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Systematics

History of classification

 References of classification of organisms are available in Upanishads and Vedas. Our Vedic literature recorded about 740 plants and 250 animals. Few other significant contributions in the field of classification are :

(1) Chandyogya upanishad : In this, an attempt has been made to classify the animals.

(2) Susruta samhita : It classifies all 'substances' into sthavara (imbobile) e.g. plants and jangama (mobile) e.g. animals .

(3) Parasara : Here, angiosperms were classified into dvimatruka (dicotyledons) and ekamatruka (monocotyledons). He was even able to find that dicotyledons bear jalika parana (reticulate veined leaves and monocotyledons bear maun laparna parallel veined leaves).

(4) Hippocrates, and Aristotle : They classified animals into four major groups like insects, birds, fishes and whales.

A. New systematic

Different systems of classification proposed from time to time have been divided into three basic categories viz., artificial systems, natural systems and phylogenetic system (However, Redford, (1986), included mechanical systems as a fourth category).

(1) Artificial system of classifications :

These systems are more or less arbitrary as the plants are classified merely on the basis of gross morophology, habit and their importance to man. The main advocates of artificial system of classifications were :

(i) Theophrastus : Father of botany. Theophrastus was a disciple of Plato and later Aristotle. In his book De Historia plantarum, he classified about 500 kinds of plants into four major group; trees, shrubs, subshrubs and herbs.

(ii) Caius Plinius Secundus : He described the biological, medicinal and agricultural aspects of plants in 37 volumes of Natural History. He used the term 'Stamen' for the first time.

(iii) Pedanios Dioscorides : He described about 600 plants of medicinal importance in his Materia Medica.

(iv) Charaka : ka Indian Scholar. He classified plants of medicinal importance in his Charaka Samhita.

(v) Andrea Caesalpino : He described 1520 species in 16 volumes of De Plantis libri grouped as herbs and trees. He further classified plants based on fruit and seed characters.

(vi) John Ray : He was a British botanist who published three volumes of his work Historia Generalish Plantarum consisting of improved classification originally proposed by him in Methodus Plantarum Noven. He was the first to divided the groups herbs, shrubs and trees into Dicots and Monocots on the basis of the presence of two or one cotyledons respecitvely. He coined the term species.  

(vii) Carolus Linnaeus : Father of taxonomy. A swedish botanist, who published an artificial system of classification based exclusively on floral characters. Linnaeus published several manuscripts including Hortus cliffortianus and Genera plantarum (1737). In his Genera plantarum he listed all the plant genera known to him. He published his best known Species plantarum in 1753. In this book he listed and described all species of plants known to him. He established binomial nomenclature.

(2) Natural System of Classifications :

These systems of classification are based not only on the characters of reproductive organs and structural morphology but used as many taxonomic characters or traits as possible to classify the plants. The advocates of natural systems of classification are listed below :

(i) Michel Adanson : A French botanist, who classified plants and animals using as many characters as possible and proposed a natural system of classification.

(ii) A.L. de Jussieu : Classified plants based on natural characters. In his system of classification he grouped the plants resembling each other in a set of characters.

(iii) A.P. de Candolle : He grouped all alike plants together and published a new classification of plants in his book Theorie elementaire de la botanique (1813).

(iv) George Bentham and Joseph Dalton Hooker : These two English botanists classified plants based on original studies of specimens. They published their well known scheme of classification in Genera plantarum (1862–83). This system of classification is still regarded as the best classification, especially from the practical point of view.

B. Basics in classification

Classification means the ordering of organisms into groups. The branch of science that deals with the study of principles and  procedures of biological classification is called taxonomy (Gk. taxis - arrangement, nomos - law; coined by A.P. de Candolle, 1813).

There are millions of organisms – plants, animals, bacteria and viruses. Each one is different from the other in one way or the other. About more than one million of species of animals and more than half a million species of plants have been studied, described and provided names for identification. Thousands are still unknown and are yet to be identified and described. It is practically impossible to study each and every individual. Also, it is difficult to remember their names, characters and uses. However, biologists have devised techniques for identification, naming and grouping of various organisms.

