2021 Solved Old Paper (BOT-101)
Ans.
Eubacteria:-
> Eubacteria is a large group of bacteria having rigid cell walls, flagella, DNA (single circular chromosome), and a single cell lacking a nucleus.> All types of bacteria are included (Gram’s positive and negative) under the eubacteria except for archaebacteria.
> They are single-celled prokaryotic microorganisms, also known as true bacteria.
> They consist of a range of characteristics and are present in various conditions throughout the world.
> All living organisms are classified into three domains of life: Domain Archeae, Domain Eukaryota, and Domain Eubacteria.
> Eubacteria are a complex domain of the kingdom Monera.
Ans.
General account of immunology:-
> Immunology is the study of the immune system and is a very important branch of the medical and biological sciences.
> The immune system protects us from infection through various lines of defence.
> If the immune system is not functioning as it should, it can result in disease, such as autoimmunity, allergy and cancer.
> The immune system consists of a complex network of cells and molecules, and their interactions.
Types of immunity:- There are two major types of immunity:
1. Innate Immunity or Natural or Non-specific Immunity
2. Acquired Immunity or Adaptive Immunity
1. Innate Immunity or Natural or Non-specific Immunity:-
> This type of immunity is present in an organism by birth.
> This is activated immediately when the pathogen attacks. Innate immunity includes certain barriers and defence mechanisms that keep foreign particles out of the body.
> Innate immunity refers to the body’s defence system.
> This immunity helps us by providing the natural resistance components including salivary enzymes, natural killer cells, intact skin and neutrophils, etc. which produce an initial response against the infections at birth prior to exposure to a pathogen or antigens.
> It is a long-term immunity in which our body produces the antibodies on its own. Our body has few natural barriers to prevent the entry of pathogens.
2. Acquired Immunity or Adaptive Immunity:-
> Acquired immunity or adaptive immunity is the immunity that our body acquires or gains over time. Unlike the innate immunity, this is not present by birth.
> The ability of the immune system to adapt itself to disease and to generate pathogen-specific immunity is termed as acquired immunity. It is also known as adaptive immunity.
> An individual acquires the immunity after the birth, hence is called as the acquired immunity.
> It is specific and mediated by antibodies or lymphocytes which make the antigen harmless.
> The main function of acquired immunity is to relieve the victim of the infectious disease and also prevent its attack in future.
> It mainly consists of an advanced lymphatic defence system which functions by recognizing the own body cells and not reacting to them.
> The immune system of our body identifies the pathogens which have encountered in the past. It is mainly caused when a person comes in contact with the pathogen or its antigen.
> Our body starts producing antibodies to engulf the pathogen and destroy its antigen.
> When it encounters for the first time, it is called a primary response. Once a body gets used to these pathogens, antibodies are ready to attack them for the second time and are known as naturally acquired immunity.
Ans.
The principal pigments are chlorophylls a (green), c-phycocyanin (blue) and c-phyco- erythrin (red). In addition, other pigments like β-carotene and different xanthophylls like myxoxanthin and myxoxanthophyll are also present.
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> The Dasycladales has perhaps the best-known fossil record of any group of green algae. More than 120 fossil genera have been described, some dating back to Cambrian and Precambrian strata. This fossil diversity contrasts remarkably with the fact that today there are only eight extant genera, including the familiar "mermaid's cup" Acetabularia. The Dasycladales began to rapidly diversify in Middle Ordovician, and are common in all strata until the lower Cretaceous. Both living and extinct species are known primarily from warm marine waters,
> Many members of the Dasycladales secrete lime (calcium carbonate) which increases their chances for preservation and later discovery as fossils. The group is easily recognized by their radial symmetry, with a central nonseptate axis to which are attached whorls of lateral appendages which may or may not be branched. Fossil forms are cylindrical, club-shaped, or have a spherical appearance from the density of their branches.
> Botryococcus is another "green alga" known from the Carboniferous onward. It is a colonial member of the Tetrasporales, and may be responsible for certain coal and petroleum deposits.
