Viruses Part 1

Viruses Part 1

About a century ago at the time of Louis Pasteur (1822-1895) and Robert Koch (1843-1910), the word “virus” was  generally referred to as a poison associated with disease and death. The present notion of virus is entirely different. Now viruses are recognized as particles of nucleic acid often with a protein coat. They replicate in living cells and cause many diseases such as influenza, hepatitis, small pox and AIDS. In this section the focus is on the properties of viruses and life cycle of bacterial viruses, also known as bacteriophages. Some diseases caused by viruses shall also be discussed in this section. The branch which deals with the study of viruses is known as virology.

The word virus is derived from Latin word venome meaning poisonous fluid. It can be defined as non cellular infectious entities which contain either RNA or DNA, normally encased in proteinaceous coat, and reproduce only in living cells. Viruses utilize the biosynthetic machinery of the host for its own synthesis and then transfer themselves efficiently to other Cell.

Some viral diseases have been known for centuries. In fact, the first infectious disease against which effective method of prevention was developed was a viral disease. In 1796, Edward Jenner first vaccinated an 8 years old boy with material removed from cowpox lesion on the hand of milkmaid. After six weeks the boy was inoculated with pus from a small pox victim, but he did not develop the disease. Later Jenner used material for vaccination from cowpox lesions and successfully vaccinated 23 persons. As the material he used was obtained from cow (latin vacca), latter the term vaccination was used by Louis Pasteur for inoculation against disease.

In 1884, one of Pasteur’s associates, Charles Chamberland, found that bacteria can not pass through porcelain filters, while agent responsible for rabies (a disease which is transferred to human by bites of rabid dogs, foxes, cats, bats and other animals) can pass through these filters. As in those days the word virus was loosely used to describe any toxic substance that caused disease, those unseen filterable agents of disease were described as filterable viruses. In 1892, Ivanowski discovered that the agent which caused tobacco mosaic disease was filterable. He obtained bacteria free filtrate from ground up infected plants and placed it on healthy leaves of tobacco. He observed that filtrate produced the disease in healthy plants. After that, presence of similar filter-passing, ultramicroscopic agents was seen in the victims of many diseses, including foot and mouth disease (1898) and yellow fever (1901).

Bacteriophages, viruses that infect bacteria were discovered independently by Twort in 1915 and D’Herelle in 1917. Twort observed that bacterial colonies sometimes undergo Lysis (dissolved and disappeared) and that this Lysis can be transferred from one colony to other. Even highly diluted material from Lysed colony can transfer the Lytic effect. However, heating the filtrate destroyed its Lytic property. From these observations he concluded that Lytic agent might be a virus. D’Herelle rediscovered this phenomena in 1917 and used the word bacteriophages meaning “bacteria eater”.

 The filterable agents were first purified in 1935, when Stanley was successful in crystallizing the tobacco mosaic virus. Chemical analysis of these particles showed that they contained only nucleic acid and protein. This suggested that, unlike other forms, viruses are of simple chemical composition.

Organisms Affecting Human Health Part 5

Organisms Affecting Human Health Part 5

Influenza

Influenza is an illness affecting upper respiratory tract symptoms of the disease are fever, headache, and pain in the limbs. Watering of eyes and running of nose and it is associated with sore throat.

Fig 20.20 influenza virus

Causative agent
Different strains of viruses are responsible for causing the disease.
The disease is spread by droplet infection (cough, sneezing, and breath).
Treatment and control

There is no particular treatment of the disease. The patient develops immunity and recovers. Rest is the only immediate treatment. Disease can be controlled by keeping patient away from other people.

Measles

 Measles is a very infectious and worldwide disease of children. The disease starts with fever with cold, cough and a watery discharge from the eyes. On the third day rashes appear first on the face and then spread over the whole body.

Causative organism

The causative organism is a virus, which is air born and is inhaled in droplets. The disease spreads through the discharge from eyes and nose, which becomes air-born and spreads rapidly.

Treatment and control

There is no specific treatment. Child develops immunity and the signs and symptoms of the disease disappear after seven days. The child should be given plenty of drink especially milk.

It is difficult to control the disease because it is highly infectious as the patient becomes infectious about 2 days before the symptom of the disease develop.

Poliomyelitis
Poliomyelitis is most common among the children but young and old people can also become its victims.

It begins slowly. The sufferer has very high fever, headache, nausea, fits and stiffness of limbs. In severe cases, the virus attacks the nerve fibers of the spinal cord causing paralysis of the limbs and muscles of the respiratory tract, sometime proving fatal, apart from permanent disabilities.

Treatment and control

There are two polio vaccines; Sabin vaccine and Salk vaccine. Sabin vaccine is more common and is given orally in the form of drops. Two or three drops are given to infants during their first year which provides life long protection against polio by developing active immunity against it. Salk vaccine is injected in to the body but it is not being practiced in our country.

Causative organism

Poliomyelitis is caused by virus, which as transmitted by infected water. It is also transmitted b droplet infection during coughing and sneezing, of the patient.

Fig 20.21 polio virus

Aids (acquired immune deficiency syndrome)
AIDS is a disease in which body’s immune system. The system which protects our body against infections is destroyed. The body becomes unable to protect itself against many other diseases i.e. immune deficiency develops.
Causative organism
The causative organism is a virus called HIV (human immune deficiency virus).
Signs and symptoms
The word syndrome is used in aids because it is a disease with many symptoms occurring at the same time. The patient may suffer from chronic fever, severe diarrhea lasting for months, pneumonia etc. and even cancer.
Aids is usually transmitted by

1. Sexual relationship with an infected person.
2. Blood transfusion from infected person.
3. By sharing contaminated injection needles. For example when drug addicts share their used syringes.
4. By using contaminated razors, surgical and dental instruments etc.
5. Virus may pass from the infected mother to the fetus during pregnancy.

Fig 20.22 HIV

Prevention and control
Aids can be prevented by talking following measures

1. By keeping to one’s own sex partner.
2. By avoiding drug abuse as drug addicts share hypodermic needless.
3. By using disposable syringe.
4. By sterilizing the medical equipment before examining the patients.
5. By avoiding the sharing of instruments that cause lesion and be contaminated with blood e.g. toothbrushes and razors.
6. By using carefully sterilized surgical instruments / equipments.

Organisms Affecting Human Health Part 4

Organisms Affecting Human Health Part 4

20. 9 some common diseases
Tuberculosis (TB)

Tuberculosis is worldwide but is most common in tropical areas. Tuberculosis takes a long time to develop but depends mainly upon the health and state of the people. Most common TB. Is of the lungs, apart from TB of intestine, bones etc.

Causative organisms

A bacillus (bacterium) mycobacterium is the cause of tuberculosis. There are several species of mycobacterium causing different types of TB. The bacterium is very resistant and can remain alive for several months under unfavorable circumstances but reproduces when circumstances become favorable. Toxins produced by the bacteria cause the disease. Te disease is spread by droplet infection (cough, sneezing, deep breath and by spitting) and unboiled milk. Over-crowding and humid and dirty conditions also promote the spread of disease.

