Chapter 2 - SEXUAL REPRODUCTION IN FLOWERING PLANTS

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.

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

(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.