Biology Notes For Class 12 | 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.

• 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.

A dehisced anther

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.

Scanning electron micrographs of a few pollen grains

• Pollen grains are generally spherical measuring about 25-50 micrometers in diameter.

• Pollen grain has a prominent two-layered wall.

1. The hard outer layer of pollen grain is called the exine.
2. 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.

• The cytoplasm of pollen grain is surrounded by a plasma membrane.

• When the pollen grain is mature it contains two cells, the vegetative cell and generative cell.
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Stages of microspore maturing into a pollen grain

• The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus.

• The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.

• In over 60 percent of angiosperms, pollen grains are shed at this 2-celled stage.

• 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).

• Pollen grains of many species cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders asthma, bronchitis, etc.

• Parthenium or carrot grass that came into India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy.

• Pollen grains are rich in nutrients.

• Many people use pollen tablets as food supplements now-a-days.

Pollen Products

• Pollen consumption has been claimed to increase the performance of athletes and racehorses.

• When once pollens are shed, pollen grains have to land on the stigma before they lose viability if they have to bring about fertilisation.

• The period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperature and humidity.

• In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release.

• While, in some members of Rosaceae, Leguminoseae and Solanaceae, they maintain viability for months.

• It is possible to store pollen grains of a large number of species for years in liquid nitrogen (-1960C). Such stored pollen can be used as pollen banks, in crop breeding programmes.

The Pistil, Megasporangium (ovule) and Embryo sac:

• The gynoecium represents the female reproductive part of the flower.

• The gynoecium may consist of a single pistil (monocarpellary) or may have more than one pistil (multicarpellary).

• When there are more than one, the pistils may be fused together (syncarpous) or may be free (apocarpous).

(b) Multicarpellary, syncarpous pistil of Papaver; (c) A multicarpellary, apocarpous

gynoecium of Michelia.

Each pistil has three parts the stigma, style and ovary.

A dissected flower of Hibiscus showing pistil (other floral parts have been removed)

• The stigma serves as a landing platform for pollen grains.

• The style is the elongated slender part beneath the stigma.

• The basal bulged part of the pistil is the ovary.

• Inside the ovary is the ovarian cavity (locule).

• The placenta is located inside the ovarian cavity.

• Arising from the placenta are the megasporangia, commonly called ovules.

• The number of ovules in an ovary may be one (wheat, paddy, mango) to many (papaya, water melon, orchids).

The Megasporangium (Ovule):

A diagrammatic view of a typical anatropous ovule

• The ovule is a small structure attached to the placenta by means of a stalk called funicle.

• The body of the ovule fuses with funicle in the region called hilum, it represents the junction between ovule and funicle.

• Each ovule has one or two protective envelopes called integuments.

• Integuments encircle the ovule except at the tip where a small opening called the micropyle is organized.

• Opposite the micropylar end, is the chalaza, representing the basal part of the ovule.