Chapter 2
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Sexual reproduction is the process of fusion Of haploid gametes resulting in production of a deployed zygote,which ultimately develops in to a new organisms.all flowering plants show sexual reproduction.
(1) Flower- Are the site of sexual reproduction in Flowering plant.
I) A flower has following part arranged in four .who is calyx (sepals) Corolla (petals) Androecium and gynoecium these are attached to Central axis called thalamus .
II) Flower may contain both male and female reproductive part or organ in it and is called bisexual .
III) In unisexual flowers only either of the representative part are present . Example corn,the tassels represent the male flower and the ears of silk represent the female flower (Styles and Stigma).
(2) Stamen- is the male reproductive unit of angiosperms.
It contains of the following two part .
The Long and slender stalk called the filament .
The terminal generally bilobed structure Called the Anther .
(3) Anther - is a bilobed structure with each lobe having two theca ,there for called dithecous.
in a cross section ,it is four sides (tetragonal) structure consisting four microsporangia , located at the corner two in each lobe . Microsporangia developed and become pollen sacs . pollen sacs contain pollan grain .
Structure of microsporengia contain following features in a transverse section.
Appears nearly circuler in outline.
It is surrounded by four wall layer .the outer three layer Epidermis ,endothecium ,and middle layer .outer three layer are protective in function and help in Dehiscence of anther to release pollen .the fourth and innermost layer are called tapetum nourishes developing pollen grain .it contains cell with dense cytoplasm and more than one nuclei.
A sporogenous tissue occupies a center of each microsporangium in young anther .
Each cell of sporogenous tissue undergo meiosis to form microspore tetrads .Each cell of tetrads is known as microspore mother cell.
(4) Microsporogenesis - the formation of microspores from a pollen. Mother cells through meiosis is called Microsporogenesis .
I) microspore are arranged as tetrads .As the anther mature and dehydrated they dissociat from each other and develop into pollen grain . pollen grain or the mail. Gametophyte are released by Dehiscence of anther .
II)pollen grains have the following characteristics features.
Generally spherical 25-50 micrometer in diameter .
Mature pollen grain comprises two layer.
(a) Outer hard layer exine made up of one of the resistant organic material. Sporopollenin that enable them to resis high temperature and action of strong acid and alkali . further no enzymes is yet knows to degrade Sporopollenin , because of witch they are well preserved as fossils .
The region of exine where Sporopollenin is absent are called germ pores .it help in the formation of pollen tube while the pollen grain germinate on stigma .
(b) Inner thin continuous layer intine made up off cellulose and pectin .
A mature pollen grain contain two cells .
(a) Vegetative cell or tube cell it is larger as compared to other cell and possess. Vacuolated cytoplasm which in rich in reserve food example- starch , protein ,fat and cell organelles .the.nucleous is large and irregular .
(b) Generative cell it is smaller cell usually spindle shaped or spherical with thin dense cytoplasm and prominent nuclei . Its divided mitotically to form two non motile male gamete prior to release pollen grain.
III) In about 60 % angiosperms pollen grain are shed of two celled stage.
IV) In about 40% flowering plants the generative cell divide mitotically to give rise to the two male gametes before pollen grain are shed at 3 cell stage.
V) pollen grain are many species causes severe allergic , chronic respiratory disorder like asthma ,bronchitis.etc.
VI) viability of pollen grain depends temperature and humidity.
VII) pollen grain are richer in nutrition and are used as food supplements in form of pollen tablet and syrup.it consumption has been claimed to increase to performance of athletes and race horses.
5) Pistil/ gynoecium - it is the female unit of flower .A flower may be monocarpellary (having one pistil) or multicarpellery(having more than one pistil) pistil may be synocarpous (fused together) and apocarpous (free).
The main part of pistil are :
Stigma receive pollen grain.
Style is the elongated slender part beneath the stigma.
Ovary the bulged part at the base of style.
Placenta is located inside the ovarian cavity or locule. megasporangia commonly called ovules arise from the placenta by a stalk called the funicle .the number of ovules in an ovary may be one ( wheat ,paddy ,mango) to many ( papaya , watermelon ,orchids ).
The main part of megasporengium are :
Hilum is a junction between ovule and funicle.
Each ovule has two or more protective envelope called integument.
Microphyl is an opening presents at the tip where integument is absent .
Chalaza to the opposite to the micropylar and the representing the basal part of ovules.
The integument enclose a mass of the cell called nucellus which has food reserve.
Embryo sac or female gametophyte is located in the nucellus .
6) Megasporogenesis - is the process of formation of megaspores mother cell .MMC .the MMC is a large cell with dense cytoplasm and prominent nuceleus.it undergo .miosis resulting in the production of four megaspores .
7) Development of female gametophyte different stage of development of female gametophyte are given below-
One of the megaspores is functional while the other three degenerate in majority of angiosperms .
Only the functional megaspores are develops into the female gametophyte or embryo sac. This is called megasporic development.
Nucleus of the functional megaspores divides mitotically to form two nuclei witch move to the opposite poles forming the 2 nucleate embryo sac.
Nuclear division result into the formation of 4 nucleat and later 8 nucleat stage of the embryo sac.
Six of the eight nuclei are surrounded by cell wall and organised into cell. the remaining two nuclei called poler nuclei are situated below the egg appratuss in the large Central cell .
Three cell group together at the micropylar end and constitutes the egg appratuss.
The egg appratuss consist of two synergids and one egg cell.
A filiform appretus made of cellular thicking of synergids at the . micropylar end play an important role in guiding the pollen tube into the synergids.
At the chalazal and three cell are present called antipodeals .
Thus a typical angiosperms embryo sac,at maturity is 8 nucleated and ,7 celled .
We hope CBSE / MP Board Class 12th Biology Chapter 2 "Reproduction in Flowering Plants" will help you.
Written By- Priyanka Sharma