Chapter 2nd of Class12 Biology

Read and learn note of biology subject

Chapter
Sexual Reproduction in Flowering Plants

Pre-Fertilisation Events
● Several hormonal and structural changes result in the
development of a flower.
● Inflorescences bear the flower buds, and then the flowers.
● Flowers are the reproductive parts of a plant.
● In the flowers, the androecium (male reproductive part) and
the gynoecium (female reproductive part) develop.

See figure in ncert Figure 2.1

Androecium
● The androecium consists of whorls of stameJn.
● The stamen consists of the filament ​(long and slender stalk) and anther​ (bilobed structure).
● Filament is attached to the thalamusI or to the petal.
● Anther​:
○ A typical anther is bilobed and each lobe is dithecous
(consists of two theca).
○ Theca are separated by a longitudinal groove running
lengthwise.
○ The microsporangia are located at the corners, two in
each theca. They further develop to form pollen sacs,
which contain the pollen grains.

See figure in ncert Figure 2.2

● Structure of microsporangium
○ The microsporangium is surrounded by four wall layers
(epidermis, endothecium, middle layers, and tapetum).
○ The outer three layers are protective and help in
dehiscence of anther to release the pollen grains. The
tapetum provides nourishment to the developing pollen
grains.
○ In the young anther, the sporogenous tissue forms the
centre of each microsporangium.

See figure in ncert Figure 2.3

Learn short trick for learn atomic number/group name  of  the periodic table elements by one click

1.You read also Short tricks For Learn Periodic Table In Easy Way Period 1 To Period 14 by one click
2.You also read Short tricks For Learn Periodic Table In Easy Way  period 15 to 18  with transitonal metal and also with Lanthanide & Actinide elements 

Learn short trick for learn atomic number/group name  of  the periodic table elements by one click

*. Read also We have another way to Learn Periodic table in Hindi Group wise of S- block & P - block elements :
*.Read also We known another way to Learn Periodic Table In Hindi
Of D - block elements

Microsporogenesis
● It is the process of formation of microspore from PMC (Pollen
Mother Cells).
● As development occurs in the anther, the sporogenous
tissue undergoes meiosis to form microspore tetrad.
● Each cell of sporogenous tissue has capacity to give rise to a
tetrad. Hence, each cell is a potential pollen or PMC.
● As the anther matures, the microspores get detached from
each other and develop into pollen grains.

Pollen grains
● Represent the male gamete and are spherical, having a
two-layered wall:
○ Exine (outer) − Hard layer made of sporopollenin,
which is extremely resistant and can withstand high
temperatures, acidic and alkaline conditions, and
enzymes
○ Intine (inner) − Thin and continuous layer made up of
cellulose and pectin
● Mature pollen grain contains two cells:

○ Vegetative cell − Large with irregular nucleus, contains
food reserves
○ Generative cell − Small and floats in the cytoplasm of
the vegetative cell

See figure in ncert Figure 2.5

● In 60% of the angiosperms, pollen grains are shed at
2-celled stage while in others generative cell undergoes
mitosis to form two male gametes (3-celled stage).
● The viability of pollen grains after they are shed depends
upon temperature and humidity. It ranges from 30 minutes
to few months.

Gynoecium and Formation of Female Gametophyte
● The gynoecium represents the female reproductive part of a
flower.
● It may be mono-carpellary (one pistil) or multi-carpellary
(many pistils). In multi-carpellary, the pistils may be fused
in one (syncarpous) or free (apocarpous).
● Each pistil consists of:
○ Stigma​ − Receives the pollen grains
○ Style​ − Elongated, slender part below the stigma
○ Ovary​ − Bulged basal part containing the placenta,
which is located inside the ovarian locule (cavity)
○ The placenta contains the megasporangia or ovules.

See figure in ncert Figure 2.7

Megasporangium
● The ovule is attached to the placenta by the funicle​. The junction of the ovule and the funicle is called hilum​.
● Each ovule has one or two protective layers, called
integuments​, which cover the rest of the ovule, except for a small opening called micropyle​.
● The chalaza​ lying on the opposite side of the micropyle end represents the basal part of the ovule.
● Nucellus​ is present within the integuments and contains reserved food. The embryo sac​ or female gametophyte is located within the nucellus.

