CBSE Class 12 Biology Revision Notes Chapter 1 Sexual Reproduction in Flowering Plants
Flowers are the reproductive organs of angiosperms where male and female gametophytes develop, fertilisation occurs and seeds and fruits are formed.
Sexual reproduction allows flowering plants to produce genetically varied offspring. The entire process begins with the formation of pollen grains and the embryo sac, followed by pollination, fertilisation and development of the embryo, seed and fruit.
These CBSE Class 12 Biology Revision Notes Chapter 1 explain the complete reproductive sequence in flowering plants. The notes cover gametophyte formation, pollination, double fertilisation, post-fertilisation events, apomixis and polyembryony.
Key Takeaways
- Male gametophyte: The pollen grain develops inside the microsporangium of the anther.
- Female gametophyte: A typical mature embryo sac is 7-celled and 8-nucleate.
- Double fertilisation: Syngamy and triple fusion occur within the same embryo sac.
- Post-fertilisation: The ovule becomes a seed, while the ovary generally develops into a fruit.
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Access Class 12 Biology Chapter 1 Sexual Reproduction in Flowering Plants Notes in 30 Minutes
These Sexual Reproduction in Flowering Plants Notes organise the chapter according to the biological sequence of events.
| Revision Time | Concepts |
| First 5 minutes | Flower, stamen and anther |
| Next 5 minutes | Pollen grain and male gametophyte |
| Next 5 minutes | Ovule and embryo sac |
| Next 5 minutes | Pollination and pollen-pistil interaction |
| Next 5 minutes | Double fertilisation |
| Final 5 minutes | Embryo, seed, fruit, apomixis and polyembryony |
The same sequence makes these Class 12 Biology Chapter 1 Notes useful for long answers, short questions and diagram-based revision.
Flower as the Reproductive Organ of Angiosperms
A flower is the site of sexual reproduction in flowering plants.
Its two main reproductive whorls are:
- Androecium: Male reproductive whorl composed of stamens
- Gynoecium: Female reproductive whorl composed of one or more pistils
The androecium produces pollen grains containing the male gametophytes. The gynoecium contains ovules in which the female gametophytes develop.
Sexual reproduction in flowering plants may be divided into three broad stages:
- Pre-fertilisation structures and events
- Double fertilisation
- Post-fertilisation structures and events
This reproductive sequence forms the core of Class 12 Biology Sexual Reproduction in Flowering Plants.
Pre-Fertilisation Structures and Events
The pre-fertilisation events include:
- Formation of male and female reproductive structures
- Development of pollen grains
- Development of the embryo sac
- Pollination
- Pollen-pistil interaction
Stamen, Anther and Microsporangium
A typical stamen consists of:
- Filament: Slender stalk
- Anther: Terminal pollen-producing structure
A typical angiosperm anther is:
- Bilobed
- Dithecous
- Tetragonal in transverse section
- Tetrasporangiate
Each anther has two lobes. Each lobe contains two thecae, and the anther contains four microsporangia in total.
The microsporangia develop into pollen sacs containing pollen grains.
Wall Layers of a Microsporangium
A typical microsporangium is surrounded by four wall layers.
| Wall Layer | Main Function |
| Epidermis | Protects the microsporangium |
| Endothecium | Helps in anther dehiscence |
| Middle layers | Provide temporary protection and support |
| Tapetum | Nourishes developing pollen grains |
The tapetum is the innermost layer. Its cells contain dense cytoplasm and may have more than one nucleus.
The centre of a young microsporangium contains compact, homogeneous cells called sporogenous tissue.
Microsporogenesis
Each cell of the sporogenous tissue can develop into a pollen mother cell or microspore mother cell.
The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
The sequence is:
Sporogenous tissue
↓
Pollen mother cell
↓ Meiosis
Microspore tetrad
↓ Separation
Individual microspores
↓ Development
Pollen grains
The pollen mother cell is diploid, while the resulting microspores are haploid.
As the anther matures and dehydrates, the microspores separate and develop into pollen grains.
Structure and Development of Pollen Grain
The structure of pollen grain reflects its function as the male gametophyte.
A pollen grain usually has a two-layered wall.
Exine
The exine is the hard outer layer.
It is composed of sporopollenin, one of the most resistant known organic materials.
Sporopollenin:
- Resists high temperature
- Resists strong acids and alkalis
- Is not easily degraded by enzymes
- Helps pollen grains remain preserved as fossils
The exine has openings called germ pores where sporopollenin is absent.
The pollen tube emerges through a germ pore.
Intine
The intine is the thin inner wall.
It is composed mainly of cellulose and pectin.
Cells of a Mature Pollen Grain
A mature pollen grain commonly contains:
- Vegetative cell: Larger cell with abundant food reserve and an irregular nucleus
- Generative cell: Smaller spindle-shaped cell with dense cytoplasm
In more than 60 per cent of angiosperms, pollen is released at the two-celled stage.
In other species, the generative cell divides before pollen release and produces two male gametes. Such pollen is released at the three-celled stage.
Pollen Viability and Pollen Banks
Pollen viability is the period during which a released pollen grain can germinate and fertilise an ovule.
The viability period varies among species and depends on temperature and humidity.
Examples include:
- Rice and wheat pollen may remain viable for only about 30 minutes.
- Pollen of some members of Rosaceae, Leguminosae and Solanaceae may remain viable for months.
Pollen grains can be stored for years in liquid nitrogen at −196°C.
Such stored pollen collections are called pollen banks and are useful in crop breeding programmes.
Pollen grains of some plants can also cause allergies and respiratory problems. Parthenium pollen is a common example.
Pistil and Structure of an Ovule
The gynoecium is composed of one or more pistils.
A pistil contains three parts:
- Stigma: Receives pollen grains
- Style: Connects stigma with ovary
- Ovary: Contains ovules
The gynoecium may be:
- Monocarpellary: One carpel
- Multicarpellary: More than one carpel
- Syncarpous: Carpels fused
- Apocarpous: Carpels free
Structure of a Typical Angiosperm Ovule
The ovule is attached to the placenta by a stalk called the funicle.
Its main parts are:
| Ovule Part | Description |
| Funicle | Stalk connecting ovule with placenta |
| Hilum | Junction between ovule and funicle |
| Integuments | Protective coverings around the ovule |
| Micropyle | Small opening in the integuments |
| Chalaza | Region opposite the micropyle |
| Nucellus | Nutritive tissue inside integuments |
| Embryo sac | Female gametophyte within the nucellus |
The nucellus contains food reserves and usually encloses one embryo sac.
Megasporogenesis
A megaspore mother cell differentiates in the micropylar region of the nucellus.
The formation of megaspores from the megaspore mother cell through meiosis is called megasporogenesis.
