NCERT Solutions Class 12 Biology Chapter 2
NCERT Solutions Class 12 Biology Chapter 2 – Sexual Reproduction in Flowering Plants
The NCERT Solutions Class 12 Biology Chapter 2 is about sexual reproduction in flowering plants. The subject of Biology revolves around learning about various organisms, their functions, anatomy, physiology etc.. It assists students in getting intrinsic knowledge about the life systems of these organisms. NCERT Solutions Class 12 Biology Chapter 2 presents a detailed and in-depth description of the sexual reproduction process in flowering plants. The study materials prepared by the Extramarks on Sexual reproduction in flowering plants have been crafted by the experts at Extramarks as per the new and updated syllabus to help students secure good grades in the examination.
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Key Topics Covered In NCERT Solutions Class 12 Biology Chapter 2
Sexual Reproduction in Flowering Plants:
- Sexual reproduction is the procedure by which new organisms are formed by fusing male and female gametes from both parents.
- The flower is the critical reproductive structure.
- Sporophylls are categorised into microsporophylls or stamen and megasporophylls or carpel.
- A carpel is an ovary that consists of an ovule, a style, and a stigma.
- There are three kinds of stamen: filament, Anther, and connective.
- Stamen is eminent as filament, Anther and connective.
Sexual reproduction in flowering plants can be fragmented into three types:
iii) Double fertilisation.
Structure and Events:
The below pre-fertilisation events can be studied:
i) Pollen grain formation.
ii) Pollen pistil interaction E.
iv) embryo sac formation.
2. Pollen Grain Formation:
Male reproductive unit (Stamen):
- A stamen is an angiosperm’s male reproducing unit. It is made of an anther and a filament.
- Several pollen grains are contained in every pollen sac. A dithecous anther’s four pollen sacs are placed in the four corners.
- Dithecous Anther: An anther with two lobes related by a non-sporangial tissue called the connective.
- The anther wall is made of four layers of cells.
- To free pollen grains, Anther dehiscence through slits.
- The growth of an anther starts with a mass of homogeneous meristematic cells surrounded by an epidermis.
- Four lobes are created.
- Archesporial cells: A group of primitive cells that split into two types of cells: a primary parietal cell and a primary sporogenous cell.
- The parietal cell divides multiple times to form the anther wall, whereas a sporogenous cell divides less frequently to create microspores.
- The tapetum is the inside layer of the cell wall that comes across contact with the PMCs. In pollen generation, the tapetum plays a crucial role.
- Tapetum: This is a tissue built within the Anther that feeds the growing spores.
- The endothelium is the layer beneath the epidermis.
Wall Layers of Anther:
The epidermis is one layer of cells that serves as a defensive layer.
Endothecium is a single-layered following wall. Cells are thickened alongside cellulose and a trace of lignin and pectin. It helps in the dehiscence of anthers.
Ranges from 1-6. When the Anther develops, the middle layer degenerates.
a) The anther wall’s innermost layer surrounds the sporogenous tissue.
b) The ubisch bodies settle in the exine of the microspore wall.
c) They have multiple nuclei and are polyploid.
d) Tapetal cells contain nutrients.
e) There are two kinds of tapetum:
(ii)Amoeboid / periplasmodial.
Microsporogenesis is the creation and differentiation of microspores.
- PMCs go through meiosis. Each forms tetrahedral tetrads.
- Cytokinesis can be either sequential or simultaneous.
- The cell wall is created in successive types after meiosis –I and meiosis –II, succeeding in an isobilateral pollen tetrad. Monocots own a distinctive trait.
- In the similar kind, each nuclear division in a microspore mother cell is followed by forming a cell wall.
Tetrads are Classified into five types:
The most common shape is Tetrahedral.
A mass of pollen grains formed on each anther lobe. The overall abundance of pollen grains is t as a unit when the pollinium is connected to pollinating agents such as insects.
- Pollen grains arrive in all forms and sizes. The pollen grain owns a haploid, unicellular body with a single nucleus. It has a double-layered exterior wall.
- It is mostly round and has a diameter of 25 – 30m.
- The wall, or sporoderm, is made of a couple of layers.
- The outer layer is quite thick. It is called the exine. It is composed of sporopollenin.
