NEET Biology Notes Reproduction in Plants
This is a biological process by which an organism produces another organism (offspring] similar to itself.
Plants exhibit several modes of reproduction, i.e. asexual and sexual reproduction. Most of the angiosperms exhibit sexual reproduction and only a few show vegetative propagation.
Reproduction in Plants
Plants reproduce asexually, vegetatively and sexually. Asexual reproduction occurs mainly by fragmentation, fission, budding, spores and by vegetative method. Sexual reproduction in plants occur by the fusion of egg and sperm.
It is uniparental and it involves various methods such as gemmules, sporulation, binary fission, budding, fragmentation and vegetative propagation in plants.
The various modes of asexual reproduction are as follows:
Gemmules are also known as propagules. These arise as modified branches. They are rich in food and germinate into new plant on detachment from the parent. These are present in plants mainly, e.g. Marchantia.
The process formation of spores is known as sporulation. It is. found in algae, fungi, bryophytes and pteridophytes.
- Binary Fission
It is the process of division of parent into two daughters of equal size. During binary fission, the cell elongates and its nucleus divides into two daughter nuclei. A transverse wall is formed in the centre of parent cell, dividing it into two daughter cells, which later on separate and lead independent lives. This method of reproduction is the characteristic feature of some yeasts algae and bacteria.
In this process, the parent cell produces one or more bud like protuberances, which detach from parent cell and grow into new individual.
Such type of reproduction is very common in budding yeast (Saccharomyces), where chain of buds may produce pseudomycelium.
It is the process of breaking down of phrent individual into small pieces or fragments accidently of through external force. Each piece of fragment develops into a new individual.
This type of reproduction is very common in lower forms like algae, fungi and lichens. Under unfavourable conditions, fragmentation is a very common method of reproduction. ‘
- Vegetative Propagation
It is the ability of plants to reproduce without sexual reproduction by producing new plants from existing vegetative structure.
Only mitotic divisions are involved in vegetative reproduction.
It takes place by following methods
- Cutting The small part of any plant organ (stem/root/leaf) used for its propagation is called cutting, e.g. stem cutting in sugarcane, grapes, cocoa, rose, Bougainvillea, etc.
- Layering A ring of the hark is removed from lower branch and bent down. This part is covered by soft soil. After 2-3 months, root develop and the branch is cut off to grow independently, e.g. lemon, grapes, strawberry, etc.
- Grafting In grafting a plant (scion) is inserted into another plant (stock), so as to be nourished by it and united with it. Grafting is especially successful in dicotyledonous plants, where stem has cambium layer, e.g. citrus, mango, rose, apple, etc.
- By leaves It is found in species of Bryophyllum (plantlets develop along the margins of intact leaves), e.g. Begonia, Saintpaulia, etc.
- Tissue culture In this case, new plants are grown by removing tissue or cells from the growing tip of a plant and placed in an artificial medium, where they divide and form a small group of cells of callus. After this the callus is transferred to another medium for growth and differentiation than the plantlets are placed in the soil, so that plantlets can grow into mature plant.
- Vegetative propagation Also takes place by runners, e.g. Cynodon (doob grass), Oxcdis, etc., bulbs e.g. Alium cepa, Tulipa, etc., rhizomes e.g. Zingiber, Marsilea, etc., offset, e.g. Eichhomia, Pistia and by roots, e.g. Murraya sp., Albizzia lebbek and Dalbergia sissoo.
- Air layering A ring of bark is removed from on aerial shoot. It is covered by grafting clay ( water, clay, cow, dung, etc.) After 1-2 months roots appear and the shoot is removed to be used for planting, e.g. litchi, pomegranate.
- Ordinary roots of Dalbergia sisso, guava, etc., can develop adventitious buds whjehgrow to form new plants.
- The culturing of cells or tissues results in the formation of an undifferentiated mass of cells, called callus, which later differentiates to produce a large number of plantlets.
Sexual reproduction involves the formation of an offspring through the fusion of egg and sperm by the process of fertilisation. Sperm cells and egg cells are known as gametes and formed as a result of meiosis. In flowering plants, sperms are produced inside the germinating pollen grains of anthers, whereas egg is produced inside an embryo sac in ovule.
- Sexual Reproduction in Algae and Fungi
Sexual reproduction involves plasmogamy (fusion of protoplasts of male and female cells), karyogamy (fusion of male and female nuclei forming zygotic nucleus) and subsequently meiosis to convert diploid structure to haploid spores.
- Sexual Reproduction in Bryophytes
Sex organs in bryophytes are multicellular and are covered by multicellular sterile jacket. The male sex organ is the antheridium. The antheridium is covered by a sterile jacket and encloses a mass of androcytes. The androcytes give rise to biflagellate, motile antherozoids. The female sex organ is the archegonium, which is a multicellular flask-shaped structure differentiated into swollen venter and elongated neck. The venter encloses the egg or oosphere. On maturity, the egg fuses with the antherozoid and forms a diploid zygote. The zygote by repeated cell divisions gives rise to embryo which forms a sporophytic plant.
- Sexual Reproduction in Pteridophytes
The male sex organ is antheridium which produces antherozoids. The female sex organ is flask shaped archegonia which give rise to egg. The egg fuses with the antherozoid and forms the diploid zygotes. The zygote develops directly into a sporophyte by mitotic divisions.
- Sexual Reproduction in Gymnosperms
Gymnosperms are heterosporous, which produce two types of spores, i.e. microspores (pollen grains) and megaspores (embryo sac). They may be monoecious (Pinus) or dioecious (Cycas and Ginkgo). They produce microspores in microsporangia born on microsporophylls.
The megasporangia are produced on megasporophylls. The megasporophylls constitute the female cone.