The sequence is:
Megaspore mother cell
↓ Meiosis
Four haploid megaspores
↓
Three degenerate
↓
One functional megaspore
The development of the embryo sac from one functional megaspore is called monosporic development.
Development of the Female Gametophyte
The functional megaspore develops into the female gametophyte or embryo sac.
Its nucleus undergoes three mitotic divisions.
The stages are:
- Two-nucleate stage
- Four-nucleate stage
- Eight-nucleate stage
- Cellular organisation of the mature embryo sac
The early nuclear divisions are free nuclear because nuclear division is not immediately followed by cell-wall formation.
Organisation of the Mature Embryo Sac
A typical mature embryo sac is 7-celled and 8-nucleate.
Its arrangement is:
- Three cells at the micropylar end
- Three cells at the chalazal end
- One large central cell
Egg Apparatus
The three cells at the micropylar end form the egg apparatus.
It contains:
- One egg cell
- Two synergids
The synergids contain a filiform apparatus that guides the pollen tube into the embryo sac.
Antipodals
Three cells located at the chalazal end are called antipodals.
Central Cell
The central cell contains two polar nuclei.
Therefore, the mature embryo sac contains:
- One egg cell
- Two synergids
- Three antipodals
- One central cell with two polar nuclei
Pollination in Flowering Plants
Pollination in flowering plants is the transfer of pollen grains from anther to stigma.
Pollen grains and female gametes are non-motile. Pollination brings the male gametophyte close to the female reproductive structure so that fertilisation can occur.
Depending on the source of pollen, pollination is classified into autogamy, geitonogamy and xenogamy.
Autogamy
Autogamy is the transfer of pollen from the anther to the stigma of the same flower.
Complete autogamy requires:
- Synchronisation between pollen release and stigma receptivity
- Close positioning of anther and stigma
Cleistogamous flowers remain closed and are invariably autogamous.
Examples include some flowers of:
- Viola
- Oxalis
- Commelina
Cleistogamous flowers ensure seed formation even without pollinating agents.
Geitonogamy
Geitonogamy is the transfer of pollen from one flower to another flower of the same plant.
It is:
- Functionally cross-pollination because it requires an agent
- Genetically similar to self-pollination because pollen comes from the same plant
Xenogamy
Xenogamy is the transfer of pollen from a flower of one plant to the stigma of a flower on another plant of the same species.
It introduces genetically different pollen to the stigma and promotes genetic variation.
Autogamy, Geitonogamy and Xenogamy
| Type | Source of Pollen | Pollinating Agent | Genetic Nature |
| Autogamy | Same flower | Usually not required | Self-pollination |
| Geitonogamy | Another flower of same plant | Required | Genetically self-pollination |
| Xenogamy | Flower of another plant | Required | True cross-pollination |
Agents of Pollination
The main agents of pollination are:
- Wind
- Water
- Animals
Wind and water are abiotic agents. Animals are biotic agents.
Pollination by Wind
Wind-pollinated flowers generally produce large quantities of pollen because transfer is uncertain.
Common adaptations include:
- Light, dry and non-sticky pollen
- Well-exposed stamens
- Large or feathery stigmas
- Numerous flowers in inflorescences
- Little or no nectar
- Inconspicuous flowers
Wind pollination is common in grasses.
Corn is a familiar example. Its long exposed styles and stigmas trap airborne pollen.
Pollination by Water
Water pollination is relatively rare in angiosperms.
Examples include:
- Vallisneria
- Hydrilla
- Zostera
In Vallisneria, female flowers reach the water surface through long stalks. Male flowers or pollen are carried by surface currents.
In seagrasses such as Zostera, pollen is released underwater. The pollen grains are often long and ribbon-like.
Water-pollinated pollen may have a mucilaginous covering that protects it from wetting.
Water hyacinth and water lily are aquatic plants but are not necessarily water-pollinated because their flowers emerge above water.
Pollination by Animals
Animals pollinate the majority of flowering plants.
Common animal pollinators include:
- Bees
- Butterflies
- Moths
- Flies
- Beetles
- Wasps
- Birds
- Bats
Animal-pollinated flowers commonly have:
- Bright colours
- Fragrance
- Nectar
- Sticky pollen
- Floral rewards
- Structures adapted to specific pollinators
Some flowers pollinated by flies or beetles release foul smells.
The Yucca plant and its moth pollinator show a specialised mutual relationship. The plant receives pollination, while the moth obtains a site for laying eggs.
Outbreeding Devices in Flowering Plants
Continued self-pollination can lead to inbreeding depression.
Flowering plants have evolved outbreeding devices that reduce self-pollination.
Different Timing of Maturity
Pollen release and stigma receptivity may occur at different times.
This condition is called dichogamy.
Different Position of Anther and Stigma
Anther and stigma may be placed at different positions so that self-pollen cannot easily reach the stigma.
This condition is often called herkogamy.
Self-Incompatibility
Self-incompatibility is a genetically controlled mechanism that prevents self-pollen from fertilising ovules.
The pistil may inhibit:
- Pollen germination
- Pollen-tube growth
Unisexuality
In monoecious plants, male and female flowers occur on the same plant.
Examples include maize and castor.
Monoecy prevents autogamy but not geitonogamy.
In dioecious plants, male and female flowers occur on separate plants.
Papaya is an example.
Dioecy prevents both autogamy and geitonogamy.
Pollen-Pistil Interaction
The pollen-pistil interaction includes all events from the landing of pollen on the stigma to the entry of the pollen tube into the ovule.
The pistil recognises whether the pollen is:
- Compatible
- Incompatible
If pollen is compatible:
- It germinates on the stigma.
- A pollen tube emerges through a germ pore.
- The tube grows through the style.
- It reaches the ovary.
- It enters the ovule through the micropyle.
- It enters one synergid through the filiform apparatus.
- It releases two male gametes.
If pollen is incompatible, its germination or pollen-tube growth is inhibited.
Pollen recognition involves chemical interactions between the pollen grain and pistil.
Artificial Hybridisation
Artificial hybridisation is used in plant breeding to combine desirable characters from selected parents.
The breeder must ensure that:
- Only the desired pollen reaches the stigma.
- Unwanted pollen does not contaminate the flower.
The main techniques are emasculation and bagging.
Emasculation
Emasculation is the removal of anthers from a bisexual flower before they release pollen.
It prevents self-pollination.
Emasculation is not required when the female parent bears unisexual female flowers.
Bagging
After emasculation, the flower is covered with a suitable bag to prevent unwanted pollen from reaching the stigma.
When the stigma becomes receptive:
- Desired pollen is applied.
- The flower is covered again.
- The fruit is allowed to develop.
Double Fertilisation in Angiosperms
After entering a synergid, the pollen tube releases two male gametes.