Treatment and control
The disease can be controlled by;

1. Vaccination of BCG to the children to encourage the development of antibodies.
2. Revaccination after three to five years.
3. Use of thoroughly boiled or pasteurized milk.
4. By keeping patients in isolated or pasteurized milk.

Cholera

Cholera is mainly found in Asia and commonly spreads in humid condition. Disease begins with fever, diarrhea and vomiting, accompanied by severe abdominal pain. Severe attacks result in dehydration and death of the patient.

Causative organism

Cholera is caused by a comma-shaped bacterium called vibro cholera with a flagellum which enables it to swim in water. Vibro cholera is ono-caplulated and cannot withstand dry condition. It is water born. The disease is spread by the use of contaminated water and food (fig. 20.19).

Control over the disease

1. By the use of boiled or chlorinated water.
2. By keeping or food covered to avoid its contamination.
3. By injecting cholera vaccine which gives an active artificial immunity for six month.
4. By keeping infected persons in isolated places as the bacteria are transmitted by flies as well as by water.
5. Good sanitation is required to prevent the contamination of water.

Fig. 20.19) disease cycle of cholera

Treatment

Saline solution is continuously injected into vein of patient to maintain the body fluid balance. This does not kill the bacteria but enables the patient to overcome the effects of dehydration and allow his own body to fight the disease. Drugs (antibiotics) are also given to kill the bacteria.

Bacillary dysentery

Bacillary dysentery is a food-born disease. Symptom of the disease is severe pain in the lower abdomen. It is followed by diarrhea and fever. There is appearance of mucous and blood (in the case of severe attack) in the stools.

Causative organism

Bacillary dysentery is caused by rod-shaped (bacilli) bacteria. The bacteria are motile and are found in the faeces of patients in large numbers. The disease is spread by the use of contaminated food and from unwashed hands after using lavatory.

Control over disease

1. By protecting cooked and uncooked food from houseflies.
2. By careful washing of hands after using toilet.
3. By adopting proper methods of sanitation to prevent flies from sitting on human faeces and breed there.

Treatment

1. The patient should take rest and eat no solid food for a day or two.
2. Patient should be given plenty of water and ORS (Oral Rehydrate Salt) to prevent dehydration.

Proper medicine should be given.

Organisms Affecting Human Health Part 3

Organisms Affecting Human Health Part 3

20.8.1 Vectors of diseases

Germs are transmitted from an infected person to a healthy person by means of animals (for example; by many insects). These animals are called vectors. Common vectors are housefly, sand fly, mosquitoes, fleas, cockroaches, dogs, mice etc. (fig 20.15).

Fig 20. 15 diagrams summarizing the spread of infections

Housefly as a vector of disease

Housefly is a mechanical vector. It feeds on decaying matter and dung which is rich source of germs causing typhoid fever, typhus, cholera, dysentery, tuberculosis and anthrax along with some tapeworm and roundworm eggs. As flies feed their feet, bodies and digestive system becomes contaminated with germs and eggs.

If the flies then settle on human food, the germs and eggs are deposited on the food. By eating this contaminated food persons become ill.

Fig 20.16 housefly

Mosquitoes as vectors

Mosquito is a biological vector because the pathogen which is responsible for causing the disease under goes a cycle of development in it. Mosquito is a vector for transmitting malaria and yellow fever.

In case of malaria, pathogen is a protozoan plasmodium. Plasmodium under goes two stages in its life cycle, one in man and one in the mosquito. In man, the parasite enters through the bits of an infected female anopheles mosquito. One stage of its life cycle is completed in man, as a result gametocytes are formed which are sucked along with blood by a mosquito. Second stage of its life cycle is complete in mosquito (fig. 20.17).

Fig. 20.17 life cycle of plasmodium

The mosquito in its life cycle is an intermediary host. Without this intermediary stage. The disease would not be passed from patient to healthy person.
Antibiotics and their importance
Antibiotics are chemicals that are extracted from one living organism and which destroy pathogens in another living organism.
Antibiotics are used to kill pathogenic bacteria but they do not harm viruses. Some of the common antibiotics are;

1. Penicillin attacks a narrow range of bacteria and are therefore called narrow spectrum antibiotics.
2. Cephlosporins are useful against bacteria which have developed resistance to penicillin.
3. Tetracyclines act against a variety of bacteria. They are broad spectrum antibiotics.
4. Erythromycins are useful against bacteria which have developed resistance to penicillin.

Fig. 20.18 penicillium chrysogenum

Organisms Affecting Human Health Part 1

Organisms Affecting Human Health Part 1

Microorganism:
Very minute living organisms not seen without the aid of microscope are called microorganisms. There are numerous microorganism but ones which concerns us are germs or disease causing microorganism (called pathogens). They are found almost everywhere in nature (in air, water and soil), on the surface of the objects and living organisms, within the living organisms.
Pathogenic microorganisms are viruses many bacteria, certain fungi and some protozoan.
Viruses

Viruses cause disease in living cells and are the smallest disease causing particles known. They are only visible under electron microscope. Chickenpox, measles, poliomyelitis, the common cold, mumps, influenza and aids are a few diseases caused by viruses.

Fig. 20.8 structure of virus

Bacteria
Bacteria are larger than viruses and are visible under the ordinary light microscope. All of the bacteria are not pathogens.
Some of the diseases caused by bacteria include boils, food poisoning, whooping coughs, cholera, diphtheria, typhoid, bacillary dysentery, TB etc.

Fig 20.9 structure of bacterium

Protozoa

Protozoa are single-celled organisms which vary in size and shape. Very few of them cause disease in humans. Some common diseases caused by protozoa are malaria, amoebic dysentery, some types of ulcers in large intestine, etc.

Fungi

Very few fungi are responsible for causing human diseases. Most of the fungi are multi-cellular but some are unicellular. Their mode of nutrition is saprotrophic but very few of them are human parasites. Common fungal human diseases are Ring worm, Athlete’s foot, histoplasmosis, candidia (thrash) etc.

Ring worm

Ring worm is a skin disease caused by fungus. The disease is spread by spores of the fungus which are passed from one individual to another by personal contact or by contact with clothing since the fungus can grow on clothing or other personal objects. Hyphae of the fungus penetrate the outer layers of the skin and grow in the normal way. The red patch as it spreads causes intense itching. As the fungus grows, the center loses its colour to normal leaving a circular red patch which is darker at the edge. The name ‘ring worm’ is given because of this appearance.

Fig 20. 10 fungus which causes ring

Athlete’s foot

Athlete’s foot is also a fungal disease. It is sort of ringworm of the foot. Fungus attacks soft skin between the toes and in bad cases spreads over the whole foot. The skin peels off leaving very sore patches.

Infected floors and mats spread the problem.

