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CHSE, ODISHA
BIOLOGY
CLASS –XII
CHAPTER – 1 REPRODUCTION IN ORGANISMS ASEXUAL REPRODUCTION
In asexual method of reproduction, the reproductive units are called spores (motile (which can move) or non motile) . Sometimes, the vegetative parts of the organisms get separated accidentally and grow into a new organism. This method is called vegetative propagation. No specialized reproductive units are formed. Asexual reproduction is very common among lower plants, animals and particularly in the kingdoms of Monera, Protista and Fungi. Most common types of asexual reproduction are - fragmentation, fission, budding, gemmule formation, sporulation etc. Fragmentation This type of vegetative propagation occurs in lower plants belongs to algae, fungi, bryophytes as well as in many highly developed angiosperms. In animals such types of reproduction occurs in lower animals like sponges, Hydra, etc. In this process fragmented or broken parts develops into new organisms. Accidental breakage may occur due to wind, water current or injury caused by animals . Fission Bacteria, certain unicellular algae etc. propagate by cell division. In this process, the nucleus and cytoplasm grows and divide into two equal parts. This is called binary fission. It is the main form of reproduction in bacteria. Other organism like Amoeba, Paramaecium and Euglena also show binary fission. In favourable conditions an Amoeba enlarges in size and withdraws its pseudopodia. It gets ready for cell division. As a result of mitosis, two cells are produced which will grow into independent organisms. Cells of certain algae like Stanieria divide into more than 2 cells and each part can develop to form new organisms. This is called multiple fission. Under unfavourable conditions Amoeba shows different type of fission. It withdraws its pseudopodia and encloses itself by 2 to 3 layered strong envelope .This process is called encystment. During this period its nucleus divides repeatedly to form a large number of nuclei. Each nucleus remains surrounded by some amount of cytoplasm. In suitable condition the cyst break and premature daughter Amoeba called Amoebula get released.( multiple fission.) Malaria causing Plasmodium also propagates by multiple fission. Budding : The unicellular fungus, yeast generally propagates by this method. Outgrowths develop at the peripheral region of the mother cell called buds. Into this, nucleus along with cytoplasm from the mother cell migrate. It remains attached to the mother cell by a narrow neck. Gradually, it loses contact from mother cell and grows into a new organism. Some other unicellular organisms and also multicellular ones like Sponge and Hydra reproduce by budding. These are of two types(a) external buds - This method of budding occurs when environmental conditions are suitable and enough nutrients are available. Initially, tiny swellings called buds develop
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in the median region of Hydra body. This gradually enlarges and at the terminal region of bud mouth tentacles develop. This is the Hydra in its infant stage which get separated from mother cell and develops into a new Hydra . (b) internal buds – When conditions become unsuitable and there is lack of the nutrient supply, sponge forms internal buds called gemmules. In the process, special types of cells called archaeocytes get aggregated. A hard coat is formed around it and the structure is now called gemmule. It remains open outside by small pore called micropyle. In suitable condition the gemmules come out through micropyle and new sponges are formed. Sporulation This is a very general process of reproduction in organisms like algae and fungi. Most of the algae produce motile asexual reproductive units called zoospores. This can float freely in water with its cilia or flagella. Example - Ulothrix. The zoospores endogenously borne inside the structures known as sporangia and are liberated from it when become mature. Lower fungi like Saprolegnia reproduce by zoospores . In higher fungi like Aspergillus, Penicillium possess nonmotile structures called conidia .These are exogenously borne (outside the body) and can be easily spread by external agent like air or water. VEGETATIVE PROPAGATION IN PLANTS When the plant is growing vegetatively, any part of it - root, stem or leaf may get separated from this plant. If this separated part can grow into new individuals it is called vegetative propagation. NATURAL METHODS 1. By roots : Tap or adventitious roots of sweet potato, Dahlia etc. become thick, swollen due to storage of food .The adventitious buds are borne on such structures. The buds produce leafy shoots, called slips. When such roots with adventitious buds are planted in the soil, they produce new plants and thus, vegetative propagation occurs. 2. By stems : The stems are efficient means of vegetative propagation. This may be of the following two types. (a) Subaerial stems : Subaerial stems may develop as lateral(side) branches from the mother plant. This may break up from the parent plant and then, grow into new plants. Ex - Runners (Oxalis), sucker - (banana, Chrysanthemum), stolon (Jasmine), offset (Eichhornia) . (b) Underground Stems : In certain plants the underground stems become modified for storage of food during the active phase of the growth. Ex - Rhizome (Ginger), tuber (Potato), bulb (Onion) and corm (Colocasia ) 3. By leaves : The fleshy succulent leaves of Bryophyllum bear adventitious buds in their notches located in the margins. When the leaves fall on moist soil, these buds develop into small plants completing the process of vegetative propagation. ARTIFICIAL METHODS :