The two male gametes participate in two separate fusion events.
This phenomenon is called double fertilisation and is characteristic of angiosperms.
Syngamy
One male gamete fuses with the egg nucleus.
Male gamete (n) + Egg cell (n)
↓
Zygote (2n)
This fusion is called syngamy.
The zygote later develops into the embryo.
Triple Fusion
The second male gamete fuses with the two polar nuclei in the central cell.
Male gamete (n) + Two polar nuclei (n + n)
↓
Primary endosperm nucleus (3n)
Since three haploid nuclei participate, the process is called triple fusion.
The central cell containing the primary endosperm nucleus becomes the primary endosperm cell.
Syngamy and Triple Fusion
| Event | Nuclei Involved | Product | Ploidy |
| Syngamy | Male gamete + egg | Zygote | 2n |
| Triple fusion | Male gamete + two polar nuclei | Primary endosperm nucleus | 3n |
The occurrence of syngamy and triple fusion in the same embryo sac constitutes double fertilisation.
Post-Fertilisation Structures and Events
The events occurring after double fertilisation are called post-fertilisation events.
They include:
- Endosperm development
- Embryo development
- Formation of seed
- Formation of fruit
Endosperm Development
Endosperm development generally begins before embryo development.
The endosperm provides nutrition to the developing embryo.
The primary endosperm cell divides repeatedly to produce triploid endosperm tissue.
In the common free-nuclear type:
- The primary endosperm nucleus undergoes repeated nuclear divisions.
- Cell walls are not initially formed.
- A multinucleate endosperm develops.
- Cellularisation may occur later.
In coconut:
- Coconut water is free-nuclear endosperm.
- The white kernel is cellular endosperm.
Endosperm may:
- Be consumed during embryo development
- Remain in the mature seed
Pea and groundnut usually lack persistent endosperm at maturity, while castor and coconut retain it.
Embryo Development
Embryo development begins from the zygote.
Most zygotes begin dividing only after some endosperm has formed. This ensures an available food supply.
The common stages of dicot embryogenesis are:
Zygote
↓
Proembryo
↓
Globular stage
↓
Heart-shaped stage
↓
Mature embryo
Dicot Embryo
A typical dicot embryo has:
- Two cotyledons
- One embryonal axis
The part above the cotyledons is the epicotyl.
The epicotyl ends in the plumule or shoot tip.
The part below the cotyledons is the hypocotyl.
The hypocotyl ends in the radicle or root tip.
The radicle is protected by the root cap.
Monocot Embryo
A monocot embryo has one cotyledon.
In grasses, the cotyledon is called the scutellum.
Important structures include:
- Scutellum: Single cotyledon
- Coleoptile: Protective sheath around the plumule
- Coleorrhiza: Protective sheath around the radicle and root cap
The scutellum lies on one side of the embryonal axis.
Seed Formation
A seed is a mature fertilised ovule.
A typical seed contains:
- Seed coat
- Cotyledon or cotyledons
- Embryonal axis
During seed development:
- Integuments become seed coats.
- The micropyle remains as a small pore.
- Water content decreases.
- Metabolic activity slows.
- The embryo may enter dormancy.
Albuminous Seeds
Albuminous seeds retain some endosperm at maturity.
Examples include:
- Wheat
- Maize
- Barley
- Castor
Non-Albuminous Seeds
Non-albuminous seeds use up the endosperm during embryo development.
Examples include:
- Pea
- Groundnut
- Bean
Perisperm
In some seeds, part of the nucellus remains after seed formation.
This persistent nucellus is called perisperm.
Examples include black pepper and beet.
Fruit Formation
During seed and fruit formation:
- Ovules become seeds.
- The ovary generally becomes the fruit.
- The ovary wall becomes the pericarp.
True Fruits
True fruits develop only from the ovary.
False Fruits
False fruits develop from the ovary along with another floral part.
In apple and strawberry, the thalamus contributes to fruit formation.
Parthenocarpic Fruits
Parthenocarpic fruits develop without fertilisation.
They are usually seedless.
Banana is a common example.
Parthenocarpy may also be induced through plant growth substances.
Importance of Seeds
Seeds offer several advantages to flowering plants.
- Reproduction does not depend on external water for gamete transfer.
- Seeds protect the embryo.
- Stored food nourishes the young seedling.
- Dormancy allows survival through unfavourable periods.
- Seeds aid dispersal and colonisation.
- Sexual reproduction creates genetic variation.
- Dry seeds can remain viable for long periods.
Apomixis and Polyembryony
Although seeds are usually formed after fertilisation, some angiosperms produce seeds without fertilisation.
Apomixis
Apomixis is the formation of seeds without fertilisation.
It is a form of asexual reproduction that resembles sexual reproduction.
Apomixis may occur when:
- A diploid egg forms without meiosis and develops without fertilisation.
- Nucellar cells form embryos directly.
Apomixis occurs in some members of Asteraceae and grasses.
Importance of Apomixis in Hybrid Seed Production
Hybrid crops often show superior characteristics, but their progeny may segregate when hybrid seeds are replanted.
Farmers may therefore need to purchase new hybrid seeds every year.
Apomixis can help maintain hybrid characters because:
- Meiosis and fertilisation are bypassed.
- Genetic segregation does not occur.
- Offspring remain genetically similar to the parent.
- Hybrid vigour can be preserved across generations.
This makes apomixis valuable in agriculture and the hybrid-seed industry.
Polyembryony
Polyembryony is the occurrence of more than one embryo in a single seed.
In Citrus and mango, some nucellar cells surrounding the embryo sac may develop into additional embryos.
Therefore, one seed may contain:
- A sexually formed embryo
- One or more nucellar embryos
The nucellar embryos are genetically similar to the parent plant.