Fig. 20. 11 athlete’s foot

Organisms Affecting Human Health Part 2

Organisms Affecting Human Health Part 2

20.9 Some common parasitic worms
Some common parasitic worms are round worms, thread worms, liver fluke and tapeworms.
Roundworms (nematodes)
Ascaris is a common roundworm. It is 20-30 cm long and is pinkish white in colour.

These worms live in the small intestine of man and move freely. The major symptoms of the disease are abdominal pain, dyspepsia, anxiety, nausea, and coughing. The eggs of ascaris are excreted in soil along with feces. These eggs can enter another person, due to unhygienic conditions. When these eggs reach small intestine, they develop into new worms and cause disease.

This disease is more common in children as compared to adults and children are source of its spread.
In this disease, as the worms feed on the food of the patient, that is why patient suffers from malnutrition.

Fig. 20.12

Thread worms

Thread worms are very small only ½ inch long. Both male and female worms live in the colon of human beings. At night the female descends the colon and lays her eggs in the folds of skin about the anus. This causes intense itching, if the anus is scratched, the eggs stick to the fingers and thus transferred to the mouth or deposited on cutlery and household utensils. The eggs also become attached to the clothing and the bedding. When these articles are shaken, the eggs are dispersed in the air and may be breathed in. the eggs pass to the stomach and to the large intestine and develop into adult thread worms. Children are mainly affected by these worms. Drugs can be given to get rid of worms.

Liver fluke (platyhelminthes)

The fluke are flat worms. They are the parasites of both man and animal. All flukes require at least two hosts to complete their life cycle. One of the fluke is liver fluke. It lives in and feeds in the liver of the human beings, where it causes much damage. The eggs of the fluke pass out in the faeces of infected person and thus they may come to the freshwater. Snails living in the freshwater eat the eggs. The eggs develop and form larva in the snails. The larva develops into small creature with a tail. At this stage they leave the snail and swim into water and seek a fresh host.

Their next host is freshwater fish. The embryos penetrate the skin and enter the muscles of the fish. In the muscles the embryo forms a hard protective layer and become cysts. These cysts are very hard and are only destroyed by a thorough cooking of the fish. When partially cooked fish is eaten by man, the cysts arrive in the stomach and there they develop. From the stomach the parasites pass to the liver, where they grow to adult flukes. The adult flukes are both male and female, they come together and fertile eggs are produced and the cycle is repeated (fig. 20.13).

Drugs for the treatment are available but the best way is to improve sanitation.

Fig. 20. 13 life cycle of liver fluke

Tapeworms (Taenia solium)

Flat worms are largest organisms to parasitize human beings. They are called flat worms because of their flattened bodies. All tapeworms are parasites and require at least two hosts to complete their life cycle.

Tapeworms have a small head on which suckers and sometimes hooks are present to attach them to their host’s intestine. Their length varies from few centimeters to 10 meters and their body is made of segments called proglottids. They are hermaphrodite i.e. each proglottid has both sexes.

The life cycle of tapeworm involves two hosts. Primary host is the one in which the adult tapeworm lives and reproduces. And secondary host is an animal, which is eaten by the primary host for example beef tape worm’s primary host is humans and secondary host is cattle. If the sewage disposal conditions are not proper, tape worm eggs may be eaten by a cow while grazing. Because of the digestive juice of cow in the gut, larvae come out of the eggs which then find their way into the muscular tissues. There they remain unchanged for several years. If the partially cooked cow meat of infested cow is eaten by humans the larvae are activated by digestive juices, where they manage to attach the intestinal wall with their suckers or hooks. There the larvae develop into adult tapeworms. After fertilization sex organs disappear and are replaced by thousands of microscopic proglottids. At this stage proglottids drop from the tape worm and pass out of the host’s body with faeces (fig 20.14).

Fig 20.14 life cycle of tapeworm

Evolution of Seed Habit Part 9

Evolution of Seed Habit Part 9

Poaceae: (Gramineae) Grass Family

Distributed throughout the world wherever vascular plants can survive. It includes about 600 genera, 10,000 species. In Pakistan it is represented by 158 genera and 492 species.

The traditional family name Gramineae takes its name from the latin Grammar which was used as a ‘ generic’ name for certain grasses, is permitted by the International Code of Binomial Nomenclature, which also provides for the use of Poaceae, based upon they type genus Poa Linn.

Familiar Plnats: Triticum vulgare, Wheat; Zea mays, Corn; Avena Sativa, Oats; Oryza sativa, Rice; Bambusa, Bamboo; Saccharam officinarum Sugar Cane etc.

Vegetative Characters: habit: annual or perennial, herbs. Stem: jointed usually hollow at the internodes, closed at the nodes. Leaves: solitary at the nodes, sometimes crowded at the base of the stem, alternate. Exstipulate, ligulate, mostly sessile, leaf-base mostly sheathing, simple.

Inflorescence: mostly compound composed of units called spikelets which are variously arranged (indense clusters as in wheat, compound spike, or loosely on branched axis-as in oats, spikelets consisting of bracts, arranged along a slender axis (called rachilla) the two lower bracts (called glumes) which are empty: the succeeding lemmas enclosing a flower and opposed by a hyaline scale called palea. The whole (lemma, palea, and flower) termed as floret: the glumes or lemmas often bearing one or more stiff bristles (called awns); this basic pattern of spikelet structure is consistent throughout the family. Spikelets of grasses vary widely in different genera, particularly as to number of fertile florets in each, and deposition of sexes with them.

                Flowers: Usually bisexual, sometimes unisexual, small and inconspicuous, sessile, bracteates, incomplete, zygomorphic, hypogynous. Perianth: Absent or represented by 2, (rarely 3), minute hyaline or fleshy scales called lodicules. Androecium: Stamens 1 to 6, usually 3. With – delic compound pistil of 3 united carpels, anthers versatile through only one is functional free stigmas usually large feather like. Fruit: graius or caryopsis (caryopsis a dry, indehiscent fruit in which fruit wall (percarp) is completed, fused with seed coat).

Economic Importance: Economically family Poaceae has greater importance than any other family of flowering plants. It has great economic importance to both man and animals. Cereals and millets which constitute the chief food stuff of mankind, belongs to this family. Most of the fodder crops, which are equally important to domestic animals, also belong to this family.

Plants providing food for man includes: Triticum sp. (wheat), Avena sativa (Oats), Zea mays (Corn, Maize), Oryza sativa (Rice), Hordeum vulgare (barley), Secale cereal (Rye), Penisetum typhoideum; Sorghum vulgare etc.

The dried stem and leaves of the cereal crops are used as fodder for the cattle. Sugar is obtained from the juice of Saccharum officinarum (Sugar Cane). Many grasses are used in the lawns e.g. Agrostis, Poa, Festuca etc. and have ornamental significance.