The art of identifying distinctions among organisms and placing them into groups that reflect their most significant features and relationship is called biological classification. Scientists who study and contribute to the classification of organisms are known as systematists or taxonomists, and their subject is called systematics (Gk. Systema = order of sequence) or taxonomy (Gk. Taxis = arrangement; nomos = law).

Explanation of terminology (Units).

(1) Taxon : The term taxon is used to represent any unit of classification. The unit (i.e., taxon) many be large (e.g., Plant Kingdom) or small (e.g., Algae, Fungi, or a single species).

(2) Category : Various sub-divisions of plants kingdom such as division, class, order, family, etc., are referred to as categories. In the hierarchy of categories kingdom is the highest and species is the lowest category. The following is hierarchial series :

(i) Kingdom : It is the highest category in biological classification. All plants are include in plant kingdom.

(ii) Division : It is a major group in the Linnean hierarchy used in the classification of plants (equivalent to phylum in animal classification). It is a taxonomic category between kingdom and class. The subcategory of division is subdivision. The suffix of division is – ophyta.

(iii) Class : A division is divided into classes. It is a taxonomic category between the division and order. Its suffix is – ae. The subcategories of class are subclass and series. In the class contain organism least similar to one another.

(iv) Order : A class includes one or more orders. It is a taxonomic category between the class and family. Its suffix is – ales. The subcategory of order is suborder.

(v) Family : An order is divided into one or more families. It is the taxonomic category between the order and the genus. Its suffix is – aceae. The subcategories of family are subfamily, tribe and subtribe.

(vi) Genus : The plural of genus is genera. A family includes one or more genera. The generic name is important and printed in Italics (If hand written, it is underlined). The subcategories of genus are subgenus, section and subsection.

(vii) Species : It is the smallest rank of taxonomic classification. The first letter of the species is denoted with small letter. The species is printed in Italics (It is underlined if hand written). A genus may include one or more species. The subcategories of species are subspecies, varieties, subvarieties, form and subform.

(3) New systematics or Biosystematics :

The term new systematics was proposed by Sir Julian Huxley in 1940. In the new systematics, the species are considered related to one another, mutable and the work of gradual modification. This is in confirmity with the facts of evolution.

Forms of new systematics : There are several forms of new systematics –

(i) Morphotaxonomy : It is based on the structural features of the organisms.

(ii) Cytotaxonomy : It is based on the somatic chromosomes of organisms.

(iii) Biochemical taxonomy or Chemotaxonomy : It is based on the protein and serum analyses and on the chemical constituents of the organisms.

(iv) Numerical taxonomy : It involves quantitative assessment of similarities and differences in order to make objective assessments. Characters of organisms are given equal weight and the relationships of the organisms are numerically determined, usually with the aid of a computer.

(v) Experimental taxonomy : It is based on the genetic relationship determined with the help of experiments.

taxonomy enfolds the following fundamental elements:

● Identification: Identification is determining the correct place in a system of classification and finding out the correct name of an organism. It is done with the help of keys. This is carried out for an organism by determining its similarity with an already known organism. Suppose there are three plants say a, b, c. All represent different species. Another plant, say d resembles b. The recognition of the plant d as identical to the already known plant b is its identification.

● Nomenclature (Latin, nomen : name ; calare : call) : It is the science of providing distinct and proper names to organisms as per the established universal practices and rules so that they can be easily recognized     and differentiated from others.

●Classification: Classification is the arrangement of organisms into groups on the basis of their affinities or relationships. It involves the placing of a kind of organisms or a group of different kinds of organisms in particular categories depending upon the system of classification but in conformity with nomenclature system.

C. Nomenclature

Plant nomenclature.

