Sexual Reproduction in Flowering Plants
Sexual Reproduction in Flowering Plants – CBSE Notes
for Class 12 Biology
- Sexual reproduction is the process of emulsion of haploid gametes, performing in the product of a diploid zygote, which eventually develops into a new organism. All flowering plants show sexual
- reproduction.
- Flowers are the point of sexual reproduction in flowering plants.
1. A flower has ensuing corridor arranged in four
curls, i.e. calyx (sepals), corolla (petals), androecium and gynoecium. These
are attached to central axis called thalamus.
2. Flowers may contain both manly (stamens) and
womanish (carpels or pistils) reproductive corridor or organs in it and is
called bisexual.
3. In androgynous flowers, only either of the
reproductive corridor are present,e.g. sludge, the tassels represent the manly
flowers (stamens) and the cognizance or silk represent the womanish flower (
styles and smirch).
- · Stamen is the manly reproductive unit of angiosperm.
- ·It consists of the following two corridor
- The long and slender stalk called the hair.
- The terminal generally bilobed structure called the anther.
- Anther is a bilobed structure with each lobe having two theca, thus called dithecous. In a cross section, it's a four sided (tetragonal) structure conforming of four microsporangia, located at the corners, two in each lobe. Microsporangia develop and becomes pollen sacs. Pollen sacs contain pollen grains.
- Structure of microsporangium contains following features in a transverse section
1. Appears nearly indirect in figure.
2. It's girdled by four wall layers. The external
three layers are epidermis, endothecium and middle layers. External three wall
layers are defensive in function and help in dehiscence of anther to release
the pollen. The fourth and inmost subcaste called the tapetum nourishes
developing pollen grains. It contains cells with thick cytoplasm and further
than one capitals.
3. A sporogenous tissue occupies the centre of each
microsporangium in a youthful anther.
4. Each cell of sporogenous tissue suffer meiosis
to form microspore tetrads. Each cell of the tetrad is known as microspore mama
cell.
- Microsporogenesis The conformation of microspores from a pollen mama cell (each cell of sporogenous tissue) through meiosis is called microsporogenesis.
- Microspores are arranged as tetrad. As the anther mature and dehydrates they disconnect from each other and develop into pollen grains. Pollen grains or the manly gametophytes are released by dehiscence of anther.
- Pollen grains have the following characteristic features
•
Generally globular, about 25-50 micrometers in
periphery.
• Mature pollen grain comprises of two layers.
- · External hard subcaste Exine made up of one of the most resistant organic material sporopollenin, that enables them to repel high temperatures and action of strong acids and alkali. Further no enzyme is yet known to degrade sporopollenin, because of which they're well saved as fuds.
- The region on exine where sporopollenin is absent are called origin pores. It helps in the conformation of pollen tube, while the pollen grain germinates on smirch.
- Inner thin, nonstop subcaste Intine made up of cellulose and pectin.A mature pollen grain contains two cells.
1. Vegetative cell or tube cell It's larger as compared to other cell and retain vacuolated cytoplasm which is rich in reserve food, i.e. bounce, protein, fat and cell organelles. The nexus is large and irregular.
2. Generative cell It's lower cell generally spindle- shaped or globular with thin thick cytoplasm and prominent capitals. It divides mitotically to form two non-motile manly gametes, previous to release of pollen grain.
3. In about 60 of angiosperms, pollen grains are
exfoliate at 2-celled stage.
4. In about 40 flowering plants, the generative
cell divides mitotically to give rise to the two manly gametes before pollen
grains are exfoliate at 3-celled stage.
5. Pollen grains of numerous species (e.g.
Parthenium) causes severe antipathetic, habitual respirtory diseases like
asthma, bronchitis, etc.
6. Viability of pollen grains depends on
temperature and moisture.
7. Pollen grains are richer in nutrients and are
used as food supplements in form of pollen tablets and bathos. Its consumption
has been claimed to increase the performance of athletes and race nags.
- Pistil/ Gynoecium It's the womanish unit of flower. A flower may be monocarpellary ( having one pistil) or multicarpellary ( having further than one pistils). Pistils may be syncarpous ( fused together) or apocarpous .
·
The main corridor of pistils are
- Smirch receives pollen grains.
- Style is the elongated slender part beneath the smirch.
- Ovary the bulged part at the base of style.
- It is located inside the ovarian depression or locule. Megasporangia, generally called ovules arise from the placenta. Ovule is attached to the placenta by a stalk called funicle. The number of ovules in an ovary may be one (wheat, paddy and mango) to numerous (papaya, water melon and orchids).
- The main corridor of megasporangium (ovule) are
- Hilum is a junction between ovule and funicle.
- Each ovule has one or two defensive envelopes called integuments.
- Micropyle is an opening present at the tip where integument is absent.
