CBSE Class 12 Biology Revision Notes Chapter 1
Introduction
Biology is a crucial subject to master for students planning to pursue a career in science and medical-oriented streams. Class 11 and 12 Biology lays a solid foundation for the preparation of the same. Students are suggested to prepare a study plan and comprehensive notes for Biology because of the extensive CBSC syllabus. Students should get into the practice of creating the chapter notes as they are studying the subject from the NCERT textbook along with other reference books. The CBSC revision notes should include all the important things in line diagrams and pointwise writing. Extramarks’ academic team has created class 12 Biology chapter 1 notes by referring to the NCERT textbook and various other books.
Students may register on Extramarks to access class 12 Biology chapter 1 notes, CBSC class 12 syllabus. In addition to the syllabus, they may also refer to CBSC sample papers, CBSC revision notes, CBSC important questions, CBSC previous year question papers and CBSC extra questions.
Key Topics Covered In Class 12 Biology Chapter 1 Notes
The span from birth to the natural death of an organism is called its life span. A Butterfly has a lifespan of 1 to 2 weeks, whereas a Parrot has 140 years. During their life span, organisms reproduce several times. Reproduction is essential because it allows the continuity of the species, generation after generation. In the absence of reproduction, there will be no existence of life on earth. In this chapter, students will learn about reproduction in organisms throughout their lifespan. The key topics covered under Class 12 Biology Chapter 1 Notes – Reproduction in Organism include
Reproduction
It is a biological process of producing new young ones (offspring) similar to parents. The young ones grow, mature, and in turn, produce new young ones. Hence, there is a cycle of birth, growth and death.
Reproducing offspring enables the continuity of the species, generation after generation. Reproduction also facilitates evolution by genetic variation through the intermingling of species and inheritance from the process of reproduction ( in the case of sexual reproduction).
There are two kinds of reproduction.
1. Asexual Reproduction:
In this mode of reproduction, progeny is produced by a single parent with or without gamete formation. The offspring are not only identical but are also exact copies of their parent (genetically and morphologically identical) because there is no intermingling of gametes. The term ‘Clone’ is used to describe such similar individuals.
2. Sexual Reproduction:
If two parents of opposite sex reproduce together and the process involves the fusion of male and female gametes to form a zygote, it is called sexual reproduction.
Asexual reproduction and its types
Under this section of class 12 Biology chapter 1 notes, we learn about asexual reproduction. Asexual reproduction is mostly seen in unicellular organisms, as well as in plants and animals with relatively simpler organisations. It is also seen in multicellular organisms. Cell division itself is a mode of reproduction in Protists and Monerans. Parent cell divides by simple mitotic division into two to give rise to new individuals.
The commonly seen modes of asexual reproduction in the case of animals are as follows:
- Fission: where a cell divides into two halves, and each rapidly grows into an adult is called Binary fission(e.g., Amoeba, Paramecium). Under unfavourable conditions, the Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed encystation. The encysted Amoeba divides by multiple fission and produces many minute amoebae; when favourable conditions return, the cyst wall bursts out, and the spores are liberated to grow up into many amoebae. This phenomenon is known as sporulation.
- Budding: In this process, a cell divides unequally, and small buds grow out and remain attached to the parent cell, which eventually gets separated and matures into new organisms (cells), e.g.Yeast cells. In the case of Hydra, the growth of the region of the body occurs called budding that separates out from the main body. In the case of sprouting corals, the bud does not fall off and reproduces as part of a new colony.
- Fragmentation: In this mode of asexual reproduction, if the body breaks into apparent pieces (fragments), each fragment grows and forms an adult that is capable of producing new young ones. A few animal species, including starfish, Flatworms, etc., reproduce using fragmentation.
Asexual reproductive structure
Kingdom Monera, Protista, Fungi and simple plants such as algae reproduce through special asexual reproductive structures. Tiny, single-celled, thin-walled spores that are extremely resistant to environmental extremes. The spores form new individuals.
For e.g.,
- Psedopodio spore( with fine pseudopodia) – Amoeba
- Zoospores( motile with flagella) – Chlamydomonas
- Conidia(non-motile) – Penicillium
- Sporangiospores(non-motile) – Rhizopus
- Gemmules( internal buds) -Sponges
Vegetative Propagation in Plants: The class 12 Biology chapter 1 notes give a detailed explanation of vegetative propagation. In plants, the term vegetative reproduction is used, although it is a type of asexual reproduction. The growth of a new plant from an organ of the mother plant that takes place under suitable environmental conditions is called vegetative propagation. The units of vegetative propagation in plants such as sucker, runner, rhizome, bulb, tuber and offset all have the capability to give rise to the new plant. These structures are called vegetative propagules. These altered organs can develop from the stem, leaf, root, or even the flower. In some filamentous algae, if the body breaks into distinct pieces (fragments), each fragment grows into an adult, e.g.Spirogyra.