- The inner wall is thin and is known as the intine. It is composed of pectin-cellulose.
- The tectum aids in pollen grain identification and categorisation by family, genus, or species.
- Pollen grains are classified as monocolpate with one germ pore, bilobate or
- with two germ pores, tricolpate or three germ pores.
- The study of pollen is termed palynology.
Development of Male Gametophyte:
- Inside the pollen grain, the nucleus develops in size. It breaks up mitotically to give rise to two unequal daughter cells: a giant vegetative cell or tube cell and a smaller generative cell.
- Pollination can occur when the pollen grain is two-celled (generative +tube )or three-celled (two male gametes + tube ).
- The generative cell’s cytoplasm contains tiny conserved food material, while the vegetative cell’s cytoplasm includes carbohydrate, fat, and protein granules.
- Pollen supplements:
The pollen grain is high in carbohydrates and unsaturated fat. They are taken in the form of tablets and syrups to improve essential body functions. Pollen consumption boosts production and is utilised by athletes as well as racehorses.
- Pollen creams:
Pollen grains give UV protection. Therefore, they are utilised in creams and emulsions to give skin protection and smoothness.
- Pollen viability indicates the amount of time pollen grains remains viable or functional.
- For 30 minutes, pollen grains are viable.
- It is estimated by temperature and humidity.
- Pollen grains cause severe allergies. It causes fever and general respiratory disorders such as bronchitis and asthma.
- Carrot grass (Parthenium hysterophorus) is a crucial allergen source.
Female Reproductive Unit (Pistil):
- The female reproducing unit of a flower is the pistil or gynoecium.
Stigma: The part of the body that acquires pollen grains.
- A carpel or pistil comprises three parts: the stigma, the style, and the ovary.
- Ovary: A swollen place at the base of the ovary. One too many ovules are found in the ovary.
- The stalk that relates the stigma to the ovary is the style.
- The ovule is a megasporangium revolved by integuments. The ovule is full-grown into a seed after fertilisation. It is whitish and oval.
- Hilum: The point at which the funicle is linked to the ovule.
- Funiculus: The stalk that connects the ovule and the placenta.
- Fusing the funiculus with the physique of the ovule results in creating a raphe.
Using only one integument. It can be located in higher dicots such as Compositae and gymnosperms.
Ovules have two integuments. It can be built in monocots and primitive dicots such as the Malvaceae and Cruciferae.
Three integuments, as in Asphodelus.
No integument. Santalum, Ziriosoma, Loranthus, and Relax are examples of this.
Forms of Ovule:
- Orthotropous (Erect):
The ovule’s body is linear and lies directly over the Hilum, funicle, chalaza, and micropyle. All share a similar phylogenetic line.
- Anatropous (Inverted):
The ovule’s body is inverted. The ovule and the funicle are connected. The raphe is created by the fusion of the ovule and the funicle. The funicle is near Hilum and Micropyle. The chalaza is found on the opposite end of the micropyle. It is the most normal ovule type.
The ovule body is at a straight angle to the funicle, as in Malpighiaceae.
The physique is curved, but the embryo sac is direct.
The ovule bodies and the embryo sac are curved, as in crucifers.
The ovule revolves at a greater than 360 degrees angle, resulting in the funicle coiling around the ovule.
- Megasporogenesis refers to the procedure of producing megaspores from megaspore mother cells.
- The MMC goes through meiosis to generate four megaspores.
- Ovules generally form a single megaspore mother cell (MMC) in the nucellar micropylar region. It’s a giant cell with many cytoplasms and a prominent nucleus.
- Monosporic development refers to forming an embryo sac from a single megaspore.
- Only the functional megaspore can grow up into a female gametophyte.
- In the majority of flowering plants, only one of the megaspores is energetic. The other three have devolved.
Formation of Embryo SAC:
- Mitosis happens in the nucleus of the functional megaspore, resulting in the formation of two nuclei.
- There are two additional sequential mitotic nuclear divisions.
- Cell wall growth does not take place immediately after nuclear division.
- Cell walls are set up after the eighth nucleate stage.
- They are located in the substantial central cell beneath the egg apparatus.
