Biology Notes For Class 12 | Chapter 2 - SEXUAL REPRODUCTION IN FLOWERING PLANTS - Important Notes From NCERT Text Book.
Chapter 2 - SEXUAL REPRODUCTION IN FLOWERING PLANTS
- Flowering plants (angiosperms).
- All flowering plants show sexual reproduction.
- End products of sexual reproduction are fruits and seeds.
FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS
- Flowers are morphological and embryological important.
- The flower is the sites of sexual reproduction.
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| A diagrammatic representation of L.S. of a flower |
PRE-FERTILISATION: STRUCTURES AND EVENTS
- Before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place so before.
- Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium.
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- Inflorescences are formed which bear the floral buds and then the flowers.
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- In the flower, the male and female reproductive structures, the androecium and the gynoecium differentiate and develop.
- The androecium consists of a whorl of stamens representing the male reproductive organ.
- The gynoecium consists of pistil/pistils represents the female reproductive organ.
Stamen, Microsporangium and Pollen Grain
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| (a) A typical Stamen; (b) A Three-Dimensional Structure of an Anther |
- A typical stamen has two parts – the long and slender stalk called the filament, and the terminal generally bilobed structure called the anther.
- The proximal end of the filament is attached to the thalamus or the petal of the flower.
- The number and length of stamens are variable in flowers of different species.
Anther:
- A typical angiosperm anther is bilobed with each lobe having two theca, i.e., they are dithecous.
- Often a longitudinal groove runs lengthwise separating the theca.
- The bilobed nature of an anther is very distinct in the transverse section of the anther.
- The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.
- The microsporangia develop further and become pollen sacs.
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- They extend longitudinally all through the length of an anther and are packed with pollen grains.
Structure of microsporangium:
- In a transverse section, a typical microsporangium appears near circular in outline.
- It is generally surrounded by four wall layers - the epidermis, endothecium, middle layers and the tapetum.
- The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen.
- The innermost wall layer is the tapetum.
- Tapetum nourishes the developing pollen grains.
- Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.
Microsporogenesis:
- When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the center of each microsporangium.
- As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
- Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad i.e. Each cell of sporogenous tissue is a potential pollen or microspore mother cell (PMC).
- The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
- The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad.
- As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.
- Inside each microsporangium, several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.
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A dehisced anther
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Pollen grain:
- The pollen grains represent the male gametophytes.
- There is a variety of architecture – sizes, shapes, colors, designs – seen on the pollen grains from different species.
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| Scanning electron micrographs of a few pollen grains |
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- Pollen grains are generally spherical measuring about 25-50 micrometers in diameter.
- Pollen grain has a prominent two-layered wall.
- The hard outer layer of pollen grain is called the exine.
- The inner wall of the pollen grain is called the intine.
Exine:
- It is made up of
sporopollenin which is one of the most resistant organic material
known.
- Sporopollenin can withstand high temperatures and strong acids and alkali.
- No enzyme can degrade sporopollenin.
- Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent.
- Pollen grains are well preserved as fossils because of the presence of sporopollenin.
- The exine exhibits a fascinating array of patterns and designs.
- can withstand high temperatures and strong acids
Intine:
- Inner wall of the pollen grain.
- It is a thin and continuous layer made up of cellulose and pectin.