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Cyanobacteria:-
1. Salient Features:-
> Most of the species are fresh water (e.g., Oscillatoria, Rivularia), a few are marine (e.g., Trichodesmium, Darmocarpa), and some species of Oscillatoria and Nostoc are grown on terrestrial habitat.
> The individual cells are prokaryotic in nature. The nucleus is incipient type and they lack membrane bound organelles.
> Both vegetative and reproductive cells are non-flagellate.
> Cell wall is made up of microfibrils and is differentiated into four (4) layers. The cell wall composed of mucopeptide, along with carbohydrates, amino acids and fatty acids.
> Locomotion is generally absent, but when occurs, it is of gliding or jerky type.
> The principal pigments are chlorophylls a (green), c-phycocyanin (blue) and c-phyco- erythrin (red). In addition, other pigments like β-carotene and different xanthophylls like myxoxanthin and myxoxanthophyll are also present.
> Membrane bound chromatophore are absent. Pigments are found embedded in thylakoids.
> The reserve foods are cyanophycean starch and cyanophycean granules (protein).
> Many filamentous members possess specialized cells of disputed function (supposed to be the centre of N2 fixation) known as heterocysts.
> Reproduction takes place by vegetative and asexual methods. Vegetative reproduction takes place by cell division, fragmentation etc. Asexual reproduction takes place by endospores, exospores, akinetes, nannospores etc.
> Sexual reproduction is completely absent.
2. Biological Importance:-
i. They are one of the early colonizers of bare and barren areas. They provide suitable conditions for the growth of other organisms even in the most hostile environment.
ii. Blue green algae function as food to several aquatic animals. Spirulina is regularly collected for human consumption in parts of Africa. Nostoc is similarly used in China. In Rajasthan Anabaena and Spirulina are collected from Sambar Lake and used as fodder and manure. Spirulina is very easily cultivated in tanks and can be used as a palatable protein rich food supplement for humans and animals.
iii. Several cyanobacteria have the ability of nitrogen fixation. The filamentous forms possess special large pale cells or heterocyst’s for this. Some of the fixed nitrogen comes out as excretion. After death of cyanobacteria the substratum becomes rich in nitrogen. Such nitrogen fixing cyanobacteria are now regularly inoculated in the rice fields. This saves consumption of nitrogen fertilizers.
iv. Nitrogen fixing cyanobacteria are often used for reclaiming usar soils, e.g., Nostoc, Anabaena. These cyanobacteria produce acidic chemicals for counteracting alkalinity of the soil and nitrogenous compounds which are generally deficient in these soils.
v. Antibiotic can be manufactured from extract of Lyngbia.
vi. Species of Anabaena and Aulosira do not allow mosquito larvae to grow nearby. Such cyanobacteria can be inoculated in village ponds and rice fields to prevent the growth of mosquitoes.
vii. Cyanobacteria can grow on the walls and roofs of buildings during the rainy seasons causing discolouration, corrosion and leakage.
viii. They produce water blooms, imparting bad odour and colour to water bodies.
ix. Some cyanobacteria produce toxins harmful to most aquatic animals. They may prove equally toxic to human beings drinking or bathing in such water. The important toxins producing cyanobacteria are Microcytic aeruginosa (= Anacystis cyanea), Anabaena flosaquae, Aphanizomenon flos-aquae.
Ans.
Serology:- In the Serology Laboratory, analysis of blood samples is performed to check for the presence of antigens and antibodies (IgG and IgM) to help with the diagnosis of diseases and test immunity status.
1. Precipitation Test:- When soluble antigens and its homologous antibody molecules react, they sometimes form large polymeric macromolecules terminating into visible precipitate.
2. Agglutination Test:- Agglutination-test is that in which visible clumping or aggregation of cells or particles takes place due to the reaction of surface-bound antigens of such cells or particles with homologous antibodies.
3. Fluorescent-Antibody Technique (Immunofluorescence):- The fluorescent-antibody technique (immunofluorescence) is often used to identify unknown antigen.