Artificial method of vegetative propagation: In this method, a small part of a plant organ (stem, root, leaf) is used to obtain a new plant. A new plant originates invariably from the nodes present in the stem or modified stem. When these nodes come in contact with water or damp soil, they give rise to roots and new plantlets e.g.rhizome of ginger, Banana and sugarcane. Similarly, in Bryophyllum, adventitious buds arise from the notches present on the margin of leaves. Farmers and gardeners use these abilities of plants for artificial propagation of plants by cutting, grafting and layering plant parts.
Looking more deeper into class 12 Biology chapter 1 notes on Vegetative propagation; it is a type of asexual reproduction seen in plants. In this, new plants arise without the involvement of sexual structure. It involves the vegetative part of plants to obtain new plants such as the rhizome, sucker, tuber, bulb, offset etc. Many plants reproduce naturally by vegetative propagation, and many are grown artificially by vegetative propagation, and the offspring produced are genetically identical. E.g., Buds- eyes of Potato and bulb – Onion.
Vegetative propagule
|
WATER HYACINTH (Terror of Bengal): The class 12 Biology chapter 1 notes give a detailed explanation of the Terror of Bengal.
- This is one of the most invasive weeds due to the phenomenal rate of vegetative propagation.
- Grows wherever there is standing water.
- Uses all the oxygen from water which leads to the death of fish
- It was introduced in India because of its pretty flowers and the shape of its leaves.
- It propagates with the help of offsets.
In an organism with simple organisation, like fungi and algae, asexual reproduction is most common, but if they are in adverse conditions, they shift to a sexual mode of reproduction. Higher plants exhibit asexual (vegetative) as well as sexual modes of reproduction, but in the case of animals, only the sexual mode of reproduction is present most of them.
Sexual Reproduction And Its Phases.
In sexual reproduction, individuals of opposite sex form male and female gametes and a “zygote” are formed by the fusion of these gametes, which develops to form a new individual. Offspring produced are not identical to each other or to the parents. All sexually reproducing animals, whether they are animals, plants or fungi, share a similar pattern of reproduction.
Phases in Sexual Reproduction
Under the class 12 Biology chapter 1 notes, the phases of sexual reproduction are explained in detail and include
- Juvenile phase- The period from birth to sexual maturity is called the juvenile phase. In the case of plants, it is known as the vegetative phase. When the juvenile/ vegetative phase comes to an end, it marks the beginning of the reproductive phase.
- Reproductive phase- In higher plants, the beginning of the reproductive stage is flowering. Few plants show flowering in a particular season, and some others show in all seasons. Few plants like Bamboo species flower only once in a lifetime(after 50 to 100 years), and Strobilanthus kunthiana (neelakuranji) flowers only once in 12 years.
Among animals, birds lay eggs seasonally, the female placental mammals exhibit cyclical changes in the activities of hormones, ovaries and accessory glands during the reproductive phase of sexual reproduction. In non-primate mammals like sheep, cows, rats, deers, dogs, tigers, etc., such cyclical changes during reproduction are called the oestrus cycle, whereas, in primates (monkeys, apes, and humans), it is called the menstrual cycle.
Many mammals exhibit such cycles only during favourable seasons, hence are called seasonal breeders. Other mammals who actively reproduce throughout their reproductive phase are called continuous breeders.
- Senescent phase- This phase is the end of the reproductive phase, and old age ultimately leads to death.
Transitions between the three phases happen in both plants and animals due to hormones. Interaction between hormones and certain environmental factors regulates the reproductive processes.
Events In Sexual Reproduction:
All sexually reproducing organisms show events and processes that have remarkable fundamental similarities. Its three different stages are; Prefertilization, Fertilisation, and Post-fertilisation.
Pre-fertilization: Under class 12 Biology chapter 1 notes, students will study pre-fertilisation. It includes gametogenesis and gamete transfer.
Gametogenesis
The procedure of development of female and male gametes. Gametes are haploid cells (having a single set of chromosomes) that may be similar or dissimilar in structure. In algae, both gametes are similar in a structure called homogametic(isogametes). In a higher organism that reproduces sexually, two morphologically different gametes are formed, called heterogametes, Female gametes are called ovum or egg, and male gametes are called antherozoid or sperm.