Megasporogenesis
● The megaspore mother cell (MMC) g ​ ets converted into megaspores by the process of megasporogenesis.
● The MMC is large and contains a dense cytoplasm and aprominent nucleus. It undergoes meiosis to produce fourmegaspores.

Female Gametophyte
● In most flowering plants, only one megaspore is functionalwhile the other three degenerate.
● The single functional megaspore develops into the femalegametophyte. This kind of development is called monosporic
development.
● The nucleus of the functional megaspore divides mitoticallyto form 2 nuclei, which move towards the opposite ends,forming a 2-nucleate embryo sac. Two more mitotic divisionsensue, leading to the formation of 4-nucleate and 8-nucleate
embryo sacs.
● After the 8-nucleate stage, the cell walls are laid down andthe typical female gametophyte (embryo sac) gets
organised.
● Six of the 8-nuclei get surrounded by the cell wall and theremaining two, called polar nuclei​, are situated below theegg apparatus in the large central cell​.
● Three of the six cells are placed at the micropylar end andconstitute the egg apparatus (2 ​ synergids + 1 ​ egg cell​).
● The synergids have special thickenings at the micropylarend. These are together called the filiform apparatus​. It
helps in leading the pollen tubes into the synergids.
● Three cells are at the chalazal end, and are called antipodalcells​.
● A typical angiosperm female gametophyte is 7-celled and8-nucleated at maturity.

See figure in ncert Figure 2.8

Read & Learn also this Physics Subjects  Article .
* Read & Learn Some Physics Formulas and Definitions In Simple Way Of Force and Motion Chapter 
* You Read and Learn also Some Physics Formulas And Definitions In simple way of electricity chapter
* You also Learn some Physics Formulas And Definitions In Simple Way About Light Chapter
* You also Read & Learn some Physics Formulas And Definitions In Simple Way  Of  SI Unit & constant

Pollination
● It is the process of transfer of pollen grains from the anther to the stigma.
● Depending on the source of pollen, pollination can be divided as follows:
○ Autogamy ​− It is the transfer of pollen grains from the
anther to the stigma of the same flower. Autogamy
requires the anther and the stigma to lie close. It also
requires synchrony in the pollen release and stigma
receptivity.
Plants like Viola, Oxalis, etc., produce two kinds of
flowers—chasmogamous flowers (​ with exposed
anther and stigma) and cleistogamous flowers
(which do not open at all and only autogamy occurs).
○ Geitonogamy​ − It is the transfer of pollens from the
anther of one flower to the stigma of another flower in
the same plant. Genetically, it is similar to autogamy,
but it requires pollinating agents.
○ Xenogamy​ − It is the transfer of pollen grains from
the anther to the stigma of a different plant. Pollination
causes genetically different types of pollens to be
brought to a plant.
Agents of Pollination
● Plants use air, water (abiotic agents) and animals (bioticagents) for pollination.
● Pollination by wind
○ It is the most common form of abiotic pollination.
○ Plants possess well-exposed stamens and large,
feathery stigma.
○ Pollens should be light and non-sticky to be carried
easily by winds.
○ Wind-pollinated flowers often have single ovule in the
ovary and numerous flowers packed in an inflorescence.
○ It is common in grass.
● Pollination by water
○ It is rare in flowering plants, except for some aquatic
plants like Vallisneria and Hydrilla.
○ In most water-pollinated plants, the pollen grains are
long and ribbon-like, and are protected from wetting by
mucilaginous covering.
○ In a majority of water plants like water hyacinth and
water lily, flowers emerge above the water level and
are pollinated by insects.
● Pollination by animals
○ Majority of flowering plants use butterflies, bees, wasps etc., for pollination.
○ Most of the insect-pollinated flowers are large,
colourful, fragrant, and contain nectar to attract the
animal pollinators. These are called floral rewards.
○ Floral reward can be in the form of providing safe
places to lay eggs (example: the tallest flower,
Amorphophallus)
○ A symbiotic relationship exists between the plant,
Yucca and its pollinator moth. The moth is dependent
on the plant since the moth deposits its eggs in the
locule of the ovary of the plant, and in return, the plant
is pollinated by the moth.
○ The pollen grains are sticky and get stuck to the body
of the pollinator.
Out Breeding Devices
● Repeated self pollination leads to inbreeding depression.
● Plants have developed methods to prevent self pollination.
Autogamy is prevented by following ways:
○ Pollen release and stigma receptivity not coordinated
○ Different positioning of the anther and the stigma
○ Production of unisexual flowers
● Ways to prevent both autogamy and geitonogamy:
○ Presence of male and female flowers on different
plants, such that each plant is either male or female
(dioecy).
○ This mechanism is present in several species of papaya.
Pollen−Pistil Interactions
● Pollination does not always ensure the transfer of compatiblepollens.
● Hence, the pistil has the ability to recognise the right type ofpollen to promote post- pollination events.
● If the pollen is of the wrong type, the pistil prevents pollengermination.
● This interaction is mediated by chemical components of thepollen and the pistil.
● Pollen−pistil interaction is a dynamic process involvingpollen recognition, followed by promotion or inhibition of thepollen.
● The pollen tube reaches the ovary and enters the ovulethrough the micropyle. Then, through the filiform apparatus,it reaches synergids. In this way, the pollen tube grows.