Plant nomenclature may be defined as the system of naming plant. Almost all plants (and animals too) are known by different common names in different parts of the world. Even within the same country people of different states and regions use different common names. Iphomoea batatas, for example, is called sweet potato in English, Shakarkandi in Hindi, Meetha alu in Assamies and Bengali, Kundmul in Telagu, Ratalu in Marathi and Jenasu in Kannad. The common names are thus quite confusion. This necessiated the need of giving scientific names so that scientists of different parts of the world could understand each other work. The earliest scientific names were polynomial, i.e., they were composed of many words (which gave the characteristics of plants), e.g., Sida acuta (a member of Malvaceae) was named as Chrysophylum foliis, ovalis superne glabris parallel striatis subtus, tomentosonitidis. Such long names were difficult to remember. Hence, to make it easier binomial system of nomenclature was introduced.

(1) Binomial system of nomenclature :

The credit of giving binomial system of nomenclature goes to Swedish naturalist, Carolus Linnaeus. He employed this system in his book Species Plantarum, published in 1753. According to this system the name of a plant or animal is composed of two Latin (or Latinised) words, e.g., potato is Solanum tuberosum. The first word (i.e., Solanum) indicates the name of the genus (called generic name) and the second word (i.e., tuberosum) denotes the name of the species (called specific name). The generic name always begins with a capital letter and the specific name with a small letter and printed in italics.

The generic and specific names always have some meaning. They are based on some special characters of the plant, on the name of any scientist or on some legend.

All plants having general similarity and relations are given a common generic name, e.g., potato, brinjal, black nightshade (makoi) have been placed in the genus Solanum. However, their specific names distinguish them from each other – potato is Solanum tuberosum, brinjal is S. melongena and black nightshade is S. nigrum.

Usually the name of the author, who names a plant, is also written in full or in abbreviated form after the specific name. Thus, in case of Mangifera indica L., the L. stands for Linnaeus and in Lychnis alba Mill., the Mill. stands for Miller.

Sometimes a single species is described under different names by different authors. These name are called synonyms. In such cases, the name under which the species is first described, is considered to be valid.

(2) Trinomial nomenclature :

Certain species are divisible into smaller units, called varieties, on the basis of finer differences. The name of the variety is written after the specific name. Thus, the name may become trinomial or three word name. e.g., Homo sapiens europeus is the name of the man of European race. Trinomial nomenclature is simply an extension of the Linnaean system.

(3) Code of biological nomenclature :

Anyone can study, describe, identify and give a name to an organism provided certain universal rules are followed. These rules are framed and standardised by International Code of Botanical Nomenclature (ICBN) and International Code of Zoological Nomenclature (ICZN). The codes help in avoiding errors, duplication, confusion and ambiguity in scientific names. The codes are established and improved upon at International Botanical and Zoological Congress held from time to time. The names of bacteria and viruses are decided by International Code of Bacteriological Nomenclature (ICBN) and International Code of Viral Nomenclature (ICVN). Similarly, there is a separate International Code of Nomenclature for Cultivated Plants (ICNCP).

D. Taxonomic hierarchy

❒ The main aim of a taxonomic study  is to assign organism an appropriate place in a systematic framework of classification. This framework is called taxonomic hierarchy by which the taxonomic groups are arranged in definite order, from higher to lower categories, depending upon their relative dimensions. The hierarchy indicates the various levels of kinship. Nearer the categories in hierarchy, the greater is the similarity between their organisms.

❒ It is also called Linnaean hierarchy because it was first proposed by Linnaeus. Linnaeus first used only five categories-Class, Order, Genus, Species and Variety. The last one was discarded and three added so that now there are seven obligate categories i.e., Kingdom, Division or Phylum, Class, Order, Family, Genus and Species. This sequence may be remembered by memorizing the sentence “Keep pots clean or family gets sick”. The botanists use Division in place of Phylum as a category in the classification of plant kingdom. In order to make taxonomic position of species more precise, certain subcategories and supercategories have been added to this list and they are called intermediate categories e.g., sub kingdom, super phylum or super division, sub division, super class, sub class, super order, sub order, super family, sub family, tribe, sub species, variety, etc.