4. Cuttings : In this method a piece of stem up to a suitable length is taken from the parent plant. This stem piece is called the cutting. It should have few nodes and internodes. The cutting is planted in moist soil. After sometime, roots arises from the nodes of the basal portions of the cuttings and the upper buds give rise to the shoot. The plants commonly grown by this
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method are China rose, sugar cane, Bougainvillea etc. 5. Grafting : In this process, a detached part of one plant (i.e. small branch or bud) is inserted into the stem or root system of another plant . This is called scion (short piece of detached shoot containing several buds) and the latter stock (lower portion of the plant which is fixed to the soil by its root system). The scion becomes part of the plant into which it is grafted. The new plant developed bears flowers and fruits having characters of scion. Mango, rose, orange, guava etc. are generally propagated by grafting. 6. Layering : In this method roots are artificially induced to grow on the branches before they are detached from the parent plant. There are three types of layering : (i) Serpentine layering : Branches at the lower portions of the stem are put in the soil at many places to form new plants from them. (ii) Mound layering : Soft lower branches are selected and a ring of epidermal layer is removed. This part is then covered in the soil with the apical portion remaining outside. After an interval of time, adventitious roots develop. Then the ringed portion is cut off to allow new plant grow independently. (iii) Air layering : Here, a ring of bark is removed from the aerial branches. It is then covered by grafting clay (water, clay, cow dung) with a little amount of root inducing promoter. The entire portion is wrapped with polythene bandage. At a particular time interval, roots are developed and when separated it can grow into a new plant. Ex Pomegranate, orange, lemon etc.
Significance of Vegetative Propagation: This is a quick method of reproduction where survival rate is very high. Endangered or threatened plant species can be saved by such propagations. Plants with desirable qualities may be developed by this method. The vegetative method of reproduction among angiosperms has a lot of agricultural and horticultural applications. MICROPROPAGATION : This method of plant propagation is different from the conventional procedure since it is carried out in aseptic condition and requires an artificial nutrient medium. A small plant cutting or explant (usually axillary bud) is sterilized and inoculated into culture vessel containing semi-solid nutrient medium. The inoculated culture vessel is incubated at room temperature. In a short span of time, a large number of shoots develop from the axillary buds by a process called axillary bud proliferation. Each growing point is then sub-cultured to give rise to shoot .This phenomenon is called adventitious shoot formation. Each shoot is stimulated by auxin to develop roots. The new plantlet is then transferred to field. Advantages of Micropropagation Disease free healthy plants can only be obtained . Rapid rates of multiplication . Development of plant materials with desired quality and their maintenance in a small space can be done. The desired genetic gains achieved through plant breeding can be multiplied several fold on an economic and rapid time scale. Examples : Mass propagation of teak, Eucalyptus, sandal wood, different species of bamboo and many other trees has been successfully done.
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commercial harvesting of medicinal plants such as Dioscorea ,Atropa, Rauwolfia serpentina etc. Some species of potato, Cassava, sugarcane and banana are severely affected by viruses. Yields of these crops can be increased significantly by planting disease free stocks.
Word Meaning Colocasia – ସାରୁ Chrysanthemum- ସସବତୀ ଫୁ ଲ Jasmine – ମଲ୍ଲୀ underground stem – ମାଟିଭିତସର Bryophyllum – ଅମରସ ାଇ China rose- ମନ୍ଦାର sugar cane – ଆଖୁ Bougainvillea - କାଗଜ ଫୁ ଲ Detached - ଅଲଗା କରାଯାଇ ଥିବା aerial branches - ମାଟି ଉ ରକୁ ଥିବା ଶାଖା axillary bud – side buds in a branch teak – ଶାଗୁଆନ Eucalyptus – ନୀଳଗିରି sandal wood - ଚନ୍ଦନ
Prepared By : Sukanta Sarkar, PGT(Botany) GHSS (SSD) Dabugam, Nabarangpur
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Prepared By : Sukanta Sarkar, PGT(Botany) GHSS (SSD) Dabugam, Nabarangpur
Mobile No :7873772090
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Prepared By : Sukanta Sarkar, PGT(Botany) GHSS (SSD) Dabugam, Nabarangpur
Mobile No :7873772090
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Prepared By : Sukanta Sarkar, PGT(Botany) GHSS (SSD) Dabugam, Nabarangpur
Mobile No :7873772090
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Prepared By : Sukanta Sarkar, PGT(Botany) GHSS (SSD) Dabugam, Nabarangpur
Mobile No :7873772090