- Three cells are located together at the micropylar end. They make up the egg apparatus.
- The egg apparatus comprises two synergids and one egg cell.
Pollination is the shift of pollen grains from the Anther to the flower’s stigma. Pollination can be categorised into two distinctive types: cross-pollination and self-pollination.
- Self-pollination indicates the transfer of pollen grains from anthers to stigmas of the same or varied flowers on the same plant. Flowers in self-pollination are genetically similar.
Self-pollination is of two kinds:
The shift of pollen grains from the Anther to the stigma of the matching flower. It is preferred because of the below adaptations:
- Chasmogamous apparatuses
It is the movement of a pollen grain from one flower’s Anther to the stigma of another flower of the same or genetically similar plant.
Benefits of Self Pollination:
- It retains the race’s purity.
- The plant does not have to produce a vast number of pollen grains.
- Self-pollination eliminates undesirable recessive traits.
- It ensures seed production.
Self-pollinated plants have many disadvantages, as shown below:
- Vitality declines, eventually leading to degeneration.
- Variable, Which reduces adaptability to changing environments.
- It is defined as the shift of pollen grains from an anther of one plant to the stigma of another plant of the same or different species. It is also referred to as allogamy.
- Pollination takes place in Xenogamy between two flowers of plants that are genetically and ecologically distinct.
Flowers are categorised into males and females. Plants could be either dioecious or monoecious.
It takes place when the Anther and stigma mature at various times.
Anthers mature at a fast rate. examples are Salvia, Sunflower, Clerodendron, and Rose.
Stigmas develop at a younger age. For example, Magnolia, Plantago, and Mirabilis.
Some crucifers and Tobacco, for example, have pollen grains that are inadequate for growing over the stigma of the same flower.
The stamens and styles are at varying heights within the flowers.
Self-pollination is avoided by the existence of a natural or physical barrier connecting androecium and gynoecium.
Advantages of Cross-Pollination:
- The race’s flawed character is eliminated and replaced by a better character.
- Cross-pollination boosts the ability of offspring to adapt to environmental changes.
- Cross-pollination causes genetic recombination and, as a consequence, variation in offspring.
Disadvantages of Cross-Pollination:
- Because an external agency is involved, the risk factor is always present.
- The very best character will most likely be spoiled.
- Plants must generate a vast number of pollen grains.
Agents of Pollination:
Characteristics of Anemophily (wind pollination):
- Pollen grains are very light in weight. They might have wings or an air sac.
- Flowers are small, colourless, and odorless.
- Stigmas are sticky and feathery in texture.
- Anthers have a long filament and are abundant.
- The pollen grains are dehydrated.
Characteristics of Hydrophily (water pollination):
The flowers are colourless, small, odorless, and nectarless, and the stigma is sticky, long, and unpalatable.
Water Pollination is of two types:
(a) Epihydrophily is a kind of hydrophilic.
(b) Hypohydrophily or inside the water, for example, Zostera and Ceratophyllum.
Characteristics of Entomophily (Insect pollination):
- The flowers are coloured. Bees are attracted to bluish–purplish–violet–yellow flowers and wasps, while butterflies are drawn to reddish flowers.
- Either nectar or pollen nourishes insects that come to visit.
- Pollen grains are sticky as an action of the pollen kit, and stigmas are sticky as well.
- Flowers have an aroma or scent.
Ornithophily (Bird pollination):
- Sunbirds and hummingbirds are two kinds of long-beaked small birds that aid in pollination.
- Bird pollination is common in bottlebrushes, coral, and silk-cotton trees.
Chiropterophily (Pollination by bats):
- They pollinate huge, dull-coloured flowers with a strong aroma.
- In Adansonia and Kigelia, bats pollinate the flowers.
Malacophily (pollination by snails):
Snails pollinate Arisaema (snake or cobra plants) and a few arum lily species.
Myrmecophily (pollination by ants):
Myrmecophily indicates ant pollination of flowers. Few members of the Rubiaceae family are examples.
Significance of Pollination:
- Pollination is required for fertilisation and, subsequently, seed and fruit production.
- The seeds and fruits are also nutritious.
- It leads to the formation of hybrid seeds.
- It promotes ovarian growth.