4. Radioimmunoassay (RIA):- Radioimmunoassay (RIA) is a widely accepted and highly sensitive serological test in which one of the reactants—antibody, antigen or hapten—is radiolabeled with radioactive isotopes of elements like iodine (125I) or hydrogen (3H) are detected in situ by radioautography.
5. Enzyme-Linked Immunosorbant Assay (ELISA):- ELISA has been pioneered by two groups of scientists, one in Sweden by Engvall and Perlmann, and the other in Holland by Van Weeman and Schurs in 1972, and developed by Clark and Adams in 1977. ELISA is based on, as the name suggests, enzyme-linked antibodies adsorbed on some solid surface.
6. Complement Fixation Test:- Complement fixation refers to the ability of antigen-antibody complex to bind complement so that the latter becomes “fixed” and “used up”. The complement fixation is used in complement fixation test (CFT), which is very versatile and sensitive and can be used to detect extremely small ; mount of an antibody (as little as 0.04 μg) for a suspect microorganism in an individual’s serum.
Ans.
Types of Vaccines:- Vaccines can be synthesized in many ways based on which they are classified:
1. Live attenuated vaccines:- Pathogens like virus or bacteria are weakened by genetic manipulations to limit its growth and thus do not cause disease to the host. In some modified versions of live vaccine an organism that is related to the pathogen is used that naturally grows poorly in humans. The weakened pathogen generates a broad immune response in the host similar to that shown by an infected individual with a natural pathogen.
Examples:-
i. Oral Sabin polio vaccine
ii. MRV Vaccine (Measles, Mumps, Rubella, and Varicella)
iii. Nasal influenza vaccine
iv. Bacille Calmette-Guerin (BCG) vaccine
v. Varicella vaccine
vi. Rotavirus vaccine
2. Inactivated or Dead vaccines:- The disease-causing pathogen is killed or inactivated, usually through a thermal (application of high temperature) or chemical (formalin etc.) process. Such vaccines, when administered, elicit a robust immune response that mimics most of the responses seen during an infection.
Examples:-
i. Typhoid vaccine
ii. Influenza vaccine
iii. Salk polio vaccine
iv. Hepatitis A vaccine
3. Acellular or Subunit vaccines:- Acellular means not containing the whole cells. Acellular vaccines do not contain the whole bacteria or viruses. Instead, they contain polysaccharides or proteins from the surface of the bacteria or virus. These polysaccharides or proteins are the parts that our immune system recognizes as ‘foreign’ and evoke immune response against them. There are many types of a cellular vaccines:
a. Toxoid Vaccine:- Some pathogenic bacteria release toxins or poisonous proteins when they attack the body. Some vaccines are made by inactivating these toxins chemically and called ‘toxoids’, because they look like toxins but not poisonous. They trigger a strong immune response.
Examples:-
i. Diphtheria vaccine
ii. Tetanus vaccine
iii. Pertussis vaccine
b. Conjugate Vaccine:- Earlier polysaccharide vaccines were made using sugar molecules present on the surface of the bacteria but it was found to be less effective in babies and young children. Researchers discovered that these vaccines can work better if the bacterial polysaccharide molecules are chemically linked or conjugated to a carrier protein. Addition of other proteins confers the immunological attributes of the carrier to the antigen and thus induces a stronger immune response effective enough for younger children also.
Examples:-
i. Haemophilus influenza type b (Hib) conjugate vaccine
ii. Pneumococcal conjugate vaccine
iii. Meningococcal C conjugate vaccine
c. Recombinant Vaccine:- A small piece of the DNA is taken from the disease-causing bacterium or virus. The particular gene is incorporated into plasmid or a carrier vehicle which enables production of large quantities of well-defined proteins, which are then used as vaccines.
Examples:-
i. Hepatitis B vaccine
ii. Human papillomavirus (HPV) vaccine
d. DNA/RNA Vaccine:- Genetic material, either DNA or RNA, from the pathogenic bacteria or virus is introduced into the human cells and then the cell machinery is employed to produce the protein encoded by the inserted gene(s) of the pathogen. Our body’s immune system detects such protein as a foreign agent and produces an immune response against the whole pathogen. At present, different types of nucleic-acid vaccines are in developmental, pre-clinical and clinical evaluation phases
Examples:- HIV vaccine
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Ans.