Fig. 9.30 Poaceae (Gramineae): Chloris barbata: A habit; B-spikelet: C-gulumes: D-fertile lamma, E-flower: F-fruit;

Bambusa (Bamboo) are used as building material for the thatching huts, making boats, carts, pipes etc. and the split stem are woven into mats, baskets, fans, hats, course umbrella. Leaves are also given to horses as a cure of cough and cold etc. certain grasses yield aromatic oils, e.g. Cymbopogon citrates (lemon grass) which yield lemon grass oil is used in perfumes and soap industry and for making infusions. Some species of the grasses are used in making papers.

Ethyl alcohol and many other kind of beverages are also prepared from cereals for example whisky from Rye, barley, corn and rum molasses from sugar cane. Fibers obtained from the leaves of Saccharum munja which is used in making ropes.

Evolution of Seed Habit Part 8

Evolution of Seed Habit Part 8

Caesalpinia Ceae: Cassia Family

This family includes about 152 genera and about 2300 species. In Pakistan the family is represented by 16 genera and about 60 species.

Familiar plants: Tamarindus India; cassia fistula. Bauhina veriegata.

Fig. 9.28 Caesalpiniaceae: Cassia senna; A twig, B-flower; C-fruit

Vegetative Characters Habit: mostly trees or shrubs, some are woody climbers; rarely herbs. Stem: Erect, woody herbaceous, or climbing. Leaves: compound, pinnate, very rarely simple, stipulate.

Floral characters: Inflorescence: Axillary or terminal raceme or panicle or spikes, rarely cymose; showy. Flowers: bisexual, zygomorphic, rarely actinomorphic, perigynous. Calyx: sepals 5, free or connate at base, often colored. Corolla: mostly 5 petals, free. Androecium: stamens 10 or fewer, rarely numerous, free or variously united. Gynoecium: a simple pistil 1-carpel; ovary superior, unilocular; placentation marginal; stigma simple. Fruit: legume.

Economic importance: the family is of great importance. Some plants are ornamental, some have medicinal importance, a few have food and other values.

The leaves of Cassia alata are used to cure ring worm and skin diseases. Cassia senna and C. obovata are cultivated for the leaves which yield the drug Senna, which is the base for a laxative. Oil extracted from the seeds of Cynometera cauliflora is applied externally for skin diseases.

Common ornamental plants are bauhinia variegate (kachnar),cassia fistula (amaltas), parkinsonia, etc.

The leaves and flower’s bud of bauhinia variegate are used as vegetable. The acidic fruit of tamarindus indica are edible and are rich in tartric acid. The bark of Bauhinia and Tamarindus Indica is used in tanning. The heartwood of Haematoxylon (longwood) yield the dye haematoxylin.

Mimosaceae: Mimosa or Acacia Family:

A family of about 56 genera and about 2800 species. In Pakistan it is represented by 11 genera and 49 species, of these only 4 genera and 18 species are native and rest are introduced.

Familiar plants: acacia nilotica, albizzia lebbek, mimosa pudica Touch me not,prosopis glandulosa, P.cineraria.

Vegetative characters: habit: mostly trees or shrubs, rarely climbers or herbs. Most of themare xerophytes. Stem: mostly woody. Leaves: pinnate by compound, alternate, stipulate, stipules modified into thorns.

Floral characters: Inflorescence: spike like or head or umbel, rarely racemose or globose umbels. Flowers: Bisexual, actinomorphic, hypogynous to slightly perigynous, bracteates. Calyx: usually sepals 5, generally fused, toothed or lobed. Corolla: petals 5, free or fused; corolla lobed. Androecium: stamens 5 to numerous, free, or adnate to the base of corolla. Gynoecium: a simple pistil of 1 carper, ovary unilocular, superior; ovules many, placentation marginal. Fruit: a legume dehiscent or indehiscent.

Economic importance: many trees of this family including species of Acacia, Albizzia and Xylia provide commercially important wood, which is used for construction purpose or for furniture or as a fuel. The wood of Albizzia lebbek is used in cabinet work, and railway carriages.

Arabic gum is obtained from Acacia nilotica and A. Senegal. Katha a dye is obtained from Acacia catechu. The tender leaves of Accacia nilotica are used as blood purifier.

Some common garden plants grwn for their beautiful flowers are mimosa pudica and Acacia melanoxylon. A few species of prosopis are planted in the arid zones for breaking the wind pressure.

Fig 9.29 Mimosaceae: Prosopsis cineraria; A-twig, B-infloresence; C-flower; -D-fruits.

Evolution of Seed Habit Part 7

Evolution of Seed Habit Part 7

Fabaceae: (Papilionaceae) Pea Family

A family of about 400 genera and 9000 species, the members of this family occurs all over the world, but particularly in the warm temperate regions. In Pakistan about 82 genera and about 587 species have been reported.

Familiar plants: Lathyrus odoratus (sweet pea), Arachis hypogeal (peanut), Cicer arietinum (chick pea) and dalbergia sissoo (shisham).

Fig 9.27 Fabaceae (Papilionaceae): Sesbania sesbar; A-twig; B-flower; B1 standard verillium; C-fruit a legume; D-carpel

Vegetative characters: habit: trees shrubs or herbs. Stem: herbaceous, or woody or climber by tendrils (wiry, coiled thread like structures). Leaves: compound or rarely simple, sometimes partially or completely modified into tendrils, alternate, stipulate; stipules mostly leafy.

Floral characters: Inflorescence: Racemose or solitary axillary. Flowers: bisexual,zygomorphic, bracteates, pedicellate, perigynous, pentamerous and papilionaceous.Calyx: 5sepals, more or less united in a tube, mostly hairy. Corolla: papilionaceous; petals 5, usually clawed, dissimilar; the upper posterior petal is large and conspicuous and is called standard or vexillum, 2-lateral ones free called wings and 2 anterior inner most that fuse to form a boat-shaped structure called the keel or carina. Androecium: stamens 10, mostly diadelphous (united by their filaments in 2 groups), 9 fused to form a sheath round the pistil, while 10^th posterior one is free. Gynoecium: a simple pistil, 1-carpeled, with 1-locule; ovary superior; ovary and style long, style bent at is bar, placentation (mono carpellary) marginal. Fruit: usually a legume or pod, showing a great variety of form in various species.

Economic importance: the family is of considerable importance as a source of high-protein food, oil, and forage as well as ornamentals and other uses. Main importance lies in the pulses, belonging to this family, which are used as food, some important and common species of pulse yielding plants are: gram, pea, kidney bean. These pulses are rich in protein contents.

Medicago sativa Alfafa is one of the world’s best forage crop for horses. Vicia, Melilotus and Trifolium are also cultivated as main fodder crops. Many trees of this family provide excellent timber for building, furniture and fuel. Main timber plants are butea. Dalbergia etc.

Seeds of arachis hypogeal peanut are edible and also used for extraction of peanut oil which after hydrogenation is used as a vegetable oil. Indigo dyes are obtained from indigofera tinctoria and butea monosperma, yielding yellow dye from flowers.

