class 12th biology chapter 1 notes
SEXUAL REPRODUCTION IN FLOWERING PLANTS
About flowers:- the myriads(bunch of flowers) of flowers that we enjoy gazing at,the scents and the perfumes.
the rich colour which attracts as an aid to sexual reproduction.
All flowering plants show sexual reproduction,and the end product of sexual reproduction is, the fruits and seeds.
In this chapter we will understand the morphology, structure and the processes of sexual reproduction in flowering plants(angiosperms).
1.1 FLOWER— A FASCINATING ORGAN OF ANGIOSPERM
human beings have had an intimate relationship with flowers since time immemorial.
flowers are objects of aesthetic,ornamental,social,religious and cultural value; they have always been used as symbols .
flowers conveying important human feelings such as love, affection, happiness, grief, mourning, etc.
flowers are morphological and embryological marvels and the sites of sexual reproduction.
1.2 PRE-FERTILISATION: STRUCTURE AND EVENTS
male
androecium
stamen
anther+filament
female
gynoecium
pistil (stigma
style+ovary)
1.2.1 stamen,microsporangium and pollen grain
the long and slender stalk called filament.
the terminal generally bilobed structure called 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 flower of differen
A typical angiosperm anther is bilobed with each lobe having two theca in simple words it called dithecous.
often a langitudinal groove runs lenghtwise separating the theca.
the bilobed nature of an anther is very distinct is a four-sided (tetragonal) structure consisting of four microsporangia located at the corner,two in each lobe.
the microsporangia develope further and become pollen sacs.
structure of microsporangium:- i
a typical microsporangium appears near circular in outline.
It is generally surrounded by four layers —The epidermis,endothecium,middle layer and the tapetum.
the innermost layer is is tapetum. it nourishes the developing pollen grain.
cells of tapetum have more the one nucleus.
cells of tapetum possess dense cytoplasm..
a group of compactly arranged homogeneous cells
called sporogenous tissue occupies the center of each microsporangium.
Microsporogenesis:-the process of formation of microspore from a pollen mother cell (PMC) through meiosis is called microsporogenesis.
the sporogenous tissue is capable of giving rise to a microspore tetrad.
microspore are arranged in a cluster of four cells—the microspore tetrad.
when the anther will matured or dehydrate the all three layers are automatically destroy.
mother cell always do meosis.
1 sporogenous cell made 4 pollen grain.
each microsporangium several thousands of microspore or pollen grain are formed that are released with the dehiscence of anther.
tapetum give food in developing male gamete.
pollen grain :-
the pollen grain represents the male gametophyte
if you touch the opened anther of HIBISCUS or any other follower you would find deposition of yellowish powdery pollen grain on your fingers.
pollen grains have variety of architecture - size,shapes,colours,design.
pollen grains are generally spherical measuring about diameter 25-50 micrometers in diameter.
#cell wall layers #
outer layer called exine
inner layer called intinw
made up of sporopollenin which is one of the most resistant organic material.
it is thin and continuous layer made up of cellulose and pectin
has prominent apertures called germ pores for transport
no need of germ pore.
when the pollen grain is matured and contain two cells the vagitative cell and generative cell
vagitative cell is bigger,has abundant food reserve and a large irregularly shaped nucleus.
generative cell is small and floats in the cytoplasm of vegitative cell.
Vegitative cell:
vegitative cell is bigger.
abundant food reserve and a large irregularly shaped nucleus
Generative cell:
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% of angiosperms, pollen grain 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 grain are shed(3-celled stage).
*pollen grain are rich in nutrients. in western countries, a large number of pollen products in the form of tablets and syrups are available in the market. pollen consumption has been claimed to increase the performance of athletes and race horses.
pollen viability :- the time or period for which pollen grains remain viable is called pollen viability.
some extent depends on the prevailing temperature and humidity.
in some cereals such as rice and wheat, pollen grain lose viability within 30minitus of their release.
and some members of rosaceae,leguminoseae and solanaceae they maintain viability for months.
such stored pollen can be used as pollen banks for a large no. of species for years in liquid nitrogen
(-196⁰C).
1.2.2.Pistil,Megasporangium
(ovule) and Embryo sac
The GYNOECIUM represent the FEMALE reproductive part of the flower.
The GYNOECIUM consists a single pistil is-monocarpellary
the GYNOECIUM which consists more than one pistil is - multicarpellary
when more than one pistil fused together- syncarpous
may be free-apocarpous
Each pistil has three parts the stigma,style and ovary.
Stigma:-serves as a landing platform for pollen grain.
Style:- elongated slender part beneath the stigma.
Ovary:- the basal bulged part of the pistil.
Megasporangium(Ovule):-
the ovule is small structure attached to the placenta by means of a stalk called funicle.
the body of the ovule fuses with funicle in the region is called hilium
Each ovule has one or two protective envelopes calles integuments.
integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised.
opposite of the micropylar end is the Chalaza.
enclosed within the integuments is a mass of cell called nucellus.
cells if nucellus have abundant food reserve
Megasporogenesis:-the process of formation of megaspores from the megaspore mother cell is called megasporogenesis.