Post Pollination Events:
- The pollen grain’s nucleus breaks up to produce vegetative and generative cells.
- The generative nucleus split to produce two male nuclei.
- The region of the arrival of the pollen tube into the ovule determines the type of entry.
They are as follows:
The approach of a pollen tube into an ovule via a micropyle, as in Ottelia.
The approach of a pollen tube into an ovule via a chalaza, such as Casuarina.
Pollen tube arrival into the ovule via the funicle or integuments,
Pollen – Pistil Interaction:
- Female parents with bisexual flowers utilise forceps to remove anthers from the flower bud before the Anther dehisces.
- Germination is linked to the result of proteins found on pollen grains and the stigma that decides compatibility.
- By manipulating pollination, plant breeders can form hybrids between different species.
- Only compatible pollen from similar species can germinate.
Fertilisation is the method by which male and female gametes fuse to form the zygote.
Post Fertilisation: Structure and Events:
- The endosperm is a nutritive tissue that enlarges from vegetative fertilisation.
- The consequence of genes from the male gamete may be seen in the endosperm. The condition is called xenia.
- The direct or indirect pollen reaction on embryo sac structure is limited to the endosperm and is not observed in the embryo.
The endosperm is broken down into three types based on how it develops:
- Nuclear endosperm.
- Cellular endosperm.
- Helobial endosperm.
Functions of Endosperms:
(i) Endosperm nutrients assist in early seedling growth in plants with albuminous seeds.
(ii) Endosperm nourishes the developing embryo.
Embryogeny (embryo formation):
- It is the creation of a mature embryo from a zygote or an oospore.
- The embryo goes through the globular stage.
- Early growth results in an axially symmetric pro-embryo.
- It is globular at first. Later, it takes on a heart shape before resuming its original shape.
- A dicotyledonous embryo is built on an embryonal axis.
Formation of Seed and Fruit:
Fruit indicates a ripened or fertilised ovary. The pericarp is a plump or dry fruit wall initiated by the ovary wall.
Seeds are assorted as follows based on the presence or absence of endosperm:
(i) Non-endospermic or exalbuminous.
(ii) Endospermic or albuminous.
Importance of Seeds:
- Seeds contain enough food reserves to feed the germinating embryo.
- Because of dispersion, seeds can colonise and populate new areas and spread and propagate their species.
- Human seed germination and sowing gave a hike to agriculture, which aided in the advancement of science, civilisation, and technology.
- Because seeds are the action of sexual reproduction, they have a wide range of variations, which aids in adaptation to various environments.
- Evolutionary success: A seed is an evolutionary success. It shields the embryo from any harm.
- The length of time the seeds preserve their ability to germinate.
- Both genetic and environmental elements influence seed viability.
- Humidity and temperature are two natural factors that can influence viability.
NCERT Solutions Class 12 Biology Chapter 2: Exercises and Answer Solutions
Extramarks provides the latest NCERT Solutions Class 12 Biology Chapter 2 as per CBSE regulations. The syllabus of all the other topics of the Class 12 Board examination is also covered by the Extramarks. Students can clarify their queries by clicking on the links below to prepare for the upcoming Biology board examinations.
Detailed Solutions for all the exercises in this Chapter are listed below:
NCERT Solutions Class 12 Biology Chapter 2 Ex 2.1
NCERT Solutions Class 12 Biology Chapter 2 Ex 2.2
NCERT Solutions Class 12 Biology Chapter 2 Ex 2.3
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Key Features of NCERT Solutions Class 12 Biology Chapter 2
The main key features of NCERT Solutions Class 12 Biology Chapter 2 are as under:
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- The theory is written by experienced subject faculty with significant experience in the relevant subject.
Q.1 Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
The male gametophyte or the pollen grains develop in the microsporangia (pollen chamber) of bilobed structure called anther. The female gametophyte (embryo sac) develops from functional megaspore in the structure called megasporangium or ovule.
Q.2 Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.