Artificial Hybridisation & Double Fertilisation

Artificial Hybridisation
● It is a method to improve crop yield.
● In this method, it is essential to ensure that the right kindsof pollen grains are used, and the stigma is protected fromunwanted pollen grains. It is achieved by:
○ Emasculation − The anther is removed from the bud ifthe female parent bears bisexual flowers.
○ Bagging − The emasculated flower is covered by a bagso as not to allow contamination of the stigma by
unwanted pollen grains.
● When the stigma of the bagged flower becomes receptive,the collected pollen grains are dusted onto the stigma, andthen the flower is rebagged.
● If the female parent is unisexual, emasculation is not
necessary. In this case, the female bud is directly bagged,and when the stigma turns receptive, suitable pollen grainsare dusted onto it so as to allow germination.
Double Fertilisation
● When the pollen grains fall on the stigma, the pollen tube
enters one of the synergids and releases two male gametes.
● One of the male gametes moves towards the egg cell and
fuses with it to complete the syngamy​ to form the zygote​.
● The other male gamete fuses with the two polar nuclei and
forms triploid primary endosperm nucleus (PEN).​ This is termed as triple fusion​.
● Since two kinds of fusion—syngamy and triple fusion—takeplace, the process is known as double fertilisation, and ischaracteristic of flowering plants.
● After triple fusion, the central cell becomes the primaryendosperm cell (PEC).
● The primary endosperm nucleus gives rise to the
endosperm, while the zygote develops into the embryo.

See figure in ncert Figure 2.13

Read also this Bank Helpfull Article

* You know also Local Sbi Bank Customer Care Toll free and Contact Number & Email Id by one click:
* You know also State Bank OD India State-wise SBI Customer Care Support Numbers:
* You also read How Check Bank Account Balance through SMS and Miss Call

Post-Fertilisation Events

It includes development of endosperm and embryo, andmaturation of ovules into seeds and ovaries into fruits.
Formation of Endosperm
● The endosperm develops before the embryo because thecells of the endosperm provide nutrition to the developingembryo.
● The primary endosperm nucleus repeatedly divides to giverise to free nuclei. This stage of development is called freenuclear endosperm.
● Cell wall formation occurs next, resulting in a cellular
endosperm.
● The endosperm may be either fully consumed by the
growing embryo (as in pea and beans) or retained in themature seed (as in coconut and castor).