Many plants of this family are important for medicines: these include Glycyrrhiza glabra for cough and cold, and Clitoria ternatea is used against snake bite. The red and white seeds of abrus precatorious are used by jewelers as weights called “ratti”. Some important ornamental plants include lathyrus, lupines, Clitoria, butea etc.

Evolution of Seed Habit Part 6

Evolution of Seed Habit Part 6

Floral Characters

Inflorescence: Typically an axillary cyme or combination of cymes, sometimes helicoids, or axillary umbellate cymes. Flowers: Mostyly bisexual, usually actinomorphic or weakly zygomorphic, hypogynous, usually pentamerous. Calyx: United 5 sepals, usually persistant. Corolla: united 5 petals, corolla rotate to tubular. Androecium: stamens 5, free but inserted on the corolla tube (epipetalours) rarely stamens 4 and didynamous ( arranged in two whorls of 2 each). Gynoecium: A compound pistil of 2 united carpels; ovary obliquely placed, superior bilocular, or imperfectly 4-locular by false septum; Placentation axile.

Economic importance: Members of the family solanaceae provides drugs and food, some are weedy, some are poisonous, and others are handsome ornamentals. The most important plant in the family is Solanum tuberosum (potato-white or Irish Potato). In Ireland people are completely dependent on potatoes.

Lycopersicum esculentum (tomato), the favorite home garden vegetable, was once believed to be poisonous.

Other important food plants are Solanum melangena (egg plant or brinjal). The fruit of capsicum annum and capsicum frutescens are rich in vitamin C and A, are used as condiment. Physalis (Ground-cherry) produces an edible fruit enclosed in a bladder like persistent calyx, the husk, giving the name husk tomatoe.

Another plant of great commercial value is Nicotiana tabacum the leaves of which are dried and made into tobacco, which is used in making cigarettes. Many members of this family yield powerful alkaloids, e.g. atropa belladonna, datura which are rich in atropine and daturine respectively are used medicinally.

Many plants are cultivated in the gardens for their beautiful flowers, these includes petunia, Nicotiana, Cestrum and Solanum etc.

Evolution of Seed Habit Part 5

Evolution of Seed Habit Part 5

Floral characters

Inflorescence is variable, solitary or may be racemose or cymose cluster. Flowers are mostly bisexual, and actinomorphic, often perigynous to some degree, usually showy and scented. Calyx: 5 sepals rarely 4, united at the base. Corolla: petals 5, or numerous in multiple of 5, which are free rosaceous, large and showy. Androecium:  numerous stamens, sometimes only 5 or 10. Gynoecium is of 1 to numerous separate carpels or variously united, ovary generally superior sometimes inferior; Placentation basal, when the carpel is one or apocarpous, but axile when the carpels are many and syncarpous (fused).

Economic importance: economic importance of this family is great in providing the pleasure and welfare to manking. The members of this family are important in temperate regions for fruit and ornamentals. Perhaps they rank third in commercial importance in the temperate zone among the families of flowering plants.

Important fruits are apple, pear, peach, almond, apricot, strawberry, etc.

A large number of plants are ornamental and are grown in gardens for their beautiful and scented flowers. The most widely cultivated genus for decorative purpose is Rosa, Rose which has been grown in gardens since ancient times and whose named cultivars are now numbered in thousands. Many other genera are also grown for their beautiful flowers in the parks and gardens.

The branches of crataeugs provide excellent walking sticks and wood. The wood of pyrus pastia is used for making tobacco pipes.

In Asian countries the petals of common rose usually called gulabs are used in making gulkand, and are also used in extraction of an essential oil (rose oil) used as perfume, when distilled with water the petals give Rose-water or Ark-Gulab, which is used for curing eye disease, and for many other purposes.

Solanaceae: Night Shade or Potato Family

It is family of about 90 genera and 2000 species of tropical and temperate distribution. In Pakistan 14 genera and about 52 species are reported, Nasir (1985).

Familiar Plants: solanum tuberosum (potato), nicotiana tabacum (Tbacco), lycopersicum esculentum( Tomato), Capsicum frutescens (Red pepper).

Vegetative Characters: plants including in this family are herbs, shrubs, sometimes trees or vines. Stem is hairy or prickly. Leaves are alternate or rarely becoming opposite in the floral region, simple, petiolate, rarely sessile.

Fig. 9.26 Solanaceae: Solanum nigrum, A – twig, B-Flower C-fruit, D-seed

Evolution of Seed Habit Part 4

Evolution of Seed Habit Part 4 

Double fertilization is a special process found in angiosperms. In this two male gametes fuses with two cells simultaneously.a male gamete (n) fuses with egg (n) to form a diploid zygote (2n) which develops later into an embryo and second male gamete (n) fuses with another female cell called fusion nucleus (2n) resulting into a triploid (3n) endosperm cell, which develops into food storing endosperm tissue. It is an important evolutionary advancement in which food storage fertilized ovule is made only on fertilization i.e. formation of zygote. This actually helps the plant to economize its food resources.

Classification of angiosperms

The class angiospermae is divided into two sub-classes, the monocotyledonae (with one cotyledon) and the dicotyledonae (with two cotyledons), according to the number of cotyledons in the embryo.

The plants included in the monocotyledonae are called monocotyledonous plants or monoocts. The plants included in the dicotyledonae are called dicotyledonous plants or dicots. A few distinguishing characters of the two classes are given below:

Fig. 9.24 comparison of dicot and monocot

Anglospermic familes

Some angoispermic families are described below:

1. Rosaceae (Rose family)
2. Solanaceae (potato family).
3. Fabaceae (pea family).
4. Caeselpiniaceae (cassia family).
5. Mimosaceae (acacia family).
6. Poaceae (grass family).

Rosaceae (rose family):
A family with about 100 genera and 2000 species is distributed over most of the earth. In pakintan 29 genera and about 213 species are reported.
Familiar plants: pyrus (pear): rosa (rose); malus (apple); fragaria (strawberry) etc.

Fig. 9.25 Rosaceae: A-twig: B-young stamen B1-enlarged open anther, showing pollen in it: C-style hairy and stigma bilabiate: C1-enlarged bilabiate stigma.

Vegetative characters

Plants are trees, shrubs and herbs. Stem of the shrubby plants usually have spines. Leaves are alternate, rarely opposite, simple or compound, with paired stipules which are sometimes adnate to the pertiole; spines may also occur on the rachis.

Evolution of Seed Habit Part 3

Evolution of Seed Habit Part 3

Class Angiospermae

          The term angiosperms literally means “enclosed seeded” (angio=close sperm=seed). In these plants fertile leaves bearing ovules are folded and joined at the margins to form ovaries. The ovary after fertilization is changed into a fruit, containing seeds.

          Angiosoperms make up 235,000 of the 360,000 known species of plants. They are heterosporous, autotrophic plants. These are highly evolved of all the plants on the earth. The plants produce flowers, fruits and seeds.