Sexual Reproduction:- Except for individuals of the Cyanophyceae class, almost all algae undergo sexual reproduction. Gametes unite to produce zygotes while sexual reproduction. The combination of gametes from different parents will result in a new genetic establishment.
1. Autogamy:-
> Fusing gametes are formed from the very same mother cell throughout this process, and then after fusion, these produce a zygote.
> For the reasons mentioned above, autogamous plants do not display the emergence of any new characteristics, such as Diatoms (Amphora normani).
2. Hologamy:-
> Vegetative cells of various strains (+ and -) act as gametes in certain unicellular members, and then after fusion, they result in the formation of a zygote.
> This seems to be an inefficient method in terms of multiplication, however, it does result in the creation of new genetic varieties, such as Chlamydomonas.
3. Isogamy:-
> It is the merger of two gametes that are physiologically and morphologically identical, resulting in the formation of a zygote. Isogametes are a form of gamete.
> These are typically flagellates, such as Chlamydomonas Eugametos, Ulothrix, and others.
4. Anisogamy:-
> The uniting gametes are physiologically and morphologically distinct during this phase.
> The microgamete (male) is small and more aggressive, while the macrogamete (female) is bigger and less active, such as Chlamydomonas braunii.
> Physiological anisogamy differs from traditional anisogamy in that the uniting gametes share morphological similarities but vary physiologically.
> Zygnema, Spirogyra, can be some examples.
5. Oogamy:-
> It is a complex process in which a small motile (non-motile in Rhodophyceae) male gamete (sperm or antherozoids) is fertilised by a large non-motile female gamete (egg or ovum).
> Male gametes grow in antheridium, while female gametes grow in oogonium, such as Polysiphonia, Oedogonium, Chara, Batrachospermum, Vaucheria, Sargassum, Laminaria, and so on.
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Ans.
Ans.
Acetabularia:-
1. Classification:-
DIvision:- Chlorophyta
Class:- Chlorophyceae
Order:- Ulvales
Family:- Ulvaceae
Genus:- Acetabularia
2. Habit and Habitat:-
> Acetabularia, also called mermaid’s wine glass, genus of single-celled green algae found in subtropical seas.
> It is found in shallow temperate and tropical seas.
> It usually grows on pebbles, shells or pieces of rock, and is often found in seagrass meadows, on mudflats and coral reefs, in estuaries and growing on the submerged roots of mangroves.
3. Thallus structure:-
> Acetabularia has an unusual structure by being large, unicellular and possessing features that might be considered organs -‘ roots, stems and leaves’.
> The single cell is attached to the substrate by root-like cellular extensions. These extensions connect to an elongate stalk that ends in an umbrella-like cap which is often 1 cm or more across.
> The single nucleus of this remarkable organism is found at the base of the stem.
> If the stalk is cut it can regenerate a new top and the top can generate a new base, although it is short-lived, presumably because it lacks a nucleus.
4. Reproduction:-
> Sexual reproduction in Acetabularia is initiated when the single (diploid) nucleus goes through multiple mitotic divisions.
> These nuclei subsequently undergo meiosis and migrate to the cap where they are released in cysts that break open to release mobile gametes.
> If these gametes find gametes from another alga they fuse to form a zygote that attaches to a substrate and grows into the mature form.
> Asexual reproduction is also possible if mobile (diploid) zoospores are released and behave like zygotes, attaching to a substrate and developing into the mature form.
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Ans.
Nitrogen fixation in Algae:-
A. Nitrogen fixing Blue Green Algae:- Both free-living and symbiotic blue-green are found to fix N2. Three groups of freeliving N2-fixing blue-green algae are recognized: heterocystous algae, nonheterocystous filamentous algae and unicellar algae.
1. Heterocystous blue-green algae:-
> These algae fix N2 aerobically and microaerobically.