Ploidy of Important Structures
| Structure | Ploidy |
| Sporogenous tissue | 2n |
| Pollen mother cell | 2n |
| Microspore | n |
| Pollen grain | n |
| Vegetative cell | n |
| Generative cell | n |
| Male gamete | n |
| Nucellus | 2n |
| Megaspore mother cell | 2n |
| Functional megaspore | n |
| Egg cell | n |
| Synergid | n |
| Antipodal | n |
| Polar nucleus | n |
| Zygote | 2n |
| Primary endosperm nucleus | 3n |
| Endosperm | 3n |
| Embryo | 2n |
| Seed coat | 2n |
| Pericarp | 2n |
Important Differences for Quick Revision
| Terms | Main Difference |
| Microsporogenesis and pollen development | Microsporogenesis forms microspores by meiosis; pollen development forms the male gametophyte |
| Megasporogenesis and embryo-sac development | Megasporogenesis forms megaspores; embryo-sac development forms the female gametophyte |
| Autogamy and geitonogamy | Autogamy occurs within one flower; geitonogamy occurs between flowers of the same plant |
| Geitonogamy and xenogamy | Geitonogamy is genetically self-pollination; xenogamy introduces pollen from another plant |
| Chasmogamous and cleistogamous flowers | Chasmogamous flowers open; cleistogamous flowers remain closed |
| Syngamy and triple fusion | Syngamy forms a diploid zygote; triple fusion forms a triploid endosperm nucleus |
| Embryo and endosperm | Embryo develops into the new plant; endosperm nourishes the embryo |
| Albuminous and non-albuminous seeds | Albuminous seeds retain endosperm; non-albuminous seeds do not |
| Perisperm and pericarp | Perisperm is persistent nucellus; pericarp is the fruit wall |
| True fruit and false fruit | A true fruit develops only from the ovary; a false fruit also contains another floral part |
| Parthenocarpy and apomixis | Parthenocarpy forms fruit without fertilisation; apomixis forms seed without fertilisation |
| Apomixis and polyembryony | Apomixis is seed formation without fertilisation; polyembryony is the presence of multiple embryos |
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Q.1 Which flower parts develop into fruit and seeds after fertilization?
Ans
Ovary and ovules respectively.
Q.2 Which events take place in pre-fertilization process ?
Ans
Gametogenesis and gamete transfer.
Q.3 The meiocyte of a potato plant contains 48 chromosomes. Workout the number of chromosomes found in its mesophyll cells.
Ans
Meiocytes (2n)= 48
Thus, mesophyll cells (2n)= 48
Q.4 Why does the process of cell differentiation occur in the organisms during embryogenesis?
Ans
It helps the groups of cells to undergo certain modifications to form specialized tissues and organs in an organism. This helps in improving the functioning, regulation and efficiency of the various lifeprocesses.
Q.5 How will you define the term embryogenesis?
Ans
The process of development of embryo is called embryogenesis.
Q.6 Name the terms used for male and female sex organs in Marchantia.
Ans
Antheridiophore and Archegoniophore.
Q.7 Which vegetative part is responsible for the formation of new plant in Bryophyllum?
Ans
Leaf buds present in the notches at the margins of the leaves.
Q.8 What is the mode to form new individuals in Protistans and Monerans?
Ans
By cell division.
Q.9 In a pond, you see green coloured long filaments, which is a type of algae consisting of single layer aggregation of vertically arranged cells. What sort of strategy can this type of cellular aggregation adopt for multiplication?
AnsFragmentation (vegetative method).
Q.10 What is the terminology for the concept of group immortality?
Ans
Reproduction
Q.11 Name vegetative propagules in the following plants:-
i) Potato
ii) Banana
iii) Guava
iv) Sugarcane
v) Bryophyllum
vi) Onion
Ans
i) Eyes (buds) of potato tuber
ii.) Rhizome (prostate thin stem)
iii.) Buds in roots
iv.) Stem
v.) Adventitious buds in the leaves
vi.) Bulb -Condensed underground stem
Q.12 By which name male gametophyte is known in plants? What is the significance of germ pore in male gametophyte?
Ans
Pollen grain.
Intine comes out through germ pore in male gametophyte.
Q.13 Define Hermaphrodites. What is the difference in the reproductive strategy of earthworm and tapeworm?
Ans
Hermaphroditism is a phenomenon where both male and female sex organs are present in the same individual.
Tapeworm |
Earthworm |
|
Both female and male gametes in fertilization are produced by same individual. |
Individual bear both types of sex organs but behave either as male or female and exhibit crossfertilization. |
Q.14 Name the cell division responsible for gametogenesis. Give one morphological difference between the two types of gametes in a heterogametic organism.
Ans
Meiosis.
Male gamete is usually small and provided with flagellum (motile), female gamete is usually big and non-flagellated.
Q.15 Define senescent phase. List some changes in the organisms during this phase.
Ans
The period from the end of the reproductive phase till death is called senescent phase.
changes during this phase are:
Slower metabolism, break down of proteins, immobilization of nutrients.
Q.16 Differentiate between seasonal and continuous breeders?
Ans
Many mammals are reproductively active only during some favourable season in their reproductive phase. These are called seasonal breeders.
Other mammals are reproductively active through out their reproductive phase.These are categorised as continuous breeders.
Q.17 Sexual reproduction is the favoured mode of reproduction under unfavorable conditions. Explain.
Ans
Sexual reproduction causes genetic variations and produce offsprings with different combination of characteristics, which help organism to adapt to unfavorable conditions and increase the chances of the survival of species in unfavorable conditions. Sexual reproduction also contributes to evolution of the species.
Q.18 Name some common structures through which asexual reproduction takes place in various organisms.
Ans
1) Zoospores-Microscopic flagellated structure, e.g. algae.
2) Conidia- Exogenously produced non motile spores, e.g. Penicillium.
3) Buds-small external projection on the parent body, e.g. Hydra.
4) Gemmules-Internal buds, e.g. sponges.
Q.19 What term is given to asexual reproduction in plants?
Ans
Vegetative propagation.
It is defined as the mode of reproduction in which any vegetative part of the plant (root, stem, leaf) is capable of giving rise to a new individual.
Q.20 Define clone. How are they produced?
Ans
Morphologically and genetically similar individuals are termed as clones. They are produced by asexual reproduction.
Q.21 Mention any two differences between sexual and asexual reproduction?
Ans
|
Sexual |
Asexual
|
|
(1) Two parents of opposite sex are involved in the process. (2) Gamete formation and fertilization takes place.
|
(1) Only one parent is involved. (2) Neither gamete formation, nor fertilization takes place. |
Q.22 Any organic material kept in moist and dark conditions gets infected with fungus, although there was no sign of fungus prior to it. How does fungus makes it possible?
Ans
Fungus reproduces through sporulation; spores are dry, light and are easily dispersed by wind.
Q.23 Sexual reproduction generally involves fusion of gametes from two different parents belonging to a species, but this is not always true. What explanation can you give in support of the statement?
Ans
Self fertilisation involves both the gametes from one parent only.
Q.24 The principle of life states that every form of life has to die; still amoeba is called as immortal. Give specific reason for it.
Ans
Amoeba multiplies by binary fission, so one parent cell divides into two daughter cells without any part of it dying. So, it is called immortal.
Q.25 In single celled organisms, the offspring are similar to one another and are the exact copies of the parent. Would it be correct to call them clones?
Ans
Yes, the term clone is used to describe such morphologically and genetically similar individuals.
Q.26 What is the terminology used for asexual multiplications when a single celled organism divides into two equal halves or when it divides into two unequal parts?
Ans
Binary fission in case of equal halves and budding in case of unequal parts.
Q.27 The changes in the body that are observed during the attainment of reproductive maturity are broadly categorised into two categories. Name these categories.