(a)

(b)

(c)

Fig. 9.22 some of the remarkable diversity of angiosperms is shown in these photographs. The species shown here are dicots (a) fragrant water lily, (b) wild geranium, (c) Indian pipe (aparsite) an angiosperm that lacks chlorophyll.

Life cycle of an angiospermic plant

            The adult plant is a diploid saprophyte mostly differentiated into roots stem and leaves. At maturity it produces flowers. A flower is a modified shoot which consists of a pedicel, thalamus or torus, and floral leaves (sepals, petals, stamens and carpals). Thalamus and floral leaves, especially the stamens and the carpals, are so modified, that they do not even look like stem and leaves respectively. The sepals and petals are non-essential or non-reproductive parts, and stamens and carpals are the essential or reproductive parts of the flower.

              The sepals and the petals protect the stamens and the carpals. They also attract insects for pollination. When the pollination is over, the sepals usually and the petals always fall off.

          The anther contains microspore mother cells which produce haploid microspores through mitosis. Each microspore germinates to produce male gametophyte. Such microspores containing male gametophytes are called pollen.

               The carpel consists of a basal broader apt, the ovary, the style and the, terminal part of the style. The stigma. The ovary contains ovules. The ovule consists of an integument (covering) and a tissue, the nucleolus present inside.

After pollination, the pollen grain is transferred to the stigma. Here it germinates to form a pollen tube. The nucleus of the microspore divides by mitotic divisions to form two male gametes and the tube nucleus. At this stage of development, the pollen grain is called male gametophyte. In the meantime certain changes occur in the ovule leading to the formation of female spore (megaspore). The megaspore develops into female gametophyte. This consists of seven cells only. One of these cells is the egg of oosphere.

The pollen tube grows through the style, enters the ovule and then reaches the female gametophyte. Here it discharges the male gametes. The egg and one of the two male gametes fuse to form the oospore. The second male gamete fuses with the secondary nucleus to form endosperm nucleus (double fertilization). The oospore develops into an embryo and endosperm nucleus develops into a multicellualr nutritive tissue, the endosperm.

Seed formation

          Meanwhile, the integuments of the ovule form testa and tegmen and ovary wall develops into the fruit. Seeds usually undergo a period of rest and then under suitable conditions, germinate and produce a seedling which gradually changes into a sporophyte (fig. 9.23).

Thus an alternation of dominant sporophyte generation (2n) occurs with inconspicuous gametophyte generation (n).

Fig. 9.23 life cycle of on angiospermic plants.

Evolution of Seed Habit Part 2

Evolution of Seed Habit Part 2

Gymnosperms are one of the successful groups of seed plants of worldwide distribution. They constitute about one-third of the world’s forests. The gymnosperms are heterosporous plants which produce seeds but no fruits. The term gymnospermae literally means ‘naked seeded’ (Gymno=naked, spermae= seed). The ovules in these plants are usually borne on the exposed surfaces of fertile leaves (megasporophylls). These ovules, unlike those of angiosperms are not enclosed but lie naked on the surface of fertile leaves.

Like filicinae, they show regular heteromorphic alternation of generations. They have independent, dominant sporophyte but less conspicuous, dependent gametophyte. The female gametophyte is permanently retained within the ovule. The two kinds of spores are microspores and megaspores which develop on microsporophylls and megasporophylls respectively. The megasporophylls bearing ovules are not folded and joined at the margins to form an ovary. For this reason the seeds lie naked on the mega sporophylls, (Fig. 9.20a).

The important genera are Cycas (sago-palm) (Figs. 9.20-a), Pinus (pine), taxus (Yew), picea (hemlock) and Cedrus (deodar) Ginkgo (Fig. 9.20=b) etc.

Fig. 9.20 (a) Cycas tree-habit and general organography

Fig. 9.20 (b) Ginkgo biloba

Pinus- Life Cycle

The pine is a conifer. The main plant body is saprophyte which produces spores after reduction division of spore mother cell in sporangia. Conifers are heterosporous. Microspores and megaspores are produced in microsporangia and megasporangia respectively. Sporangia (i.e, micro and megasporangia) are produced on respective cones (male cones and female cones) on the same plant.

               The male cones are small in size and are produced in clusters on an axis. Each male cone consists of microsporophylls which contain microsporangia. Microspore germinates to form a small inconspicuous male gametophyte (also called as microgametophyte) within the spore wall. Such a microspore of seed plants that contains the microgametophyte including the gametes is called a pollen grain (plural = pollen).

               Pollen are produced in great numbers and are transported by wind. Pollen grain in pinus has two wings attached to its lateral sides. Due to wings, pollen can float in air for a longer period of time and can travel long distances. The gymnosperms have successfully evolved this totally new mechanism of transfer of male gamete to the female gametophyte through wind which has made them independent of water for this purpose. This is an important improvement and evolutionary adaptation to survive in the harsh dry terrestrial (land) environment.

               During pollination the pollen land directly on the ovules. Only few pollen are able to germinate to form pollen tubes through which male gametes are transferred to the embryo sac for fertilization.

               More then one egg can be fertilized to form several zygotes, but one zygote usually survives to form a single embryo. After fertilization the ovule becomes the seed. The seeds now contains an embryo along with some stored food material. The seed upon germination gives rise to a new sporophyte plant.

               In the life cycle of piuns, the dominant diploid sporphyte generation alternates with inconspicuous haploid gametophyte generation (Fgi. 9.21)

Fig. 9.21 Life Cycle of pinus

Evolution of Seed Habit Part 1

Evolution of Seed Habit Part 1

A review of the kingdom plantea indicates that the seed-plants (spermatophytes predominate over non-seed vascular plants.

One of the most significant events in the history of land plants was the development of seed habit. It was an important change in the reproductive system of the vascular plants which occurred approximately 390 million years ago. First complete seeds appeared approximately 365 million years ago during late Devonian times. Technically a seed may be defined as a fertilized ovule. An ovule is an integument indehiscent megasporangium. Integuments are specialized protective coverings around megasporangium which vary in number. All seed producing plants are called spermatophytes. Various steps involved in the evolution of seed habit are as follows.

1. Evolution of heterospory.
2. Retention and germination of megaspore within the megasporangium.
3. Development of protective layers around megasporangium.
4. Reduction to a single functional megaspore per sporangium.
5. Development of an embryo sac within the sporangium.
6. Modification of distal end of megasporangium for pollen capture.

1. Evolution of heterospory

Primitive vascular land plants produced one kind of spores, a condition called homospory. All groups of land plants up to pteridophytes are homosporous. During the early phase of evolution some plant groups started producing two different types of spores, the smaller ones called microspores and the larger ones known as megaspores.

The mivropores produced inside microsporangia germinate to form male gametophyte or the microgametophyte, whereas the megaspores germinated to form female gametophyte or megagametophyte.