The differences between microsporogenesis and megasporogenesis are:
|1. The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.||1. The process of formation of megaspores from the megaspore mother cell (MMC) is called megasporogenesis.|
|2. It the process of formation of the male gametophyte.||2. It the process of formation of the female gametophyte.|
|3. It occurs inside 4 microsporangia located inside the anther.||3. It occurs inside the ovules of an angiosperm flower.|
|4. Several thousands of microspores or pollen grains are formed inside each microsporangium.||4. A single MMC differentiates in the micropylar region of the nucellus to form 4 megaspores; out of which three degenerate while one develops into female gametophyte (embryo sac).|
Both microsporogenesis and megasporogenesis involve meiosis which results in the formation of haploid gametes from microspore and megaspore mother cells.
Microsporogenesis results in the formation of haploid microspores from the diploid microspore mother cells. Whereas, megasporogenesis results in the formation of haploid megaspore from the diploid megaspore mother cells.
Q.3 Arrange the following terms in the correct development sequence:
Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Male gametophytes are produced in stamens. An anther has 4 pollen sacs or microsporangia. When the anther is young, a group of compactly arranged homogenous cells called sporogenous tissue occupies the centre of each microsporangium. Each cell of the sporogenous tissue acts as a pollen mother cell and is capable of giving rise to a microspore tetrad. Each cell of the microspore tetrad is called a microspore. The microspore divides further by mitosis and forms a structure called pollen grain, which is an immature male gametophyte (containing a vegetative cell and a generative cell). This generative cell divides mitotically to give rise to two male gametes.
So, the correct developmental sequence is:
Sporogenous tissue – pollen mother cell – microspore tetrad – pollen grains – male gametes
Q.4 With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
The ovule or megasporangium is a part of the gynoecium that represents the female reproductive part of a flower. The gynoecium consists of single or multiple pistils at the end of which is a bulged part called the ovary. Inside the ovary is the ovarian cavity where the placenta is located. Megasporangia, which are also called as ovules, arise from these placental cells inside the ovary.
Structure of 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, thus serving as a junction between the ovule and funicle. Each ovule has one or two protective envelopes called integuments. The integuments encircle the ovule except at the tip where a small opening called the micropyle is organized. Opposite to the micropylar end is a region that represents the basal part of the ovule and is called chalaza.
The integuments enclose a mass of cells called nucellus. These are filled with abundant food reserve. The embryo sac or the female gametophyte is located in the nucellus. The single embryo sac located inside the ovule arises from a megaspore through reduction division.
Q.5 What is meant by monosporic development of female gametophyte?
The process of formation of embryo sac of the female flower from a single megaspore is called the monosporic development of female gametophyte. Out of the four megaspores produced as a result of the meiotic division of the megaspore mother cell (by megasporogenesis), only one remains functional while the other three degenerate. The functional megaspore develops into a female gametophyte or an embryo sac.
Q.6 With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
The female gametophyte (embryo sac) develops from one of the functional megaspores arising from meiotic division while the remaining three degenerate. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, thus forming a 2-nucleate embryo sac. Mitotic division continues and forms 4-nucleate and later the 8-nucleate stages of the embryo sac. These mitotic divisions result in the division of nucleus however there is no wall formation. Only after the 8-nucleate stage, cell walls are laid down leading to the formation of a typical female gametophyte or embryo sac. Here, six out of eight nuclei are surrounded by cell walls. The remaining two nuclei, called polar nuclei are situated below the egg apparatus in a large central cell.
The cells are distributed in a very typical fashion within the embryo sac. Three cells are grouped at the micropylar end and constitute the egg apparatus. Out of these three cells, two are synergids and one is the egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes of the pollen grains into the synergids. Three cells located at the chalazal end are called antipodals. The large central cell has two polar nuclei. Thus, a typical angiosperm embryo sac, once matured, is called 7-celled, 8-nucleate structure.
Q.7 What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
A flower that opens up to expose the anthers and the stigma is called a chasmogamous flower. A chasmogamous flower opens at maturity, exposing anthers (stamens) and stigma to allow fertilization.
No, cross-pollination cannot occur in cleistogamous flowers. The reason is that these flowers stay always closed which allows self-fertilization. A cleistogamous flower is usually smaller and requires less energy. The structures are such that they allow reproductive organs to make close contact, making fertilization more likely. Since the flower never opens up, no cross-pollination can occur in cleistogamous flowers, e.g., Oxalis, Viola, etc.