> The most common N2-fixing species belong to the genera:
i. Anabaena
ii. Aulosira
iii. Calothrix
iv. Cylindrospermum
v. Nostoc
vi. Scytonema
vii. Tolypothrix
viii. Fischerella
ix. Mastigocladus
x. Stigonema.
> These algae usually differentiate vegetative cells into heterocysts only when grown in the absence of combined nitrogen.
> This observation led Fogg (1949) to suggest that heterocysts are the sites of N2 fixation.
> Nitrogenase is located in the heterocysts under aerobic growth conditions.
> Heterocysts are suitable sites for nitrogenase because oxygen evolving Photosystem II is absent in them.
2. Nonheterocystous filamentous blue-green algae:-
> Most of these algae fix N2 only under microaerobic conditions.
Exceptions:- Trichodesmium and Microcoleus are exceptions to this general observation, since they were found to fix N2 in aerobic conditions.
> The most common N2-fixing species belong to the genera:
i. Lyngbya
ii. Phormidium
iii. Plectonema
iv. Oscillatoria
v. Trichodesmium
3. Symbiotic blue-green algae:-
> Some heterocystous and unicellular blue-green algae develop in symbiosis.
> They are found in association with other organism:
i. With Bryophytes:- Nostoc in Anthoceros.
ii. With Ferns:- Anabaena in Azolla
iii. With Gymnosperms:- Nostoc in Macrozamia
> The importance of Azolla containing Anabaena azollae has been recognized by farmers of the Southeast Asian countries for centuries. Azolla is commonly used as a green manure to improve the nitrogen balance in rice fields.
B. Biochemistry of Nitrogen fixation:-
- The nodule serves as site for N2 fixation.
- Nodule contains nitrogenase and leghaemoglobin.
- The nitrogenase has 2 components:
i. Molybdoferredoxin (Mo-Fe protein)
ii. Azoferredoxin (Fe-protein)
- The free di-nitrogen first bound to MoFe protein and is not released until completely reduced to ammonia.
- In this process ferredoxin serves as an electron donor to Fe-protein (nitrogenase reductase) which in turn hydrolyzes ATP and reduce Mo-Fe protein, the Mo-Fe protein in Turn reduce the substrate N2. The electrons and ATP are provided by photosynthesis and respiration of the host cells.
- Many intermediates are formed to form ammonia (NH3).
Dinitrogen → Hydrazine → Diamine → Ammonia
- Ammonia (NH3) is immediately protonated at physiological pH to form ammonium ion (NH4+). As NH4+ is toxic to plants, it is rapidly used near the site of generation to synthesize amino acids.
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Ans.
The principal pigments are chlorophylls a (green), c-phycocyanin (blue) and c-phyco- erythrin (red). In addition, other pigments like β-carotene and different xanthophylls like myxoxanthin and myxoxanthophyll are also present.
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Ans.
Division - Bacillariophyta:-
General characteristics:-
> Diatoms are cosmopolitan and ubiquitous in distribution, present in freshwater and oceans. Diatoms can also be found in terrestrial environments in the soil.
> Grow epiphytically on other algae and higher plants.
> Exist in two major forms-
i. Benthic:- Which are non planktonic, attached to some substratum such as rocks, sand or mud or be epiphytic, epizoic or endozoic.
ii. Planktonic:- Free floating.
> They are commonly unicellular and free- living but some members form colonies of various shapes like filaments, mucilaginous colonies etc.
> Microscopic cells are of different shapes. They may be oval, spherical, triangular, boat- shaped etc.
> Plant bodies are either bilateral or radial in symmetry.
> The cells are surrounded by a rigid cell wall, called frustule, consisting of upper epitheca and lower hypotheca; arranged in the form of a box with its lid.
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> The cell wall is composed of pectic substances impregnated with high amount of siliceous substance.
> The wall may have secondary structures like spines, bristles etc.
> The frustules show beautiful sculpturing and ornamentations for this diatoms are described as “jewels of the sea" and "living opal” and "Gems of the plant kingdom".