❒ Both in animals and plant kingdoms, the lowest category is Species and highest is the Kingdom. The placement of group of individuals or organisms in species, genus and up to phyla or divisions is determined by the similarities in their characters and the relationships. The categories in the hierarchy are thus in ascending order. As we go from the lowest rank Species towards Kingdom the number of similar characters decreases.

❒Of various categories of classification, only the species have a real existence in nature, others are merely man made conveninent and arbitrary groups without actual existance in nature.

E. Taxon

Taxon (Gk. Taxis : arrangement) 

❒The word ‘taxon’ signifies a taxonomic group of any rank which represents the real biological organisms included in a category like maize (species), roses (genus), grasses (family), conifers (order), dicots (class), seed plants (division) etc.

❒The term was introduced by Adolf Meyer (1926) for animal groups. H. S. Lam first proposed the term ‘taxon’ for plants. Mayr (1964) defined taxon to be a taxonomic group of any rank that is sufficiently distinct to be worthy of being assigned to a definite category.

❒There is some confusion in the use of taxon and category. Bryophyta is a taxon while division is a category. Similarly Zea mays is a taxon while species is a category. While category represents an abstract term, taxon represents the real organisms.

❒Depending upon the closeness of its members, a taxon may be monophyletic or polyphyletic. A monophyletic taxon or clad generally belongs to a lower ranking category, e.g., tigers representing a species. Polyphyletic taxon or grade usually belongs to higher category, e.g., mammals representing a class.

F. Obligate categories

The obligate categories used in classification are explained below :

Species

❒ Species occupies a key position in classification. It is the lowest or basic taxonomic category.

❒ It is a basic unit for understanding taxonomy as well as evolution. It is a natural population of individuals or group of populations which resemble one another in all essential morphological and reproductive characters, carry same type and amount of genetic material so that they are able to breed freely among themselves under natural conditions in order to produce fertile offspring.

❒The species is also called genetically distinct and reproductively isolated natural population e.g., mango (Mangifera indica), potato (Solanum tuberosum), tulsi (Ocimum sanctum), lion (Panthera leo), etc. In this case, indica, tuberosum, sanctum, and leo are species of genera Mangifera, Solanum, Ocimum and Panthera, respectively. A genus may have more than one species as in Panthera tigris where tigris is another species.

❒The individuals of species also represent population of species and they do not breed with individuals of other species.

❒As per rules of binomial nomenclature, a species can be named only if it is assigned to a genus.

❒A species may have subgroups, called subspecies or varieties, showing certain distinct features of their own.

Genus

❒ It is the first higher category above the level of species. It is a group of species which are related and have less characters in common as compared to species. For example, potato (Solanum tuberosum) and brinjal
(S. melongena) though they constitute different species, belong to the same genus Solanum. Similarly lion (Panthera leo), leopard (P. pardus), tiger (P. tigris) and jaguar (P. onca) have several common features and are included in the same genus Panthera.

❒A genus may have a single species (monotypic), e.g., Homo sapiens, or it may have several species (polytypic), e.g., Panthera, Solanum, etc.

❒All the species of a genus have a number of common features called correlated characters. The close resemblance indicates a common ancestry for all the species of a genus.

❒There is no rule for suffix in genus and species.

Family

❒Family is represented by a group of related genera that are more similar to each other than with the genera of other families.

❒All the genera of a family resemble one another in certain correlated characters indicating a common ancestry. The genera like Solanum, Petunia, Datura, Atropa, etc. based on the similarities are placed in the family Solanaceae. In animals such as lion, leopard, tiger, jaguar from genus Panthera, and cat from genus, Felis are included in the family Felidae. However, the feathers of a cat (Felis domesticus) and a dog (Canis familiaris) show distinct differences between them. Therefore, they are placed in different families, Felidae and Canidae, respectively.