Ans
Morphological and physiological.
Q.28 Both vegetative and asexual reproductions do not involve any sexual method so they come under a common category, yet they are different in one specific point. Elaborate that specific point.
Ans
Vegetative methods have direct involvement of somatic cells but asexual methods generally do not involve complete somatic cells.
Q.29 The chromosome number in meiocyte of a butterfly is 380. What is the number of chromosomes in its gamete?
Ans
Chromosome number in meiocyte is diploid (2n) and that of the gamete is haploid (n). Hence, the number of chromosomes in its gamete would be 190.
Q.30 Water hyacinth is one of the most invasive aquatic weeds, resulting in the degradation of aquatic ecosystem and thereby in the death of fishes. Does it directly damage the fish life? Give your answer with proper justification.
Ans
Rapid growth and death of water hyacinth increases heterotrophic activity of microbes. Increased microbial activity results in the reduction of dissolved oxygen (DO) or increase in BOD in water, thereby resulting in the death of fishes.
Q.31 Why runners, suckers, tubers, offsets and bulbs are called as vegetative propagules?
Ans
Runners, suckers, tubers, offsets and bulbs are units of vegetative propagation. They are all capable of giving rise to new offspring. Hence, these structures are called vegetative propagules.
Q.32 In case of a monoecious species having bisexual flowers with self-fertilisation, is there any need of a mediator for pollination?
Ans
No mediator is required for self fertilisation.
Q.33 A student while crossing across a papaya orchard observed that some papaya plants had flowers with a very small papaya like structure at its base, while there were other papaya plants that had flowers without such a swollen portion. What information do you get concerning the type of plant and the flower from the above data?
Ans
The papaya plant is dioecious and its flower is unisexual.
Q.34 Fill in the vacant columns with reference to asexual reproduction.
|
Column A |
Column B |
|
Paramecium |
|
|
Penicillium |
|
|
|
Bulbil |
|
|
Leaf buds |
|
|
Gemmules |
|
Water hyacinth |
|
Ans
|
Column A |
Column B |
|
Paramecium |
Binary fission |
|
Penicillium |
Conidia |
|
Agave |
Bulbil |
|
Bryophyllum |
Leaf buds |
|
Sponges |
Gemmules |
|
Water hyacinth |
Offset |
Q.35 What is vegetative reproduction?
Ans
Asexual reproduction is also known as vegetative reproduction or agamogenesis. It is the simplest form of reproduction and does not involve meiosis, gamete formation, or fertilisation. There is only one “parent” involved. This form of reproduction is common among simple organisms such as amoeba and other single-celled organisms.
Q.36 Complete the flow chart with reference to the reproduction in flowers.
Ans
Q.37
(i) Identify the given plants.
(ii) How are they different from each other with respect to their sexuality?
Ans
(i)<span “> A – Chara B – Marchantia
(ii) Chara is a monoecious plant and Marchantia is a dioecious plant. Monoecious plants are those in which male and female reproductive structures are present on the same individual plant. Dioecious plants are those in which male and female reproductive structures are present on different plants/thalli. In case of Chara, antheridium (male sex organ) and oogonium (female sex organ) are present on the same thallus. However in Marchantia, antheridiophore (male sex organ) and archegoniophore (female sex organ) are present on the different thallus.
Q.38 Both amphibians and reptiles are oviparous animals, yet they differ in certain aspects of reproduction. What are the differences? What is the disadvantage to oviparous animals?
Ans
Amphibians (such as frogs) exhibit external fertilisation (outside the body of the organisms) while reptiles exhibit internal fertilisation (inside the body of the organism).
Amphibians release large number of gametes into the external medium (water) to enhance the chances of fertilisation. In reptiles, egg is formed inside the female body where they fuse with the male gamete. They lay fertilised eggs. The fertilised eggs are laid in a safe place in the environment and after a period of incubation, the young ones hatch out.
The major disadvantage in case of oviparous animals is that the Offspring are extremely vulnerable to predators threatening their survival up to adulthood.
Q.39 Fill in the column B whether the structures given in column A are haploid, diploid, triploid or polyploid.
|
Column A |
Column B |
|
Sperm |
|
|
Ovum |
|
|
Zygote |
|
|
Endosperm |
|
|
Pollen |
|
|
Embryo |
|
Ans
|
Column A |
Column B |
|
Sperm |
Haploid |
|
Ovum |
Haploid |
|
Zygote |
Diploid |
|
Endosperm |
Triploid |
|
Pollen |
Haploid |
|
Embryo |
Diploid |
Q.40 What are the advantages and disadvantages of asexual reproduction?
Ans
Advantages:
i. Numerous offspring can be produced without spending great amount of time and energy.
ii. Animals that remain in one particular place and are unable to look for mates can reproduce asexually.
iii. Stable environments that experience very little alterations are of advantage for asexually reproducing organisms.
Disadvantages:
i. It does not produce genetic variation.
ii. All of the organisms are genetically identical and therefore, share the same defect.
iii. If the stable environment changes, the results could be deadly for all of the individuals.
Q.41 Successful gamete transfer and fusion of gametes is essential for the most critical event in sexual reproduction.
i. Give the technical term for the fusion of gametes.
ii. What are the events that occur during gamete transfer in plants?
iii. What is formed as a result of fusion of gametes/syngamy?
iv. How will you categorise syngamy based on the fusion of gametes inside or outside the female body?
v. What would happen if syngamy does not occur?
Ans
i. Fertilisation.
ii. In plants, pollen grains are the carriers of male gametes and ovules have the egg. These pollen grains have to reach the stigma before fertilisation. The process of pollination facilitates transfer of pollen grains to the stigma. Pollen grains germinate on stigma and the pollen tubes carrying the male gametes reach the ovule and release the male gametes.
iii. Fusion of gametes or syngamy results in the formation of a diploid zygote.
iv. If syngamy occurs outside the body of the organism, i.e., in the external medium (water), then it is called external fertilisation. If syngamy occurs inside the body of the organism, then it is called internal fertilisation.
v. In sexual reproduction, meiosis halves the diploid (2n) number of chromosome to the haploid number (n) of chromosome for forthcoming gametes. The two types of gametes (egg and sperm) fuse at fertilisation and the chromosome number is restored, giving the new individual two sets of chromosomes, one from each parent. If syngamy does not occur, two haploid gametes will not fuse and the chromosome number will not be restored.
Q.42 Distinguish between oestrus cycle and menstrual cycle.