2. Retention and germination of megaspore within the megasporangium.

During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores are used to be shed and dispersed soon after their formation in order to germinate into female gametophyte. However in some plants (e.g. Selaginella) the megaspore is not allowed to escape from megasporangium immediately after its formation. In others the megaspore is permanently retained within the megasporangium. Here, within the confines of the megasporangium wall the megaspore germinates to form egg containing female gametophyte.

3. Development of protective layers around megasporangium

Some branch like structure of sporophyte surrounding the megasporangium fused around to megasporangium to form protective envelope or integument. The megasporangium tightly locked by integuments becomes totally indehiscent. This important change led to the evolution and formation of the ovule, which is nothing but an integumented indehiscent megasporangium. In this way more protection is accorded to the egg-containing apparatus in terrestrial environment.

4. Reduction to a single functional megaspore per sporangium

Each megaspore mother cell within a megasporangium used to produce four gametophytes. There was a competition for space and food among the four gametophytes. Soon the early vascular plants adopted a new strategy i.e., only one megaspore is selected for further development into a healthy female gametophyte while the remaining three are aborted.

5. Development of an embryo sac within the sporangium

The single healthy megaspore retained within the megasporangium germinates to form an egg containing female gametophyte called an embryo sac.

6. Modification of distal end of megasporangium for pollen capture

When most of the structural and functional changes leading to the development of seed habit were completed, another important modification took place in the megasporangium which was now integumented, indehiscent and permanently attached to the sporophyte. The distal end of the megasporangium became modified for capturing pollen (microspore containing male gametophyte).

Pollen after being trapped in the distal cavity of the megasporangium produces pollen tube which carry male gametes deep into the embryo sac to fertilize the egg, forming a zygote, that forms an embryo. The megasporangium (ovule) after fertilization is transformed into a seed, the integuments becoming the seed coats. The seed offers maximum degree of protection to a developing embryo under the unfavorable terrestrial environment. The development and evolution of seed habit was a great success and a giant leap which ultimately enabled plants to colonize land permanently.

Life Cycle Of Flowering Plant

Life Cycle Of Flowering Plant

The flowering plants show two generations i.e. sporophyte and gametophyte, whichalternate with one another, during life cycle.A sporophyte is a dominant multicellular, diploid generation which is formed from zygote or fertilized egg. This generation produces spores (microspores and megaspores) by meiosis. The gametophyte is a small multicellular, haploid generation, which is formed from spores. The male gametophyte develops from microspore or pollen grain; and the female gametophyte develops from megaspore. The male and female gametophytes produce male and female gametes (sperms and egg) by mitosis. The gametophytes in flowering plants are short lived, much reduced and dependent for protection on sporophyte.

Double fertilization
The pollen tube grows via style through micropyle into the ovule.

Division Bryophyta

Division Bryophyta

The first plants to colonize land were the bryophyta. They are generally thought to have evolved from green algae.
The bryophyta are poorly adapted to live on land and are mainly confined to damp shady places (Fig. 9.1)

Fig. 9.1  A moss bug, lacking rigid supporting tissue, bryophytes are low-profile plants they are most common in damp habitats.

            These plants are devoid of specialized conducting (xylem and phloem) and strengthening tissues. only  the process of diffusion and osmosis helps in the transportation of water and minerals as well as in transportation of prepared food and other substances. The plant body is with a proper cuticle, or has 2 very thin one. The water is absorbed by the general surface of the plant. The bryophyta are said to be the amphibians of the plant world because they cannot live away from water. They need water for reproduction (Fig. 9.2).

Fig. 9.2 Mosses often grow at wet places as seen here in a small water fall.

            The bryophytes are non-vascular flowerless plants. These plants show a regular alternation of heteromorphy (morphologically different) generation. They have a dominant independent free living gametophyte. This may be tabloid as in many liverworts or is differentiated into structures resembling to stem, leaves and absorbing and anchoring organs, rhizoids, as in mosses and some liverworts. The gametophyte produces a sprophyte, which is a less conspicuous generation, partially or totally dependent upon the gametophyte for its nutrition. The sporophyte generally consists of foot, seta and capsule.

            The sporophyte is diploid (2n) which produces in sporangia one kind of haploid spores (i.e. it is homosporous) by meiosis. The spores germinate and give rise to gametophyte which is also haploid. Multicellular male and female exe organs i.e. antheridia and archegonia respectively, are born on gametophyte either on same or different plants. These sex organs are multicellular and protected by a sterile covering of cells (Fig. 9.3).

Fig. 9.3 Sex organs, male (antheridium) female (archegonium)of a bryophytic plant

            Gametes are produced by mitosis. Male gameters produced within antheridia are called antherozoids; antherozoids are motile and always produced in large number . female gametes formed within archegonia are termed as eggs. A single egg is formed in each archegonium. Fertilization takes place in water. Antherozoids (n) are attracted towards archegonia (n) chemotactically. A single antherozoid fuses with an egg(n) thus accomplishing fertilization which results in the formation of the diploid zygote (2n). the zygote is retained within the female sex organ (archegonium) for some time. After a resting period the zygote develops through mitotic divisions into a diploid embryo. The embryo ultimately develops into a sporophyte which is also diloid.

            The entire development of sporophyte thus takes place within the gametophyte plant body. Even which the sporophyte is fully developed it remains attaché d to the gametophyte for nourishment and protection because it does not contain chloroplasts and is unable to perform photosynthesis. There is an alternation of generations in the life cycle of bryophytes i.e. multicellular haploid gametophytic (gamete producing) generation (Fig. 9.6). it is a very important phenomenon, which provides continuous genetic variabilities and selection for the best genetic make up for survival and adaptation in the changing environment (s)(as explained in a later section).

            An view of the above mentioned discussion, bryophyta can therefore be defined more precisely as plants with the distinguishing characters as follows:
            “Vascular system absent; gametophyte dominant; saprophyte attached of gametophyte; homosporous.”

Classification Part 2

Classification Part 2

Anthoceropsida (Horn Worts)

This group of bryophytes differs in many respects and is slightly advanced than bryopsida and Hepaticopsida. The gametophyte is highly lobed and irregular in outline.except for a little early stage of development, the sporophyte is not dependent upon gametophyte for nourishment and protection. Antheridia and archegonia are partially sunken in the gametophytic tissue. The sporophyte exhibit many advanced characters due to which it can thrive better on alnd as compared to other groups. The sporophyte has stomata and chloroplasts in the epidermis and can thus photosynthesize its own food rather than obtaining it from ghametophyte. It also has a waxy cuticle to check excessive loss of water (desiccation). Furthermore. It also has a waxy cuticle to check excessive loss of water (desiccation). Furthermore, at the junction of foot and spore producing region there is a band of meristematic tissue. This tissue keeps on adding cells towards the spore-producing region during the formation, maturation and dispersal of spores from the opposite end. Due to the fast growth rate of this meristematic tissue the sporophyte keeps on increasing in length for an indefinite period of time. Due to these chareacters the sporophyte continues to survive as such even after the death and decay of the gametophyte. One good example of anthoceropsida is anthoceros which is also found in the hilly areas of Pakistan (Fig. 9.9).