❒In plants, the family ends in the suffix - aceae and subfamily in - oidae while in animals the suffixes are - ideae for family, - inae for sub-family, - ini for tribe (between sub-family and genus).

G. Systems of classification

                                                                                   Differences between Natural and Artificial classification

No.

Characters

Natural classification

Artificial classification

1.

Number of characters

Almost all the characters are considered.

Only few characters are considered.

2.

Hereditary constitution

Members are mostly alike in hereditary pattern of different groups.

Members of different groups are usually not similar in hereditary pattern.

3.

Flexible

May change with advancement in knowledge.

Stable classification.

4.

Phylogeny

Closely related phylogenetically.

Not related phylogenetically.

5.

Information

Provides plenty of useful information.

Provides only limited information.

6.

Recent advances

Recent useful research can be easily incorporated.

Cannot incorporate new work.

7.

Convenience

Identification of plants easy.

Difficult.

8.

Little known plants

Got the place at definite place.

Not certain about position and identification.

H. Number of kingdoms

Modern system of classification.

(1) Two kingdom system of classification :

This system of classification is the oldest it was suggested by Carolus Linnaeus in 1758. He divided the living word (organism) in to two kingdoms, Plantae (for all plants like tree, shrubs, climbers, creepers, moss and floating green algae) and Animalia (For animals).

(2) Three kingdom system of classification : Ernst Haeckel, a German biologist and philosopher, suggested a third kingdom protista in 1866 for –

(a) Unicellular organisms such as bacteria, protozoans and acellular algae.

(b) Multicellular organisms without tissue such as algae and fungi.

(3) Four kingdom system of classification : It was proposed by Copeland in 1956. The two additional kingdoms were Monera for the bacteria and blue green algae and Protista for protozoans, algae and fungi.

(4) Five kingdom system of classification : R.H. Whittaker, an American ecologist. He proposed five kingdom system of classification in 1969.

This system replaced the old, two-kingdom grouping of living organisms. As already discussed, a division of living world merely into plant and animal kingdoms is too simple. It does not take into account the gradual evolution of distinct plant and animal groups and it allows no place for those primitive organisms that even now are neither plants nor animals nor that are both. In this classification eukaryotes were assingned to only four of the five kingdom.

Five-kingdom classification is based on the following four criteria :

(i) Complexity of cell structure.

(ii) Complexity of organism's body.

(iii) Mode of obtaining nutrition.

(iv) Phylogenetic relationship.

The five kingdom are : Monera, Protista, Fungi, Plantae and Animalia.

I. Kingdom Monera

Kingdom Monera (The prokaryotes).

Monera (Monos – single) includes prokaryotes and shows the following characters :

(1) They are typically unicellular organisms (but one group is mycelial).

(2) They lack nuclear membranes.

(3) Ribosomes and simple chromatophores are the only subcellular organelles in the cytoplasm. The ribosomes are 70 S. Mitochondria, plastids, Golgi apparateus, lysosomes, endoplasmic reticulum, centrosome, etc., are lacking.

(4) The predominant mode of nutrition is absorptive but some groups are photosynthetic or chemosynthetic.

(5) Reproduction is primarily asexual by fission or budding.

(6) Protosexual phenomenon also occurs.

(7) The organisms are non-motile or move by beating of simple flagella or by gliding.

(8) Flagella, if present, are composed of many intertwined chains of a protein flagellin. They are not enclosed by any membrane and grow at the tip.

(9) Moneran cells are microscopic (1 to few microns in length).

(10) Most organisms bear a rigid cell wall.

(11) The kingdom Monera includes true bacteria, mycoplasmas, rickettsias, actinomycetes (ray fungi) etc. Microbiologists also include blue green algae (i.e., Cyanobacteria) under the group bacteria because of the presence of prokaryotic cell structure. Studies have established that the members of archaebacteria group are most primitive and have separated from eubacteria group very early in the process of evolution. Furthermore, these studies have also concluded that the archaebacteria and eubacteria possibly originated from a more ancient form of life called Progenote.  