Ans
Differences between oestrus cycle and menstrual cycle:
|
S.No. |
Oestrus cycle |
Menstrual cycle |
|
1. |
Interval between two consecutive cycles varies between a few months to over a year. |
Interval between two consecutive cycles is about 4 weeks. |
|
2. |
Females exhibiting this cycle are normally sexually active only during the oestrus phase of their cycle. This is also referred as being “in heat”. |
Females exhibiting this cycle can be sexually active throughout their cycle, even when they are not about to ovulate. |
|
3. |
Reabsorbs the endometrium (inner membrane of the mammalian uterus) if conception does not occur during that cycle. |
Shed the endometrium through menstruation if conception does not occur during that cycle. |
|
4. |
No blood loss occurs. |
Blood loss occurs. |
|
5. |
This cycle continues until death. |
This cycle ends at menopause. |
|
6. |
Example: Non primate mammals – cows, sheep, rats, deer, dogs etc. |
Example: Primates -humans, apes |
Q.43 What is asexual reproduction? Discuss briefly the forms of asexual reproduction in animals.
Ans
Asexual reproduction: It is the production of Offspring that are genetically identical to each other and to their parents. It does not involve two parents (male and female) and formation of gametes. Offspring are produced by the process of mitosis.
Various forms of asexual reproduction are:
1. Budding: Small buds are produced on the parent body that remains attached initially to the parent cell which ultimately gets separated and matures into adult organism. E.g., hydra
2. Regeneration: A piece of a parent is detached, which grows and develops into a completely new individual. E.g., star fish
3. Fragmentation: The body of the parent breaks into discrete pieces, each of which can produce an offspring. E.g., planaria
4. Gemmules: A parent releases a specialised mass of cells that can develop into offspring. E.g., sponges
5. Parthenogenesis: Development of an egg into an individual without fertilisation. E.g., wasps, bees, ant and some reptiles
Q.44 Potato is a food source but small potatoes are generally called as potato seeds, so they are directly used for cropping purposes. How does a farmer get a new plant from such a potato seed?
Ans
The method used by the farmer is vegetative propagation. Potato has small eyes bearing buds that grow to form daughter plants.
Q.45 Differentiate the following diagrams based on the process shown.
AnsThe process is Binary fission which is irregular in amoeba (1a and 1b) and longitudinal in euglena (2a and 2b).
Q.46 When multiplication of species can occur with the help of asexual methods, then why do the organisms adapt sexual methods of multiplication?
Ans
Organisms adapt to sexual methods just before the onset of adverse conditions. Sexual reproduction enable these organisms to survive during unfavourable conditions. Moreover, it brings variations and increases the chances of survival and continuity of species.
Q.47 The population of humans on this earth would not have been much if humans had oestrus cycle rather than the menstrual cycle. Give reasons for your justification?
Ans
Oestrus cycle is exhibited by cows, sheep, rats, deer etc., only during the favourable seasons in their reproductive phases and are therefore called the seasonal breeders. If humans had oestrus cycle rather than the menstrual cycle, then they would have been seasonal breeders. So, it might have some reducing effect on the population.
Q.48 Both the prefixes (Uni and Mono) have the same meaning i.e. one in number. Does it mean that unisexual and monoecious species are the same?
Ans
Unisexual is used in reference to the flower (presence of either anther or carpel), whereas Monoecious is used in reference to the plant (morphologically one plant bearing both the sexes in its flowers).
Q.49 In case of sexual reproduction, the number of female gametes is generally limited, but that of the male gametes is innumerable. Give reason for this observation.
Ans
Male gametes have to travel through the female reproductive tract and tolerate its physical and bio-chemical conditions. A large number of male gametes die and fail to reach the female gamete. To compensate this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.
Q.50 Why is internal fertilisation considered to be more advanced method than external fertilisation?
Ans
In internal fertilisation, the egg is formed inside the female body where it fuses with the male gamete. It ensures more chances of fertilisation with less wastage of gametes. Moreover, in external fertilisation, the offspring are extremely vulnerable to predators threatening their survival up to adulthood.
Q.51 What is the difference between the type of gametes and their transfer from male to female organisms of higher animals and higher plants?
Ans
In higher animals, the male gamete is motile and the female gamete is stationary. The male gamete is transferred to female by Insemination. In higher plants, the male gamete is generally transferred by the growth of a pollen tube.
Q.52 Zygote is a vital link that ensures continuity of species between organisms of one generation to its next. Justify.
Ans
It is the end product of sexual reproduction (Zygote) which further develops into next generation, so it ensures continuity of species. Every sexually reproducing organism, including human beings begin life as a single cell – the zygote.
Q.53 Which one is more recent in origin- Ovipary or Vivipary? Support your answer with a valid reason?
Ans
Vivipary (advanced) is more recent in origin. The zygote develops into a young one inside the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones are greater in viviparous organisms.
Q.54 Terror of Bengal, a most invasive aquatic weed, is widely spread and expands rapidly in the growing season.
a. Name the weed.
b. Which multiplication strategy would you attribute to this type of observation?
c. How does it affect the aquatic life?
Ans
a.<span “> The weed is ‘Water Hyacinth’.
b.<span “> Vegetative Propagules.
c.<span “> Rapid growth and death of water hyacinth increases heterotrophic activity of microbes. Increased microbial activity results in the reduction of dissolved oxygen (DO) or increase in BOD in water, thereby resulting in the death of fishes.
Q.55 Sexual reproduction involves syngamy (fusion of male and female gametes), thereby forming a zygote which develops into next generation.
a. What can happen if syngamy fails?
b. Is there any possibility of any exception to this principle?
Ans
a.<span “> Generally if syngamy fails, the zygote would not be formed.
b.<span “> In some organisms like rotifers, honeybees, lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called as Parthenogenesis.
Q.56 In oogamy, female gamete is large and non-motile but the male gamete is reverse in its properties. Why such type of adjustment is there in higher organisms?
Ans
In oogamy, female gamete being larger and non motile, is an adaptation for having more food reserves that may be required for the future development. The male gamete has to move to reach the counterpart, so it has the machinery for its transportation and delivering the chromosomes. A lot of cytoplasm will mean extra weight also. So, both the sex cells have specialised themselves for their functions.
Q.57 Give a brief account of sporulation.
Ans
Sporulation is a type of asexual reproduction in which a resistant coat develops around the spores. The spores are formed through the process of multiple fission followed by development of cyst around them.
During unfavourable environmental conditions, these spores remain inactive and on return of favourable conditions, the cyst is hatched which gradually develops into an adult.
Thus, sporulation also helps an organism to resist unfavourable environmental conditions apart from being a method of reproduction.
The dispersal of spores takes place by various means like air, water, insects, etc.
Q.58 What is regeneration? State any two examples.
Ans
Regeneration is defined as the ability of some organisms to re-grow severed parts. It can occur within their cells, tissues or organs.