> The cells generally have many discoid or two large plate-like chromatophores. Some cells possess stellate chromatophore.
> The photosynthetic pigments are chlorophyll a, chlorophyll c along with xanthophylls like fucoxanthin, diatoxanthin and diadinoxanthin.
> Reserve food is oil, volutin and crysolaminarin.
> Some vegetative cells show gliding movement.
> Motile structure (antherozoid) has single pantonematic flagellum.
> Vegetative multiplication takes place by cell division, which is very common. Some of the cells become very much reduced in size.
> They produce characteristic spore, the auxospore which develops to regain the normal size.
> Sexual reproduction takes place by isogamy and oogamy.
Ans.
Replication of Viruses:- During the process of viral replication, a virus induces a living host cell to synthesize the essential components for the synthesis of new viral particles. The particles are then assembled into the correct structure, and the newly formed virions escape from the cell to infect other cells.
1. Attachment:-
> The first step in the replication process is attachment.
> In this step, the virus adsorbs to a susceptible host cell.
> High specificity exists between virus and cell, and the envelope spikes may unite with cell surface receptors.
> Receptors may exist on bacterial pili or flagella or on the host cell membrane.
2. Penetration:-
> The next step is penetration of the virus or the viral genome into the cell.
> This step may occur by phagocytosis; or the envelope of the virus may blend with the cell membrane; or the virus may “inject” its genome into the host cell.
> The latter situation occurs with the bacteriophage when the tail of the phage unites with the bacterial cell wall and enzymes open a hole in the wall. The DNA of the phage penetrates through this hole.
3. Replication:-
> The replication steps of the process occur next.
> The protein capsid is stripped away from the genome, and the genome is freed in the cell cytoplasm.
> If the genome consists of RNA, the genome acts as a messenger RNA molecule and provides the genetic codes for the synthesis of enzymes.
> The enzymes are used for the synthesis of viral genomes and capsomeres and the assembly of these components into new viruses.
> If the viral genome consists of DNA, it provides the genetic code for the synthesis of messenger RNA molecules, and the process proceeds.
>In some cases, such as in HIV infection (as discussed below), the RNA of the virus serves as a template for the synthesis of a DNA molecule. The enzyme reverse transcriptase catalyzes the DNA's production. The DNA molecule then remains as part of the host cell's chromosome for an unspecified period. From this location, it encodes messenger RNA molecules for the synthesis of enzymes and viral components.
4. Assembly:-
> Once the viral genomes and capsomeres have been synthesized, they are assembled to form new virions. This assembly may take place in the cytoplasm or in the nucleus of the host cell.
> After the assembly is complete, the virions are ready to be released into the environment.
5. Release:-
>For the release of new viral particles, any of a number of processes may occur. For example, the host cell may be “biochemically exhausted,” and it may disintegrate, thereby releasing the virions.
> For enveloped viruses, the nucleocapsids move toward the membrane of the host cell, where they force themselves through that membrane in a process called budding. During budding, a portion of cell membrane pinches off and surrounds the nucleocapsid as an envelope.
Lytic Cycle and Lysogeny:-
Lytic cycle:-
- The replication process in which the host cell experiences death is called the lytic cycle of reproduction.
- The viruses so produced are free to infect and replicate in other host cells in the area.
Lysogeny:-
- Not all viruses multiply by the lytic cycle of reproduction.
- Certain viruses remain active within their host cells for a long period without replicating.
- This cycle is called the lysogenic cycle.
- The viruses are called temperate viruses, or proviruses, because they do not bring death to the host cell immediately.
- In lysogeny, the temperate virus exists in a latent form within the host cell and is usually integrated into the chromosome.
- Bacteriophages that remain latent within their bacterial host cell are called prophages. This process is a key element in the recombination process known as transduction.
- An example of lysogeny occurs in HIV infection. In this case, the human immunodeficiency virus remains latent within the host T-lymphocyte.
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Division - Rhodophyta:-
Salient Features:-
> Red Algae are an ancient group of algae with over 5,000 living species.