(12) Nutrition : They show both autotrophic and heterotrophic modes of nutrition.

(i) Autotrophs : These are able to form their own food by one of the following methods.

(a) Photoautotrophs : They prepare their own food by reducing CO2 using light energy.

(b) Chemoautotrophs : They form their food by energy derived from chemical reaction.

(ii) Heterotrophs : A few live in symbiosis while others form association of commensalism. Saprophytes also called 'saprobes' cause decay, fermentation or putrefaction of dead organic matter. Some bacteria are facultative sasprophyte (= facultative parasites). In the process of fermentation there is anaerobic break-down of carbohydrates into CO2, alcohol and some energy. Putrefaction or decay is anaerobic break-down of proteins accompanied by foul smell due to evil smelling gases produced in the process.

The saprobes produce enzymes which convert non-diffusible food substrates (carbohydrate fats, proteins, etc.) into simpler diffusible form which diffuses into the cytoplasm and is assimilated, i.e., converted into body cytoplasm or stored as reserve food.

Still others live on other living organisms (animals, plants or man) in the form of parasites directly absorb their food from the body of host. Some of the parasites are non-pathogenic i.e., cause no ill-effect or disease in the host, while some are pathogenic causing diseases in the host.

J. Kingdom Protista

Protista (Protistos = Primary) includes unicelluar eukaryotes and show the following characters :

(1) Protists include solitary unicellular or colonial unicellular eukaryotic organisms which not form tissues.

(2) Simple multinucleate organisms or stages of life cycles occur in a number of groups.

(3) The organisms possess nuclear membranes and mitochondria.

(4) In many forms, plastids, (9+2 strand) flagella and other organelles are present.

(5) The nutritive modes of these organisms include photosynthesis, absorption, ingestion and combination of these.

(6) Some protists possess contractile vacuole for regulation of their water content.

(7) Their reproductive cycles typically include both asexual divisions of haploid forms and true sexual processes with karyogamy and meiosis.

(8) The organisms move by flagella or by other means or are non-motile.

(9) Nutrition : It is may be photosynthetic, holotrophic, saprotrophic and parasitic. Some have mixotrophic nutrition (holotrophic + saprobic). Chemosynthetic nutrition is lacking. Certain protozoans decompose organic matter, such as cellulose, in the gut of termites and woodroaches. They live as symbionts. The photosynthetic, floating protists are collectively called phytoplankton. They usually have a cell wall. The free-floating, holozoic protozoans are collectively termed zooplankton. They lack cell wall to allow ingestion of particulate food.

(10) Reproduction : It is occurs by both asexual and sexual methods :

(i) Asexual reproduction :  It is the most common method of reproduction in protists in which the genetic constitutions of young ones remains the same as that of the parent. Under favourable environmental conditions, they reproduce asexually several times a day resulting in population explosions. The major types of asexual reproductions are as follows :

(a) Binary fission : The parent cell divides into two approximately equal daughter cells either transversely (e.g., Paramecium), longitudinally (e.g., Euglena) or axially (e.g., Amoeba) by mitosis.

(b) Multiple fission : Division of parent cell into a number of daughter cells is called multiple fission. It occurs in Amoeba.

(c) Plasmotomy : Fission of multinucleate protist into two or more multinucleate offsprings by the division of cytoplasm without nuclear division is called plasmotomy. It occurs in Opalina.

(d) Budding : In this type of asexual reproduction, a small bud is formed from the parent body which separates and develops into new individual. e.g., Paracineta, Arcella, etc.

(e) Spore formation : Sessile or stalked sporangia containing spores are formed in slime moulds. They liberate the spores which can withstand a prolonged period of desiccation. On germination, each spore gives rise to new individual. e.g., Slime moulds.