Development of Embryo

● The embryo develops at the micropylar end of the embryosac where the zygote is situated.
● The zygote gives rise first to the pro-embryo, and then tothe globular, heart-shaped, mature embryo.
● A typical dicot embryo​ consists of an embryonal axis andtwo cotyledons.
● The portion of the embryonal axis above the level of
cotyledons is called epicotyl. It contains the plumule (shoottip). The portion below the axis is called hypocotyl. Itcontains the radicle (root tip). The root tip is covered by theroot cap.

See figure in ncert Figure 2.14 (a)

● In a monocot embryo,​ there is only one cotyledon. In
grass, it is known as the scutellum, and is situated at oneside of the embryonal axis. At its lower end, the embryonalaxis has the radicle and the root cap enclosed in the coleorrhiza.
● The epicotyl lies above the level of the scutellum, and hasthe shoot apex and leaf primordia enclosed in hollowstructures called coleoptiles.

See figure in ncert Figure 2.14 (b)

पढ़िए कुछ खास जानकारी वाले आर्टिकल सिर्फ एक क्लिक से
* जाने तुरंत12th के बाद किन्हे बना सकते है आप करियर ऑपशन्स
* जानिये कौन सी गाड़ी किस शहर की है। एक बहुत महत्वपूर्ण मैसेज।
उत्तर प्रदेश के शहरों के व्हीकल यूनिक नंबर:-
* जाने कबीर दास का जीवन परिचय 
* यदि हो पेट में गैस प्रॉब्लम तो क्या करे जाने आसान तरीके
* जाने कुछ वैज्ञानिक तर्क आपके साथ होने वाली घटनाओ में।
* पेट गैस से तुरंत राहत पाने के असरदार टिप्स
* कभी नहीं होगा चेचक अगर करेंगे ये रामबाण उपाय.

Seeds and Fruits
Development of Seeds
● It is the last stage of sexual reproduction in angiosperms.
● Seeds are the fertilised ovules that are developed inside a fruit.
● A seed consists of:
○ Seed coat
○ Cotyledons
○ Embryonal axis
● Seeds may be albuminous​ (endosperm present; as in
wheat and maize) or non-albuminous​ (endosperm absent;since it is consumed by the growing embryo; as in pea andbeans).
● Some seeds such as black pepper and wheat have remnantsof nucellus known as perisperm​.
● The integuments of ovules harden to form the seed coat,and the micropyle facilitates the entry of oxygen and waterinto the seed.
● As it loses moisture, the seed may enter dormancy, or iffavourable conditions exist, it germinates.
Development of Fruits
● The ovary of a flower develops into a fruit.
● The walls of the ovary transform into the walls of the fruit(pericarp).
● Fruits may be fleshy, as in mango and orange, or can bedry, as in groundnut and mustard.
● In some plants, floral parts other than the ovary take part infruit formation, as in apple and strawberry. In these, thethalamus contributes to fruit formation. Such fruits are called false fruits​. Fruits that develop from the ovary are called true fruits​.
● Some fruits develop without fertilisation, and are known as parthenocarpic fruits​ (example: banana).
Apomixis and Polyembryony
● Some plants produce seeds without fertilisation. This process of seed formation is known as apomixis.
● Apomixis is a form of asexual reproduction mimicking sexual reproduction.
● In some species, apomixis occurs as the diploid egg cell isformed without meiosis, and develops into embryo withoutfertilisation.
● In some varieties of citrus and mango, the nucellus cellsdivide and protrude into the embryo sac to develop intoembryos. In such cases, each ovule may contain severalembryos and this condition is called polyembryony.
● Apomixis is important for producing hybrid varieties of fruitsand vegetables, and also for increasing crop yield multifold.

You also read
Read Chapter 1st Of Class 12th Reproduction in Organism

Please do the comment on this article
For read more article
If this article is helpful
So write on comment 👉 Yes
If this article is helpless
So write on comment 👉 No

For contact to me search at Facebook
    Maddheshiya Ji

Check out Maddheshiya Ji And follow on twitter (@Maddheshiyaji):

📝 Written by
     Maddheshiya Ji

<script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<script> (adsbygoogle = window.adsbygoogle || []).push({
google_ad_client: "ca-pub-4409819426646995",
enable_page_level_ads: true
});
</script>