Q.8 Mention two strategies evolved to prevent self-pollination in flowers.
The two strategies evolved by the flowers to prevent self-pollination are:
- In some species, pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.
- In some other species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower.
Q.9 What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Self-incompatibility is a genetic mechanism of preventing self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules. This is achieved by inhibiting pollen germination or pollen tube growth in the pistil. Self-incompatibility is a widespread mechanism in flowering plants that prevents inbreeding and promotes outcrossing. This genetic response is controlled by one or more multi-allelic loci and relies on a series of complex cellular interactions between the self-incompatible pollen and pistil. Example: The flowers of Brassicaceae family are self-incompatible due to the presence of a receptor kinase signalling pathway that is activated in the pistil leading to pollen rejection.
In self-incompatible species, seeds are not formed in the event of self-pollination because the flowers are genetically programmed to inhibit pollen germination or pollen tube growth in the pistil. There will be no fertilization and thus, no further development leading to seed formation in such flowers.
Q.10 What is bagging technique? How is it useful in a plant breeding programme?
There is always a need to improve the quality of food crops to achieve higher-level production with better qualities. This is achieved by artificial hybridization techniques where it is made sure that only the desired pollen grains are used for pollination and the stigma is protected from contamination (from unwanted pollens). This is achieved by the bagging technique. The female reproductive part of the flower is covered with a bag of suitable size, usually made of butter paper. This prevents contamination of its stigma with unwanted pollen. In case of a bisexual flower, anthers are removed from the flowers using a pair of forceps (emasculation) before covering the flower with bags. Once the stigma of the bagged flower attains maturity, mature pollen grains collected from anthers of the desired male parent is artificially dusted onto the stigma and the flowers are rebagged. Fruits are allowed to develop.
This is a very useful technique and has been widely utilized in plant breeding programme. This allows careful selection of parent plants with desired traits.
Q.11 What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
The fusion of one of the male gametes of the pollen grain with the two polar nuclei located in the central cell to produce a triploid, primary endosperm nucleus is called triple fusion. It takes place in the central cell of the embryo sac.
Process of triple fusion: The pollen grains have two cells, a vegetative cell and a generative cell. The latter divides and forms the two male gametes during the growth of the pollen tube through the style. Once it reaches the ovary, pollen tube enters the ovule through the micropyle and releases the two male gametes into the cytoplasm of synergid. One of the male gametes moves towards the egg cell and fuses with the egg cell. This results in the formation of a diploid cell called the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid, primary endosperm nucleus. As this involves the fusion of three haploid nuclei, it is termed as triple fusion.
Three haploid nuclei; one male gamete and two polar nuclei are involved in the triple fusion.
Q.12 Why do you think the zygote is dormant for sometime in a fertilized ovule?
The zygote is dormant for sometime in a fertilized ovule as it waits till endosperm is developed. This is because endosperm ensures an ample supply of nutrition for the embryo. The primary endosperm cell starts its division immediately after double fertilization occurs in the embryo sac. The cells of this endosperm tissue are filled with reserve food material and are used for the nutrition of the developing embryo. This tissue becomes the food that the young plant will consume until the roots have developed after germination.
Q.13 Differentiate between:
(a) hypocotyl and epicotyl;
(b) coleoptile and coleorhiza;
(c) integument and testa;
(d) perisperm and pericarp.