(ii) Sexual reproduction : Sexual reproduction is believed to have originated in primitive protists. It involve meiosis (reduction division) and syngamy. It occurs following types.

(a) Isogamy : The two fusing gametes are structurally and functionally similar, e.g., Monocystis.

(b) Anisogamy : The two fusing gametes are similar but differ only in their size and/or motility, e.g., Ceratium.

(c) Oogamy : Large non-motile gametes are fertilized by smaller motile gametes, e.g., Plasmodium.

(11) Major group of protists : Unicellular protists have been broadly divided is to three major grous

(i) Photosynthetic protists : Protistan algae e.g. Dinoflagellates (i.e. Ceratium, Glenodinium, Gymnodinium, Gonyaulax, Noctiluca and Peridinium), Diatoms (Navicula, Nitzchia, Melosira, Cymbella, Amphipleura, Pinnularia) and Euglenoids or Euglena like flagellates (Euglena, Eutreptia, Phacus, Peranema).

(ii) Consumer protists : Slime moulds or Myxomycetes, e.g., Physarum, Physarella.

(iii) Protozoan protists : It is include four phyla – Zooflagellata (e.g., Trypanosoma, Giardia, Trichonympha, Trichomonas, Leishmania etc.), Sarcodina (e.g., Amoeba, Entamoeba, Pelomyxa, Mestigamoeba etc.), Sporozoa (e.g., Plasmodium, Monocystis, Eimeria etc. all are endoparasites) and Ciliata (e.g., Paramecium, Vorticella, Opalina, Podophyra etc.).

K. Kingdom Fungi

(1) Introduction : The science dealing with the study of fungi is called as mycology. The knowledge of fungi to mankind dates back to prehistoric times. Clausius, 1601 may be regarded as one of the earliest writers to describe fungi. Bauhin (1623) also included the account of known fungal forms in his book Pinax Theatric Botanica. The fast systematic account of fungi came from Pier Antonio Micheli (1729) who wrote 'Nova Plantarum Genera'. He is described by some workers as founder or mycology. Linnaeus (1753) also included fungi included fungi in his 'Species Plantarum'. Elias Fries (1821-31) gave a more detailed account of fungi in his 'Silloge Fungorum' in 25 volumes describing some 80,000 species of fungi. This work remains unparalleld even today.

(2) Thallus organization : The plant body of true fungi (Eumycota), the plant body is a thallus. It may be non-mycelial or mycelial. The non-mycelial forms are unicellular, however, they may form a pseudomycelium by budding. In mycelial forms, the plant body is made up of thread like structures called hyphae (sing. hypha). The mycelium may be aseptate (non-septate) or septate. When non-septate and multinucleate, the mycelium is described as coenocytic. In lower fungi the mycelium is non-septate e.g., Phycomycetae. In higher forms it is septate e.g., Ascomycotina, Basidiomycotina and Deuteromycotina. In some forms the plant body is unicelled at one stage and mycelial at the other. Their organization is sometimes described as dimorphic.

Holocarpic and Eucarpic : When the entire mycelium is converted into reproductive structure, the thallus is described as holocarpic. However, if only a part of it becomes reproductive, the thallus is called as eucarpic. The eucarpic forms may be monocentric (having a single sporangium) or polycentric (having many sporangia).

(3) Specialised formation : In higher forms the mycelium gets organised into loosely or compactly woven structure which looks like a tissue called plectenchyma. It is of two types :

(i) Prosenchyma : It comprises loosely woven hyphae lying almost parallel to each other.

(ii) Pseudoparenchyma : If the hyphae are closely interwoven, looking like parenchyma in a cross-section, it is called as pseudoparenchyma.

In addition to above, the fungal mycelium may form some specialized structures as under :

(a) Rhizomorphs : Its a 'root-like' or 'string-like' elongated structure of closely packed and interwoven hyphae. The rhizomorphs may have a compact growing point.

(b) Sclerotia : Here the hyphae gets interwoven forming pseudoparenchyma with external hyphae becoming thickened to save the inner ones from desiccation. They persist for several years.