(a)

(b)

Fig. 9.9 Anthoceros, a hornwort (a) Gametophyte with attached horn-shaped sporophyte (b) V.S. of sporophyte.

In the life history of liverworts, mosses and hornworts there are two distinct multicellular phases or generations. These generations are haploid gametophyte and diploid sporophyte, which regularly alternate with each other. The gametophyte is the dominant generation because it is more conspicuous. It produces gametes called spermatozoids or antherozoids and eggs, therefore called gamete-producign generation. A haploid spermatozoid fuses with a haploid egg to produce diploid oospore.

The oospore does not produce the gametophyte directly but produces a totally different plant called sporophyte. The sporophyte in bryophytes is a less conspicuous generation, which is usually differentiated into foot, seta and capsule (also called sporogonium). Spores develop within the capsule by reduction division (meiosis) from spore mother cells. The sporophyte produces spores and is, therefore, called spore producing generation. The spore on germination does not develop into a sporophyte but gives rise to the gametophyte. Thus in the life-history of a bryophytic plant, the two generations, the gametophyte and the sporophyte, regularly alternate with each other. The phenomenon of alternation of gametophyte and sporophyte in the life history of a plant is called alternation of generations (Fig. 9.10).

Fig. 9.10 graphic representation of the alternation of gametophytic and sporophytic generation

It should be noted that the gametophyte or haploid stage begins with spores and ends at gameters, whereas the sporophyte begins with oospore and ends at spore mother cell.

The significance of alternation of generations

During the formation of spores from spore mother cells by meiotic division reshuffling of genes occurs. As a consequence, a great variety of spores with different genetic make-up are produced. These spores in turn produce gametophytes with different genetic combinations. The gametophytes with better genetic make-up will have a better chance for survival in the environment where they occur. On the other hand, the gametophytes with less advantageous characteristics will be eliminated. There is no reshuffling of genes during gametogenesis in the gametophyte as gametes are produced after mitosis.

The oospore developing after fertilization now has a new genetic make-up as compared to the parent. This genetic variation passes to the new sporophyte which on maturity once again produces further genetic recombination which are transferred to the gametophyte. In this natural process the sporophyte thus rovide a large amount of genetic variability and nature selects the best genetic combinations. In the long run, this will allow the populations to become increasingly better adapted to their environment.

Classification Part 1

Classification Part 1

Bryophytes are divided into three subdivisions : hepaticopsida, bryopsida and anthoceropsida.
                Hepaticopsida (Liverworts)
Tryophytes belonging to this subdivision are called liverworts. It includes about 900 species. Liverworts are the siplest of all bryophytes (Fig.9.4).
They are usually found on moist rocks and on wet soil, since they live near water therefore chances of drying out are greatly reduced.

(a)

(b)

Fig.9.4 (a) Marchantia, a typical liverwort, the gemma cups function in asexual reproduction (b) Porella, a leafy liverwort showing lateral antheridia bearing branch.

The plant body is a gametophyte. It may be thalloid i.e. flat, or ribbon-like, usually dichotomously branched. It is attached to soil by means of rhizoids e.g. marchantia. Other species tend to grow upright and are falsely leafy i.e., differentiated into a false stem, and leaves e.g., porella (fig. 9.4b). the saprophyte is dependent upon gametophyte for nourishment and protection.

                The sex organs develp on the upper surface of the thallus near the tips of the branches. Scmetimes they develop on special branches on gametophyte called the antherdiophores and the archegoniophores as in marhcantia (fig. 9.5).
Fig. 9.5 A Liverwort, Marchontia bearing sex organs, antheridia and archegonia, on special branches called antheridiophores and archegoniaphores.
Fig. 9.6 A generalized life cycle of a bryophyte showing alternation of generation.
Bryopsida
Like liverworts most mosses inhabit damp places. In contrast to other bryophytes they grow equally well in fairly dry places. However, water is essential in the reproduction of mosses, thus they usually grow to form cushions or mats.
Each adult moss plant, a gametophyte, is always differentiated into structures which resemble stem and leaves. Multicellular rhizoids are also present. Examples of mosses are funaria and polytrichum (Fig. 9.7). Archegonia and antheridia, develop on the tips of different branches on the same plant e.g. , Funaria, or on different plants as in polytrichum. The archegonia and antheridia form clusters and are mixed with sterile hairs, the paraphyses.
Fig 9.7 polytrichume, a hair cup moss plant.
Formation of diploid sporophyte and haploid spores follow the same sequence of events of alternation of generations as in liverworts (Fig. 9.6). however, the spore of a moss, unlike that of liverworts, develops into an alga like structure, the protonema. Haploid moss plants (gametophyte) develop from buds on the protonema and the life cycle is completed (Fig. 9.8)

Fig. 9.8 Moss life cycle

Grade Radiate Cells Part 2

Grade Radiate Cells Part 2

The life cycle of coelenterates is characterized by the presence of alternation of generations. There are two generations; one reproduces by sexual means and the other by asexual means. Both generations are diploid. Often the two generations consist of one free-living and one attached stage. Therefore asexual generation and sexual generation alternation with one another. This is known as alternation of generations e.g., Obelia.

Polymorphism – A Characteristic Feature of Coelenterates (Cnidaria)

The occurrence of structurally and functionally more than two different types of individuals, called the zooids within the same organism is called polymorphism.

For example, in Obelia there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusa…..

The common examples of coelenterates are:

1. Hydara:  A freshwater – coelenterate. It exists only in polyp form; therefore alternation of generations is absent.
2. Obelia: Amarince colonial                     that exhibits alternation of generations.
3. Aurelia (jelly fish): The polyp is reduced and medusa is dominant in jellyfish.
4. Actinia (sea anemone): The body consists of polyp only enteron is divided by large partitions called mesenteries.
5. Madrepora: The body is covered with hard calcareous skeleton formed of calcium carbonate. They are commonly called corsal. The skeleton forms large coral reefs and even small islands.

Fig. 10.5 coelenterates (Cnidarians)

Coral reefs

Corals are formed from the secretions produced by specialized polyps that are present in certain coelenterates. These polyps become covered by stony cups due to hardening of their secretions. From the mouth of the stony cup a polyp can pass out its tentacle for the purpose of feeding and withdraw itself where not feeding. Most such coelenterates are colonial. They stony net-work or mass of such coelenterates are called corals. Living polyps are found on the surface layer of corals whereas underneath the mass are dead stony structures only and there are no polyps inside. The stony masses that are formed in this way are called coral reefs. These are mostly formed of calcium carbonates (lime-stone).

The corals because of their massive structure serve as living place for a variety of sea life.

Coral reefs are found in the coastal waters of Florida. West Indies, east coast of Africa, Australia and Island of Coral sea.