> Majority of them are marine. A few (example- Batrachospermum, Lemnaea) are freshwater aquatic or subaquatic terrestrial (example- Porphyridium).
> Their size varies from unicellular microscopic forms to half a metre in length. Several species of red algae are graceful and lace-like plants. Some red algae secrete CaCO3 on their walls and form coralline algae.
> They are red in colour due to phycoerythrin and phycocyanin pigments. They have chlorophyll a molecule to capture the blue-green sunlight.
> These are photo-autotrophic, but some others are colourless, lack photosynthetic pigments and live as parasites on other photosynthetic red algae (Example- Harveyella).
> Reserve food is in the form of floridean starch and a soluble starch, floridoside.
> Flagellate forms and stages are completely absent.
> Cell Wall consists of cellulose and pectic compounds. Sulphated geloses or phycocolloids occur in most red algae. The important ones are agar, carrageenin and funori.
> Vegetative reproduction occurs by fragmentation, gemmae and regeneration of holdfast.
> Reproduction occurs by both sexual and asexual methods. The spores are non-motile. Sexual reproduction is highly specialised. Male gametes are non-motile and called spermatia. Spermatia are carried by water currents to elongated tip trichogyne of female sex organ carpogonium. Following fertilisation, the zygote may produce carpospores. Example- Poryphyra by division directly or indirectly from zygote as in Batrachospermum and Polysiphonia.
> The life cycle of many red algae like Polysiphonia has an alternation of haploid and diploid multicellular generations.
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Nitella:-
1. Classification:-
Division:- Charophyta
Class:- Charophyceae
Order:- Charales
Family:- Characeae
Genus:- Nitella
2. Habit and Habitat:-
> Nitella cosmopolitan, present in fresh or slightly brackish water; endemic species present on all continents (except Antarctica).
> Unisexual species tend to be endemic; bisexual species more widely distributed.
> Oospores of N. furcata with temperature dependent annual cycle of germination.
> Subfossil oospores can be identified from lake sediments.
> Nitella used as a model system for studies of cytoplasmic streaming, and cell wall synthesis.
3. Thallus structure:-
> Mature plants 30-100 cm height, attached with numerous basal rhizoids, these multicellular, colorless and branched.
> Primary axes with alternating sequence of nodal and internodal cells developed from apical cell.
> Each apical cell derivative divides transversely and the daughter cells develop into multicellular nodal complex with 6 (rarely 7-8) peripheral cells, and its internodal cell, respectively.
> Each peripheral cell divides periclinally; the inner cell develops into a basal node, whereas the outer becomes an apical cell and produces the branchlet; stipulodes absent.
> Each node with whorl of four to many determinate, ecorticate branchlets. Branchlets furcate, with sympodial growth, appearing di- or trichotomously branched to 3-4 orders, with acuminate branch tips.
> Indeterminate axes up to two or more per node, developing in axils of branchlets and scattered on plant.
> Cortication on internodal cells absent.
> Cytoplasm distinctly layered with external stationary layer with rows of helicoidally aligned chloroplasts (ectoplasm) and internal streaming layer with nuclei, mitochondria (endoplasm); cells with large central vacuole.
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4. Reproduction:-
> Sexual reproduction oogamous with gametes produced in multicellular gametangia.
> Species uni- or bisexual, in former antheridia single and terminal at point of furcation, oogonia lateral and subtending furcation. Antheridium is called globule and oogonium is called nucule.
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> In bisexual species antherida as in unisexual forms with one to several oogonia subtending node.
> Antheridia similar to Chara except for presence of two rather than one stalk cell, developing from terminal cells of branchlets.
> Antheridial stalk cells formed after the development of antheridial mother cell.
> Sperm released from antheridial filament by pore in cell wall; pore more specialized than in Chara.
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> Oosporangia encrusted with calcium carbonate, with two, 5-celled tiers of crown cells; at maturity compressed laterally.
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> Oosphere with three sterile cells.
> Zygote germination associated with development of protonemal stage that undergoes divisions to form large and small cells.
> The primary axes develop as lateral branches to the protonema.
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