For example:
1)<span “> Tail regeneration in lizard: A lizard is capable of regenerating its lost tail. This act is known as autotomy or self amputation. It is usually a self-defense mechanism designed to escape from a predator’s grasp or from any other danger.
2)<span “> Damaged liver regeneration in humans: The liver is the only internal human organ capable of natural regeneration. Even 25% of the liver can regenerate into complete liver.
Q.59 What is fragmentation? Give two examples.
Ans
Fragmentation is a method of asexual reproduction in which an organism gets divided into fragments and each each fragment is capable of growing into a complete organism.
Each fragment is able to develop into a mature individual. For example:
1)<span “> Fragmentation in Planaria
2)<span “> Fragmentation in sea stars
Q.60 Jack conducted an experiment to find out the concentration of sugar which is required for the pollen grains of a plant to produce pollen tubes.
The below given table illustrates the result of his experiment.
Based on the given data, try to answer the following questions:
- What percentage of pollen grains produced pollen tubes in each solution?
- Draw a graph to show how growth of pollen tubes varies with concentration of sugar.
- What would you expect the concentration of sugar in the stigmas of the flowers to be?
- How does sugar concentration affect the growth of pollen tube?
Ans
a)
b)
c)
The experiment is showing best results with 10% concentration of sugar solution. Thus, the concentration of sugar in the stigmas of the flowers should be 10% to grow pollen tubes down to the ovule.
d)
When a pollen grain lands on the stigma of another plant, the tube cell uses stored nutrients and sugars to grow a pollen tube down to the ovule. The ability of the pollen grains to grow tubes is affected by the concentration of sugar.
As the sugar concentration increases, the growth of the pollen tube increases; up to a concentration of 10%; after which any increase in sugar concentration inhibits the growth of pollen tubes.
Q.61 Suppose a new unisexual plant species has invaded an area ‘X’, where it will be pollinated by new pollinating agents. The distribution of length of red coloured petal at this time is illustrated in graph (a).
Over 30 generations, the flower of this plant species evolved in order to adapt to the new pollinating agents in the environment. The distribution of length of petal after 30 generations is illustrated in graph (b).
Based on your analysis, answer the following questions:
- What can you interpret from these data? [3 Marks]
- Suppose this plant species is covered with net. What will happen? Justify your answer. [3 Marks]
Ans
- From these data, it can be interpreted that the range of petal length decreased over the course of 30 generations. This could be because pollinating agents avoided to visit the flowers with extremely long or short petals. Also, the number of different pollinating agents could have decreased with the passage of time due to decrease in petal size.
- If this plant species is covered with net, pollinating agents like insects, birds or animals will not be able to visit the flowers. Since the flowers of this plant species are brightly coloured, their pollen grains are not dry, light weighted and powdery, they cannot be pollinated by fast blowing wind.
Q.62 Ramanathan is a part of a land rehabilitation program operated by an NGO. He has to choose any one of the below given plant species for rehabilitating a large area of land damaged as a result of frequent fire caused naturally. Which species will he choose? Give three reasons to support your answer.
- Dalbergia sissoo (Shisham): tree that can reproduce with tuberous adventitious roots
- Elymus repens (Quackgrass): perennial grass that can reproduce with rhizomes
- Begoniella Oliv (Begonia): herb that can reproduce with adventitious buds on leaves
Ans
He shall choose the Elymus repens species; as
- It will regrow in case of a subsequent fires; because it is able to reproduce with the help of underground stem
- Grass can grow in short period of time with minimum or no post-plantation cost
- It is perennial plant hence can regrow for many years and serve as a permanent solution for the area
Q.63 Name the agents of pollination in the following cases:
- Flower 1: Bright coloured with scent and nectar glands present [ 1.5 Marks]
- Flower 2: Colour, scent and nectar are absent but pollen grains are dry, light weight and powdery. Its stigma is feathery. Also give two examples for (i) & (ii). [ 1.5 Marks]
Ans
|
Cases |
Agents of pollination |
Examples |
|
Flower 1 Bright coloured with scent and nectar glands present |
Insects |
Lily and Rose |
|
Flower 2 Colour, scent and nectar are absent but pollen grains are dry, light weight and powdery. Its stigma is feathery. |
Wind |
Grass and pine |
Q.64 Surinder has a garden with the Jasmine, Nerium, Gulmohar, Rose, Lotus, Corn, Pearl Millets, Barley, Chrysanthemum, Dahlia, Grass, Coconut and Pea plants. A swarm of bees visited his garden.
- Will the bees visit all the flowers?
- Name the flowers which will attract the bees.
- Give reasons to support your answer.
Ans
- No, the bees will not visit all the flowers
- A swarm of bees will visit the following flowers:
- Jasmine
- Nerium
- Gulmohar
- Rose
- Lotus
- Chrysanthemum
- Dahlia
- Pea
- Reason: These flowers have bright petals and sweet fragrance which attract insects thus, show pollination by insects also known as Entamophily.
Q.65 Two farmers ‘A’ and ‘B’ cultivated peas (Pisum sativum) in their fields. Farmer A covered his fields with nets to restrict the entry of birds and insects. Farmer B left his field uncovered.
- Name the type of pollination that would occur in field ‘A’ and field ‘B’.
- Which of these fields will give a higher yield?
- To raise the next crop, from which field should the seeds be chosen by the farmers. Give reason to support your answer.
Ans
- The type of pollination that would mainly occur in field ‘A” is self-pollination. The type of pollination that would mainly occur in field “B” is cross pollination.
- Field ‘B’ will give a higher yield because of cross pollination.
- To raise the next crop, seeds should be chosen from field ‘B’ because cross-pollination leads to the production of new varieties and more viable seeds.
Q.66 Mango and Coconut are both drupes. The mesocarp of mango is edible, while it is not so in coconut. Based on this fact, answer the following:
- Which part of the coconut is edible?
- Why does the coconut have a fibrous mesocarp?
- Can you mention any other use of the fibrous mesocarp?
Ans
- The endospermic tissue of the coconut seed is its edible part.
- The fibrous mesocarp of coconut helps in its dispersal by water. It facilitates floating of fruit in water and carrying away by water currents.
- Fibrous mesocarp forms a thick protective covering around the coconut seeds. It is used to produce coir which is used to make ropes, mats and carpets etc.
Q.67 (a) Everybody loves the fragrance of Jasmine flowers, which are white and small in size. Do you know why these flowers are white, fragrant and appear in clusters?
(b) You must have seen Gloriosa flower in bouquets with its bright orange coloured single accessory whorl. In Gloriosa flower, stamens are exerted that is extending beyond the corolla.
What is the evolutionary purpose of exerted stamen?