Are meiosis and gametogenesis always interlinked? The class 12 Biology chapter 1 notes provide answers to this. Gametogenesis is the process of gamete formation; during gametogenesis, a diploid organism(2n) needs to reduce its chromosome number to half (n) so that both the parents can contribute equally( n+n=2n). Meiosis is the kind of cell division that reduces the number of chromosomes by half that occurs during gametogenesis. Hence, meiosis and gametogenesis are said to be interlinked.
In fungi and plants, homothallic monoecious terms are used to denote the bisexual condition and heterothallic and dioecious are used for the unisexual condition. The unisexual male flower is staminate in flowering plants,i.e., bearing stamens, while a female is pistillate or bearing pistils.
In animals, species that possess both female and male reproductive organs in the same individual are called bisexual or hermaphrodites( earthworms, sponges, tapeworms etc.), and having either male or female reproductive organs are called unisexual (human, cockroach).
Students will also learn about Cell division in the course of gamete formation in class 12 Biology chapter 1 notes.
Cell division in the course of gamete formation: In all heterogametic species, gametes are of two types, namely, male and female. Gametes are always haploid though the parent plant body may be either diploid or haploid. A haploid (n)parent produces gametes by mitotic division.
Many organisms belonging to monera, algae, fungi, and bryophytes have haploid plant bodies, but in organisms belonging to pteridophytes, gymnosperms, angiosperms and most animals, including human beings, the parental body is diploid. In diploid(2n) organisms, specialised cells called meiocytes (gamete mother cells) undergo meiosis and form haploid gamete. One set of chromosomes gets incorporated into each gamete at the end of meiosis.
For example,
- Ovary – Diploid (2n)
- Anther – Diploid (2n)
- Pollen – Haploid (n)
- Egg – Haploid (n)
- Male gamete – Haploid (n)
- Zygote – Diploid (2n)
Gamete Transfer
Students can learn about Gamate Transfer by referring to class 12 Biology chapter 1 notes. In a majority of organisms, the male gamete is motile, which exhibits movement and the female gamete is stationary. In a few algae and fungi, both types of gametes are motile. In lower plants like algae, bryophytes and pteridophytes, water is the medium through which male and female gametes move. The number of male gametes is much more than female gametes as most of the male gametes fail to reach the female gametes.
In angiosperms and gymnosperms, pollen grains are the carriers of male gametes, and the ovule has the egg. Pollen grains must be transmitted to the stigma( receptive part of the female reproductive organ) to facilitate fertilisation. The transfer of pollen from the anther to the stigma is called pollination.
Pollination may be self (pollen transfer from anther to the stigma of the same flower) or cross (pollen transfer from anther to the stigma of a different flower). Pollen grains germinate and give rise to a tube-like structure on the stigma, called pollen tubes, which carry the male gametes to the ovule and discharge male gametes near the egg. Coming together of both the gametes is essential for fertilisation.
The Extramarks class 12 Biology chapter 1 notes provide more information to enable students to understand the concepts better.
Bisexual Flowers: The flowers having both female and male reproductive parts are considered bisexual flowers. The androecium is the male reproductive structure( anther), while the female reproductive structure is the gynoecium( pistil). For instance, Solanum Lycopersicum (Tomato), Hibiscus (Gudhal), Brassica (Mustard), Helianthus (Sunflower), and Narcissus (Daffodil) are some examples of bisexual flowers.
Unisexual flowers: Flowers that bear stamens( male reproductive structure) are called staminate flowers, while the flowers that bear pistil( female reproductive structure) are known as pistillate flowers. Cucurbit plants bear unisexual flowers, i.e., separate male and female flowers but are on the same plant.
The staminate flowers of cucurbit have bright yellow petals. It obviously has stamens which function as the male reproductive structures of a flower. The female reproductive structure is present on pistillate flowers, and the mode of pollination is cross-pollination. If a female flower is fertilised, a small fruit may be visible. Another plant that bears unisexual flowers is Papaya.
Fertilization:
The class 12 Biology chapter 1 notes details on fertilisation. The fusion of male and female gametes to form a diploid zygote is called syngamy or fertilisation. When a female gamete develops into a new organism without fertilisation, it is called Parthenogenesis.
