogs, and because the gametes are released into an open medium the organisms produce enormous numbers of both gametes, often released at the same time, and the eggs and young are left unprotected, so many are eaten by predators. In internal fertilisation the gametes fuse inside the body of the female; the egg is retained inside, the motile male gamete is delivered to it, only a few eggs are produced though sperm are still numerous; it occurs in fungi, bryophytes, pteridophytes, gymnosperms, angiosperms, reptiles, birds and mammals. Internal fertilisation is more advantageous because it does not depend on an external watery medium, so it works on land; the chance of gametes meeting is much higher, so fewer gametes are wasted; and the zygote and embryo are protected inside the body or the ovule, giving the offspring a far greater chance of survival. / बाह्य निषेचन में नर और मादा युग्मक जीव के शरीर के बाहर, आसपास के जल में संलयित होते हैं; यह शैवालों, अस्थिल मछलियों और मेंढकों जैसे अधिकांश जलीय जीवों में होता है, और युग्मक खुले माध्यम में छोड़े जाने से जीव दोनों युग्मक अत्यधिक संख्या में, प्रायः एक साथ, छोड़ते हैं तथा अंडे और शिशु असुरक्षित रहते हैं जिससे कई परभक्षियों द्वारा खा लिए जाते हैं। आंतरिक निषेचन में युग्मक मादा के शरीर के भीतर संलयित होते हैं; अंड भीतर रखा रहता है, गतिशील नर युग्मक उस तक पहुँचाया जाता है, केवल कुछ अंडे बनते हैं यद्यपि शुक्राणु फिर भी अनेक होते हैं; यह कवकों, ब्रायोफाइटों, टेरिडोफाइटों, अनावृतबीजियों, आवृतबीजियों, सरीसृपों, पक्षियों और स्तनधारियों में होता है। आंतरिक निषेचन अधिक लाभदायक है क्योंकि यह बाहरी जलीय माध्यम पर निर्भर नहीं करता, अतः स्थल पर संभव है; युग्मकों के मिलने की संभावना बहुत अधिक होती है, अतः कम युग्मक व्यर्थ जाते हैं; और युग्मनज तथा भ्रूण शरीर या बीजांड के भीतर सुरक्षित रहते हैं, जिससे संतति के जीवित रहने की संभावना कहीं अधिक होती है।
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What are the post-fertilisation changes in a flower? List the fate of each part of the flower after fertilisation. / पुष्प में निषेचन के बाद के परिवर्तन क्या हैं? निषेचन के बाद पुष्प के प्रत्येक भाग की परिणति सूचीबद्ध कीजिए।
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After fertilisation the flower undergoes a series of changes that convert it into a fruit containing seeds. The sepals, petals and stamens wither and fall off, although the calyx persists in brinjal, tomato and chilli. The pistil remains attached. Within the ovule the zygote develops into the embryo by mitotic divisions and differentiation (embryogenesis), and the triploid primary endosperm nucleus develops into the nutritive endosperm. The ovule as a whole becomes the seed, its integuments becoming the seed coat (testa and tegmen), the micropyle remaining as the micropyle of the seed, the funicle as the seed stalk and its scar as the hilum; the nucellus is usually consumed but persists as perisperm in black pepper and beet; synergids and antipodals degenerate. The ovary enlarges into the fruit and the ovary wall becomes the pericarp, fleshy in mango and tomato or dry in pea and mustard. In false fruits such as apple the thalamus also becomes fleshy. The fruit protects the seeds and helps disperse them, and on germination the embryo in the seed grows into a new plant. / निषेचन के बाद पुष्प में परिवर्तनों की एक शृंखला होती है जो उसे बीजयुक्त फल में बदल देती है। बाह्यदल, दल और पुंकेसर मुरझाकर गिर जाते हैं, यद्यपि बैंगन, टमाटर और मिर्च में बाह्यदलपुंज बना रहता है। स्त्रीकेसर जुड़ा रहता है। बीजांड के भीतर युग्मनज समसूत्री