[1.5+1.5=3 marks]
Ans
(a)The white colour of the flowers of Jasmine makes them more readily visible at night to insects (pollinating agents). Jasmine flowers produce fragrance. Insects detect the fragrance easily and they follow the scent trail to flowers. As the flowers are small, a number of flowers remain clustered to appear like inflorescence to make them conspicuous to insects.
(b) Exerted stamen is the way to prevent self-pollination as the pollen grains from the flower cannot pollinate itself.
In this plant, pollen grains of one flower transfer to the stigma of another flower of a different plant.
This type of pollination is called cross pollination.
This ensures better variation; hence, better chances of perpetuation.
Q.68 Two farmers cultivated two different types of crops namely, A and B. Crop ‘A’ reproduces by asexual means and crop ‘B’ reproduces by sexual means.
Both the farmers got their crops insured. The premium paid for crop A was more than the crop B.
What is the possible reason behind the difference in the premium?
Ans
Genetic variation in asexual mode of reproduction is low as compared to the sexual mode of reproduction.
Plant breeding through sexual mode of reproduction will have more variants.
The emergence of any disease is more likely to devastate asexually reproducing plant species as compared to that of sexually reproducing species.
Hence, the premium of crop A was more than the crop B.
Q.69 Following diagram shows an experiment which was set up to investigate the conditions required for germination.
The seeds in flask A are in dry cotton wool; while the seeds in flasks B, C, and D are in wet cotton wool. Some of the seeds will germinate and some will not. Now complete the table by stating whether the seeds in flasks A, B, C and D will germinate or not. Also explain the answer.
|
Flask |
Will the seeds germinate? write YES or NO |
Explanation |
|
A |
|
|
|
B |
|
|
|
C |
|
|
|
D |
|
|
Ans
|
Flask |
Will the seeds germinate? write YES or NO |
Explanation |
|
A |
No |
Test tube does not possess water and water is required for germination. |
|
B |
No |
Test tube does not possess oxygen and oxygen is required for germination. |
|
C |
Yes |
Both oxygen and water are available in the test tube. |
|
D |
Yes |
Both oxygen and water are available in the test tube and light is not required for germination. |
Q.70 A scientific study was conducted to know how flowers of white deadnettle were pollinated. For this study, they counted the number of visits made by different insects to flowers of white deadnettle. The results are exhibited by the below given pie chart.
Analyse the data and answer the following questions:
- What attracts insect pollinators towards the flowers of white deadnettle?
- Why bumblebees paid most visits to the flowers?
- How bees facilitate cross-pollination in white deadnettle?
- What prevents the bees from accidentally self-pollinating the flower?
- What makes it easier for bees to locate the flowers of white deadnettle?
Ans
- To attract insect pollinators, following structural adaptations are found in the flowers of white deadnettle:
- The flowers of white deadnettle emit a fragrance and are equipped with nectar guides.
- The corolla and stamens had glandular trichomes and papillae which secrete essential oils.
- In the flowers of white deadnettle, the nectaries are present in the shape of an irregular disc at the base of the corolla tube surrounding the base of the ovary. Thus, the nectar is only accessible to long-tongued insects like bumble bees.
- In the flowers of white deadnettle, the anthers are positioned in such a way that sticky pollen from them brush against the bee’s back as it pushes its head down the corolla tube. When the bee enters another flower, it brushes some of the pollen against the ripe stigma and cross-pollination is achieved.
- In the white deadnettle, the anthers ripen before the stigma, which prevents the bees from accidentally self-pollinating the flower.
- A number of deadnettle flowers cluster together and form a more conspicuous inflorescence. It is easier for bees to locate this inflorescence.
Q.71 Lumbricina and Crassostrea virginica are hermaphrodites that bear both male and female reproductive organs. So, ideally, they must reproduce by self fertilisation.
Read the given information and justify the given statements:
- Why does Lumbricina follow the mode of cross fertilisation?
- Why does Crassostrea virginica follow the mode of cross fertilisation?
- If Lumbricina and Crassostrea virginica follow the mode of cross fertilisation, then why have hermaphrodites evolved in nature? Support your answer by suggesting two possible conditions.
Ans
- Though Lumbricina (earthworm) is a hermaphrodite, it follows the mode of cross fertilisation because its male reproductive part matures prior to the female reproductive part. This condition is popularly known as protandry.
- Though Crassostrea virginica (American oyster) is a hermaphrodite, it follows the mode of cross fertilisation because it produces sperms in the first year and ova in the next year of attaining maturity. This regular alternation of sexes continues once the maturity is attained.
- Hermaphrodites have evolved in nature under the following conditions:
- Inability to find a suitable mate
- Presence of small and genetically isolated population
Q.72 In Apis species, sex is determined by set of chromosomes received by an organism.
Analyse the given information and answer the following questions:
- Identify the gender of Apis species if an ovum is not fertilised. Also, mention the process by which this species is formed.
- Under what condition, female species is produced?
- Suppose the number of chromosomes in a female species is m, work out a rough cross to depict the occurrence of species in F1 generation.
Ans
- If an ovum is not fertilised, male species (drone) is formed by the process of parthenogenesis.
- Female species (worker or queen) is formed when an ovum is fertilised by a sperm.
- If a female has m number of chromosomes, then a male will have m/2 number of chromosomes.
Q.73 Ramanathan is a part of a land rehabilitation program operated by an NGO. He has to choose any one of the below given plant species for rehabilitating a large area of land damaged as a result of frequent fire caused naturally.
Which species will he choose? Give three reasons to support your answer.
- Dalbergia sissoo (Shisham): tree that can reproduce with tuberous adventitious roots
- Elymus repens (Quackgrass): perennial grass that can reproduce with rhizomes
- Begoniella Oliv (Begonia): herb that can reproduce with adventitious buds on leaves
Ans
He shall choose the Elymus repens species for the area of land damaged due to frequent fire caused naturally as:
- It will regrow in case of subsequent fires; because it is able to reproduce with the help of underground stem.
- Grass can grow in short period of time with minimum or no post-plantation cost.
- It is perennial plant hence; can regrow for many years and serve as a permanent solution for the area.
FAQs (Frequently Asked Questions)
It contains three cells of the egg apparatus, three antipodals and one central cell, making seven cells. The central cell contains two polar nuclei, so the total number of nuclei is eight.
In both cases, the pollen comes from the same plant. Geitonogamy requires a pollinating agent because pollen moves between two flowers, but it does not introduce pollen from a genetically different plant.
Two fusion events occur in one embryo sac. One male gamete forms the zygote through syngamy, while the other forms the triploid primary endosperm nucleus through triple fusion.
The endosperm forms the nutritive tissue required by the developing embryo. Its early formation ensures that food is available when the zygote begins active development.
Apomixis allows seeds to form without meiosis and fertilisation. It prevents segregation of hybrid characters and may allow farmers to reuse hybrid seeds without losing desirable traits.