External fertilisation is a mode of reproduction where syngamy( fusion of male and female gamete) occurs in the external medium(water), i.e., outside the body of the organisms. External fertilisation is observed in most algae, fishes and amphibians such as frogs. However, there is a certain disadvantage of external fertilisation:
- The possibility of survival of the gametes is low.
- Not all the eggs are fertilised and are wasted
- The predators usually eat the eggs.
- The offspring are extremely vulnerable to predators.
- A lack of adequate parental care results in a low survival rate.
Difference between External Fertilisation and Internal Fertilisation.
| External Fertilisation
Syngamy occurs outside of the body of the organism. A large number of gametes are released in the surrounding medium. e.g., Amphibians, Bony fishes |
Internal Fertilisation
Syngamy takes place inside the organism’s body. There is less ova produced, but a large number of male gametes are released, which travel towards the ovum. The male gamete is motile and hence reaches the egg. E,g., Mammals, Birds. |
Vegetative reproduction is considered to be a type of asexual reproduction as it does not involve the fusion of male and female gametes. In this, a new plant grows from some vegetative part of the plant, such as root stem or leaves and is genetically identical to the parent plant.
Post-fertilisation: As explained in class 12 Biology chapter 1 notes, the post-fertilisation process includes
- The formation of the diploid zygote is universal in all sexually reproducing organisms.
- The zygote is the vital link that ensures continuity of species between organisms of one generation and the next. Every sexually reproducing organism, including human beings, begins life as a single cell–the zygote.
- In organisms with external fertilisation, a zygote is formed in the external medium, whereas in those exhibiting internal fertilisation, the zygote is formed inside the organism’s body.
- In fungi and algae, the zygote is thick-walled and resistant to desiccation and damage. It germinates after a period of rest.
- In organisms with a haplontic life cycle, the zygote divides by meiosis to form haploid spores that grow into haploid individuals.
For example, the post-fertilisation changes in a flower include:
- After the process of fertilisation, the sepals, petals and stamens of the flower fall off, but the pistil remains attached to the plant.
- The zygote develops into an embryo through the process of embryogenesis.
- The ovule turns into the seed.
- The ovary grows into the fruit, which develops a thick wall called the pericarp, which is protective in function.
Embryogenesis
As per class 12 Biology chapter 1 notes, embryogenesis is the process of development of an embryo from the zygote. The zygote undergoes cell division (mitosis) and cell differentiation. Cell divisions increase the number of cells; cell differentiation means groups of cells go through certain modifications to create specialised tissues and organs to form an organism.
Following are the main differences between gametogenesis and embryogenesis as mentioned in class 12 Biology chapter 1 notes.
| Gametogenesis | Embryogenesis |
| It is the process of formation of haploid male and female gametes from diploid meiocytes. | It is the process of the development of an embryo from a diploid zygote. |
| Both mitosis and meiosis occur during the process. | Repeated mitosis occurs during this process. |
| Spermatogenesis and oogenesis are the two processes of gametogenesis. | Embryogenesis leads to organogenesis by cell division and cell differentiation. |
| Occurs before fertilisation. | Occurs after fertilisation. |
| It occurs inside the ovaries and testes in animals, and in plants, inside the antheridia and archegonia. | It happens inside the female reproductive system of animals and in female gametophytes in plants. |
Oviparous: The development of the zygote occurs outside the body of the female parent; they lay fertilised or unfertilised eggs. After a certain period of incubation, young ones hatch out. e.g., Reptiles and Birds.
Viviparous: The development of the zygote takes place inside the female parent’s body. The offspring are delivered out of the body of the female after a certain time. e.g., Human beings and the majority of mammals.
Furthermore, in class 12 Biology chapter 1 notes, students learn that Oviparous animals lay eggs(fertilised or unfertilised), and the foetus gets mature inside the eggs. Eggs laid are at a bigger risk of being destroyed by other animals (predators) or environmental factors. In the case of viviparous organisms, the foetus develops inside the female, and because of proper embryonic care and protection, the chances of survival of offspring are greater in viviparous organisms.
In flowering plants, the zygote develops inside the ovule. After fertilisation, the sepals, petals and stamens fall off. The zygote develops into the embryo (progenitor and the next generation) ovules develop into the seed. The ovary develops into the fruit within a thick-walled protective covering called pericarp. After dispersal, seeds germinate in the presence of favourable conditions to give rise to new plants.
Despite its complex nature, higher organisms undergo sexual reproduction because the organisms produced by the fusion of two different gametes are genetically different. Due to these differences, they are well adapted to changing and challenging environmental conditions and usually have higher survival rates.