- Chalaza is contrary to the micropylar end representing the rudimentary part of the ovule.
Sexual Reproduction in Flowering Plants – CBSE Notes
for Class 12 Biology
- · The integuments encloses a mass of cells called the nucellus which have food reserves.
- · Embryo sac or female gametophyte is located in the nucellus ( generally one formed from megaspores througth reductional division).
- Megasporogenesis is the process of conformation of megaspores from the Megaspore Mother Cell (MMC). The MMC is a large cell with thick cytoplasm and prominent nexus. It undergoes meiosis performing in the product of four megaspores.
- Development of female Gametophyte Different stages of development of womanish gametophyte are given below:
- One of the megaspores is functional, while the other three degenerate in maturity of angiosperms.
- Only the functional megaspore develops into the womanish gametophyte or embryo sac.This is called monosporic development.
- Nexus of the functional megaspore divides mitotically to form two capitals, which move to the contrary poles forming the 2-nucleate embryo sac.
- Nuclear divisions affect into the conformation of 4-nucleate and latterly 8-nucleate stagesof the embryo sac.
7. Six of the eight capitals are girdled by cell
walls and organised into cells.
8. The remaining two capitals called polar
capitals, are positioned below the egg outfit in the large central cell.
9. Three cells group together at the micropylar end
and constitute the egg outfit.
- The egg outfit consists of two synergids and one egg cell.
- A filiform outfit made of cellular thickenings of synergids at the micropylar end,
- plays an important part in guiding the pollen tubes into the synergid.
- At the chalazal end, three cells are present called antipodals.
- Therefore, a typical angiosperm embryo sac, at maturity is 8-nucleate and 7-celled
Sexual reproduction in flowering plants class 12 ppt
Seed
- · Seed is a fertilized ovule. Integuments of ovule harden to form Seed fleece, still micropyle is still present on the seed fleece.
- · Introductory structure of a seed consists of the following corridor
1.
Seed fleece Hard external covering of the seed
2.
Radicle Root tip which latterly gives rise to
the entire root system
3.
Plumule Shoot tip which latterly gives rise to
the entire shoot system
4.
Cotyledons Seed leaves
- Grounded on the number of cotyledons, there are 2 types of seeds
- · Dicotyledonous seed Seed with 2 cotyledons
- · Monocotyledonous seed Seed with 1 cotyledon
- A dicotyledonous seed has the following important corridor
- · Seed fleece
- External covering of the seed which is composed of 2 layers
- Testa thick external subcaste
- Tegmen thin inner subcaste
Hilum
- Scar on seed fleece through which the seed was attached to the fruit
Micropyl
- A small severance on the seed through which oxygen & water exchange occurs during seed germination
- Cotyledons
- Seed leaves
- Food storehouse
- · Give aliment to the developing radicle & plumule
- · Radicle
- · Embryonic root
Sexual Reproduction in Flowering Plants – CBSE Notes
for Class 12 Biology
Plumule
- · Embryonic shoot
- · A monocotyledonous seed has the following important corridor
- · Seed fleece
- · Fused with pericarp
Endosperm
- · Big part within which lies the small embryo
- · Massive & stiff endosperm
- · Aleurone subcaste
- · Special tissue girding the endosperm
Cotyledon
- · Single cotyledon nominated as Scutellum
- · In contact with endosperm through an epithelial subcaste
Plumule
- Embryonic shoot covered by defensive subcaste Coleoptile
ALSO CHECK:
Sexual reproduction in flowering plants notes
Radicle
- Embryonic root covered by defensive subcaste Coleorrhiza
- Seeds are extremely useful in husbandry. Some of the important operations of seeds are
- Help factory species to be settled in different areas
- Give aliment to youthful seedlings
- Insure protection to the youthful embryo
- Results in variations with new inheritable combinations
- Can be fluently stored for unborn operation
- Long term viability of utmost of the seeds
Sexual Reproduction in Flowering Plants – CBSE Notes
for Class 12 Biology
Apomixis
- Apomixis is a medium to produce seeds without fertilization. This medium produces duplicates, hence can be considered as a form of asexual reproduction.
- Apomixis can do in a number of ways, some of which are mentioned below
- Nucellar cells which are diploid & located outside the embryo sac continuously divide and enter inside embryo sac and latterly develop into embryos. In this way, multiple embryos can live inside one ovule. This is nominated as Polyembryony. It's seen generally in Citrus, mangoetc.
- Occasionally, the egg cell isn't formed as a result of reduction division, hence diploid. This diploid egg cell latterly directly develops into an embryo.
- Apomixis is extremely useful these days because of the following reasons:
- .Clonal reproduction through seeds
- .New mongrels produced in lower time
- Complaint free plants can be produced
- Cost-effective