sexual reproduction in flowers (ncert) class 12 exam 2024/2025
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Sexual reproduction in flowers class 12
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Sexual Reproduction In Flowers Class 12
sexual reproduction in flowers (ncert) class 12 exam 2024/2025
define floriculture - ANSWERSthe cultivation of flowers
describe the pre-fertilization changes (3 points) - ANSWERS1. several hormonal and structural changes are initiates which lead to the differentiation and further developmen...
sexual reproduction in flowers (ncert) class 12 exam 2024/2025
define floriculture - ANSWERSthe cultivation of flowers
describe the pre-fertilization changes (3 points) - ANSWERS1. several hormonal and structural changes are initiates which lead to the differentiation and further development of the floral primordium 2. inflorescences are formed which bear the floral buds and then flowers 3. androecium and gynoecium differentiate and develop
parts of a stamen - ANSWERS1. long and slender stalk called filament 2. terminal generally bilobed structure called the anther 3. the proximal end of the filament is attached to the thalamus or the petal of the flower 4. number and length of stamens are variable in flowers of different species
transverse section of anther reveals - - ANSWERS1. a typical angiosperm anther is bilobed with each lobe
having 2 theca - dithecous 2. longitudinal groove separating the theca 3. bilobed nature of an anther is very distinct in the transverse section of the anther 4. the anther is a four-sided (tetragonal) structure consisting of 4 microsporangia locates at the corners, two in each lobe
structure of microsporangium (in transverse section) - ANSWERS1. appears near circular in outline 2. generally surrounded by 4 wall layers - epidermis, endothecium, middle layers, tapetum 3. outer 3 layers perform the function of protection and help in dehiscence of anther to release the pollen 4. when the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the centre of each microsporangium tapetum? - ANSWERS1. the innermost wall layer of microsporangium 2. nourishes the developing pollen grains 3. cells of the tapetum possess dense cytoplasm and generally have more than one nucleus
explain the process of microsporogenesis - ANSWERS1. As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads
2. As each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad. Each one is a potential pollen or microspore mother cell
3. The microspores, as they are formed, are arranged in a cluster of four cells-the microspore tetrad. 4. As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains 5. Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther
define microsporogenesis - ANSWERSThe process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis
structure of pollen grains - ANSWERS1. represent the male gametophyte 2. Pollen grains are generally spherical measuring about 25-50 micrometers in diameter.
3. It has a prominent two-layered wall. 4. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known. 5. Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent
6. The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin.
7. The cytoplasm of pollen grain is surrounded by a plasma membrane. 8. When the pollen grain is mature it contains two cells, the vegetative cell and generative cell .
9. The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. 10. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with
dense cytoplasm and a nucleus.
how are pollen grains well preserved - ANSWERSsporopollenin differentiate between 2-cell and 3-cell stage - ANSWERS1. In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage.
2. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
what are the allergies caused by pollen grains - ANSWERSchronic respiratory disorders - 1. asthma 2. bronchitis
what is Parthenium - ANSWERScarrot grass that came into India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy
benefits of pollen grains - ANSWERS1. Pollen grains are rich in nutrients and are used as food supplements.
2. In western countries, a large number of pollen products in the form of tablets and syrups are available in the market. 3. increase the performance of athletes and race horses
4. pollen banks in crop breeding programmes
how long can pollen grains retain their viability - ANSWERS1. the period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperatures and humidity 2. cereals like rice and wheat - pollens lose viability within 30 minutes of their release 3. Rosaceae, Leguminoseae, Solanaceae - viable for months 4. it is possible to store pollen grains of a large number of species for years in liquid nitrogen (-196 degrees) 5. such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes
what is meant by pollen viability - ANSWERSwhen once they are shed, grains have to land on the stigma before they lose viability if they have to bring about fertilization
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