विभाजनों और विभेदन से भ्रूण बनता है (भ्रूणजनन), और त्रिगुणित प्राथमिक भ्रूणपोष केंद्रक पोषक भ्रूणपोष बनता है। पूरा बीजांड बीज बनता है, उसके अध्यावरण बीजचोल (टेस्टा और टेग्मेन) बनते हैं, बीजांडद्वार बीज का बीजांडद्वार बना रहता है, बीजांडवृंत बीज का डंठल और उसका निशान नाभिका; बीजांडकाय प्रायः खर्च हो जाता है पर काली मिर्च और चुकंदर में परिभ्रूणपोष के रूप में बना रहता है; सहायक कोशिकाएँ और प्रतिव्यासांत कोशिकाएँ नष्ट हो जाती हैं। अंडाशय बढ़कर फल बनता है और अंडाशय भित्ति फलभित्ति बनती है, जो आम और टमाटर में गूदेदार तथा मटर और सरसों में शुष्क होती है। सेब जैसे कूटफलों में पुष्पासन भी गूदेदार हो जाता है। फल बीजों की रक्षा करता है और उनके प्रकीर्णन में सहायक होता है, और अंकुरण पर बीज का भ्रूण नए पौधे में बढ़ता है।
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Why is the offspring formed by asexual reproduction referred to as a clone? Give the advantages and disadvantages of clonal propagation for a farmer. / अलैंगिक जनन से बनी संतति को क्लोन क्यों कहा जाता है? किसान के लिए क्लोनी प्रवर्धन के लाभ और हानियाँ बताइए।
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In asexual reproduction a single parent produces offspring by mitotic divisions only, without gamete formation or fusion, so every offspring receives an exact copy of the parent's genetic material; the offspring are therefore morphologically and genetically identical to the parent and to one another, and such a group of identical individuals derived from one parent is called a clone. For a farmer, clonal propagation through tubers, setts, suckers, cuttings, grafts or tissue culture has clear advantages: a high-yielding or disease-resistant variety is reproduced exactly, the crop is uniform in size, quality and time of ripening, seedless varieties such as banana and seedless grapes can be multiplied, the plants establish quickly and fruit earlier than seedlings, and no pollinators or seed viability are needed. The disadvantages are equally real: there is no genetic variation, so the entire crop is equally susceptible to a new disease or pest, as happened with Panama disease of banana; viruses are carried from parent to offspring in the propagule; the stock may lose vigour over many generations; propagules are bulky, perishable and costly to transport and store compared with seeds; and no improvement is possible without going back to sexual reproduction for breeding. / अलैंगिक जनन में एक ही जनक युग्मकों के निर्माण या संलयन के बिना केवल समसूत्री विभाजनों से संतति उत्पन्न करता है, अतः हर संतति को जनक की आनुवंशिक सामग्री की सटीक प्रतिलिपि मिलती है; इसलिए संतति जनक और आपस में आकारिक तथा आनुवंशिक रूप से समान होती है, और एक जनक से बने ऐसे समान व्यक्तियों के समूह को क्लोन कहते हैं। किसान के लिए कंद, सेट, अंतःभूस्तारी, कलम, ग्राफ्ट या ऊतक संवर्धन से क्लोनी प्रवर्धन के स्पष्ट लाभ हैं: अधिक उपज देने वाली या रोगरोधी किस्म की सटीक प्रतिलिपि बनती है, फसल आकार, गुणवत्ता और पकने के समय में एकसमान होती है, केला और बीजरहित अंगूर जैसी बीजरहित किस्में बढ़ाई जा सकती हैं, पौधे जल्दी स्थापित होकर बीजू पौधों से पहले फलते हैं, और परागणकर्ता या बीज की अंकुरण क्षमता की आवश्यकता नहीं होती। हानियाँ भी उतनी ही वास्तविक हैं: आनुवंशिक विविधता नहीं होती, अतः पूरी फसल किसी नए रोग या कीट के प्रति समान रूप से संवेदनशील होती है, जैसा केले के पनामा रोग में हुआ; विषाणु प्रवर्ध के माध्यम से जनक से संतति में जाते हैं; कई पीढ़ियों में स्टॉक का ओज घट सकता है; प्रवर्ध बीजों की तुलना में भारी, नाशवान और परिवहन-भंडारण में महँगे होते हैं; और प्रजनन के लिए लैंगिक जनन पर लौटे बिना कोई सुधार संभव नहीं है।