Difference between Sexual and Asexual reproduction.
Studying deeper into class 12 Biology chapter 1 notes, students must understand the difference between sexual and asexual reproduction. Learning about the difference can help students understand the two modes of reproduction better.
Chances of survival: Sexual reproduction better promotes genetic diversity through a new combination of genes during gametogenesis( meiosis ), and progeny inherit a mixture of genes from both male and female parents, so they are different from each other and their parents. So they can adapt to new or changing environments, which gives them a survival advantage. e.g., the disease is less likely to infect all the individuals in a population. However, asexual reproduction is faster and consumes less energy and time. So sometimes, offspring produced by asexual reproduction can survive better.
The difference in offspring: The progeny originated by asexual reproduction involves a single parent and is genetically identical(clone) to the parent, whereas the progeny originated by sexual reproduction are formed when male and female gametes ( with a new combination of genes) fuse together, and offspring are genetically and morphologically unique.
Other dissimilarities between sexual and asexual reproduction as per class 12 Biology chapter 1 notes are mentioned below:
| Sexual Reproduction | Asexual Reproduction |
| It occurs by the fusion of male and female gametes. (two parents involved) | The organism arises from a single parent. |
| The offsprings produced are different from the parents due to:
i) The Recombination of genes during gametogenesis. ii)Two different sets of chromosomes come together to form a zygote. |
The offspring produced are genetically similar to their parents and are considered clones. |
| It is in higher invertebrates and all vertebrates. It involves the process of fertilisation. | It is found in lower organisms. Budding, Fragmentation, Binary fission, spore formation and even cloning are considered asexual reproduction. |
| It is a time taking process. | It is a time and energy-efficient process compared to sexual reproduction. |
The Extramarks class 12 Biology chapter 1 notes differences between a zoospore and a zygote are mentioned below:
| Zoospore | Zygote |
| Result of asexual reproduction. | Result of sexual reproduction. |
| These are produced inside the zoosporangium. | These are formed by the fertilisation of male and female gametes. Either inside the female parent or outside the organism. |
| Motile and flagellated spore. | Non-motile. |
| Can be diploid or haploid. | Diploid. |
| It takes part in dispersal. | Does not take part in dispersal. |
| It is found in fungi, algae and protozoans. | Found in higher organisms. |
Class 12 Biology Chapter 1 Notes: Exercises & Answer Solutions
Extramarks provides a Biology syllabus where class 12 students can get proper knowledge of every chapter with ease. Referring to the syllabus helps students to be prepared with the necessary study materials. Subjects like Biology will become so easy to study once students access the syllabus, refer to the notes and concentrate on important questions. Students may register on Extramarks to access the Class 12 Biology chapter 1 notes any time they need them on Extramarks.
Class 12 Biology chapter 1 notes Reproduction in Organism is provided here with a simple explanation. The notes are based on the CBSE syllabus and can benefit the students immensely. All questions and solutions are from the NCERT books of class 12 chapter 1. In addition to Class 12 Biology chapter 1 notes, students may access study materials by below.
The Class 12 Biology chapter 1 notes and study materials provided by Extramarks are prepared by experts and are 100% accurate.
NCERT Exemplar for Class 12 Biology Chapter 1 Notes
Extramarks platform provides NCERT class 12 Biology chapter 1 Exemplar, which has a wide range of CBSE extra questions for students to increase their knowledge. With the help of these notes, students would have all the study material related to chapter 1 Biology in one place.
Solving CBSE sample papers will also help the students to understand how much and which content should be given more time to study. Study material such as class 12 Biology chapter 1 notes enable quick and thorough revision for subjects like Biology. Students can score really well in class 12 CBSE board exams by getting access to academic materials from Extramarks.
Key Features of Class 12 Biology Chapter 1 Notes
Key features of Class 12 Biology Chapter 1 Notes are as under.
- It follows the CBSE Syllabus.
- It is prepared by the experts at Extramarks.
- It provides a broad understanding of all the concepts.
- It is focused on preparing short and pointwise notes for quick revision.
- It provides authentic knowledge in an easy language.
- It enhances time management skills.
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)
1. What is a ‘Hydra’?
Hydra is a freshwater invertebrate animal of phylum Cnidaria and class Hydrozoa.
2. What is the Oestrous cycle?
The females of placental mammals consist of cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase of sexual reproduction. In non-primate mammals like cows, deers, dogs, tigers, etc., These cyclical changes during reproduction are called the oestrus cycle.