(a) Hypocotyl and Epicotyl
|1. The cylindrical portion of the embryonal axis below the level of cotyledons is called the hypocotyl.||1. The portion of the embryonal axis above the level of cotyledons is called the epicotyl.|
|2. It is the portion of the embryonal axis that lies between the radicle and point of attachment of cotyledons.||2. It is the portion of the embryonal axis that lies between the plumule and cotyledons.|
|3. It terminates within the radicle or root tip.||3. It terminates within the plumule or stem tip.|
|4. It forms an important part of the embryonic root system.||4. It forms an important part of the embryonic shoot system.|
(b) Coleoptile and Coleorhiza
|1. It is the conical protective sheath which encloses the shoot apex and a few leaf primordia in a monocot seed.||1. It is the undifferentiated sheath which encloses the radicle and root cap in a monocot seed.|
|2. It forms the portion of the embryonal axis above the level of attachment of scutellum.||2 It forms the portion of the embryonal axis below the level of attachment of scutellum.|
|3. It forms the shoot system in adult plant by coming out from the soil and performs photosynthesis.||3. It remains inside the soil and is non-green in colour.|
(c) Integument and Testa
|1. The integument is the outermost layer of the ovule. It acts as a protective covering of the ovule.||1. The tesla is the outermost protective covering of the seed.|
|2. There are two thin integument layers called inner and outer integuments. The cells of the integument layer are living.||2. It is thick and made of dead cells.|
(d) Perisperm and Pericarp
|1. Perisperm is the persistent remains of the nucellus (ovule) in the seed.||1. Pericarp is the wall of fruit formed by the ovarian wall.|
|2. It is the part of a seed.||2. It is the part of a fruit.|
Q.14 Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
An apple is called a false fruit because the thalamus (an accessory part) along with ovary contributes to the fruit formation. It develops from a compound ovary but much of the fruit’s edible part comes from the thalamus, which grows around the ovary.
Q.15 What is meant by emasculation? When and why does a plant breeder employ this technique?
Emasculation is the removal of stamens from bisexual flowers of the female parents to avoid self-pollination in these flowers. It is done before the anthers mature. This is a very useful technique and has been widely utilized in plant breeding programme.
Plant breeders employ this technique for crop improvement programmes where they wish to select parent plants with desired traits or combine desirable characters to produce commercially ‘better’ varieties. They make sure that pollen grains with desired traits are used for pollination and the stigma is protected from contamination by other pollens. Emasculation is done in cases where the plant produces bisexual flowers.
The female reproductive part of the flower is covered with a bag of suitable size, usually made of butter paper. This prevents contamination of its stigma with unwanted pollen. Anthers are removed from the flower using a pair of forceps before covering the flower with bags. Once the stigma of the bagged flower attains maturity, mature pollen grains collected from anthers of the desired male parent are artificially dusted onto the stigma and the flowers are rebagged. Fruits are allowed to develop. This allows the development of the desired traits.
Q.16 If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
If one can induce parthenocarpy through the application of growth substances, the fruits which contain a large number of hard seeds would be selected to induce parthenocarpy. Such fruits resemble a normally developed fruit but are seedless. Seedlessness is a desirable trait in edible fruit with hard seeds. This trait increases the commercial value as consumers prefer to eat fruits without seeds. Thus, varieties of pineapple, banana, cucumber, grape, orange, grapefruit, etc. can be selected to induce parthenocarpy. Parthenocarpy is also desirable in fruit crops that may be difficult to pollinate or fertilize, such as tomato.
Q.17 Explain the role of tapetum in the formation of pollen-grain wall.
Tapetum is the innermost layer that surrounds the pollen grains inside the anther. The lobes of anther of a male flower develop to form pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains. This is surrounded by four layers namely epidermis, endothecium, middle layers and the tapetum. The three outer layers are protective whereas the tapetum, which constitutes the innermost layer, nourishes the pollen grains. The cells in this layer are large with densely packed cytoplasm and generally have more than one nuclei. This layer surrounds the sporogenous tissue occupying the centre of microsporangia. Tapetum helps in pollen wall formation, transportation of nutrients to the inner side of anther and is involved in the synthesis of callase enzyme for the separation of microspore tetrads.
Q.18 What is apomixis and what is its importance?
The method devised by flowering plants to produce seeds without fertilisation is called apomixis. There are numerous ways by which apomixis happens:
- A diploid egg cell is formed without reduction division and develops into an embryo without fertilisation.
- The nuclear cells surrounding the embryo sac start dividing, protruding into the embryo sac and developing into the embryos; for example, Citrus and Mango.
Importance of apomixis:
Large varieties of food crops under cultivation are hybrid varieties and maintaining the hybrid characteristics in these plants is very difficult as they segregate in subsequent divisions. This is a costly process. If these hybrids are converted into apomicts (plants that carry out apomixes), there will be no segregation of characteristics as there will be no reduction division. This will allow the hybrid character to remain as it is and farmers need not buy seeds every year as they can raise hybrid seeds by themselves without losing the character.
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