(c) Stroma : It is thick mattress of compact hyphae associated with the fruiting bodies.

(4) Cell organization : The cell wall of fungi is mainly made up of chitin and cellulose. While chitin is a polymer of N-acetyl glucosamine, the celulose is polymer of d-glucose. Precisely, the cell wall may be made up of cellulose-glucan (Oomycetes), chitin chitosan (Zygomycetes) mannan-glucan (Ascomycotina), chitin-mannan (Basidiomycotina) or chitin-glucan (some Ascomycotina, Basidiomycotina and Deuteromycotina). Besides, the cell wall may be made up of cellulose-glycogen, cellulose-chitin or polygalactosamine-galactan.

In higher fungi, where the mycelium is septate, the septa are of several types :

(i) Solid septum : It has no perforations.

(ii) Perforated septum : It has several perforations.

(iii) Acomycetean septum : It has a single large pore in the centre of the septum.

(iv) Bordered pit type septum : It has a perforation in the septum resembling the bordered pit of tracheary elements.

(v) Dolipore septum : It has a single barrel shaped pore in the septum due to thickened rim. The pore has a cap of ER called parenthosome.

L. bryophyata

(1) Introduction : Bryophyta (Gk : Bryon = moss ; phyton = plants) includes the simplest and primitive land plants. De Jussieu (1789) placed mosses etc. under acotyledons along with algae and fungi. De Candolle (1813) placed liverworts and mosses in the class Aetheogamous of the division Cellulare. Robert Broun include algae, fungi, lichen and mosses under bryophyta. It occupies a position intermediate between algae and pteridophyta. Due to peculiar type of their habitats, they are regarded as 'the amphibians of the plant kingdom'.

(2) Habitat : Bryophytes usually grow in moist and shady places. The plants grow densely together and form green carpets or mats on damp soil, rock, walls, barks of trees and on decaying logs in forests, especially during the rainy season.

(3) Specialized habitats : Some bryophytes grow in diverse habitats such as – aquatic (e.g., Riccia fluitans, Ricciocarpus natans, Riella), epiphytes (e.g., Dendroceros, Radula protensa and many mosses), saprophytes (e.g., Buxbaumia aphylla, Cryptothallus mirabilis), and in dry habitats such as dry heaths (e.g., Polytrichum juniperinum), deserts (e.g., Tortula desertorum) and dry rocks (e.g., Porella platyphylla).

(4) Gametophytic plant body

(i) The life cycle of bryophytes consists of two distinct phases – the gametophytic phase and the sporophytic phase. The haploid gametophyte is dominant, long lived, green and independent whereas the diploid sporophyte is short lived and dependent upon the gametophyte. The two phases come one after the other in alternating manner and both are morphologically distinct.

(ii) The plants are small, range from few millimetres (e.g., Zoopsis) to 30–40 centimetres. The tallest species may reach upto 70 cm in length (e.g., Dawsonia).

(iii) The gametophytes are either thalloid (i.e., not differentiated into true roots, true stem and true leaves) or leafy shoot having stem-like central axis and leaf-like appendages.

(iv) The roots are completely absent and they are replaced by unicellular or multicellular thread like rhizoids. In some higher forms the multicellular rhizoids form cords.

(v) The vascular tissue (i.e., xylem and phloem) are completely absent. In few mosses (Polytricum) the xylem like hydroids, which conduct water and phloem like leptoids, which conduct the assimilates, have been reported.

(5) Apical growth : The apical growth in bryophytes take place by a single apical cell or a group of meristematic cells arranged in a transverse row. In Riccia, Marchantia and many jungermanniales the apical growth takes place by a transverse row of apical cell. In mosses, it occurs by single pyramidate apical cell. In Anthoceros, on the other hand, there may be a single apical cell or a transverse row of such meristematic cells.

(6) Reproduction : The bryophytes reproduce vegetatively, asexually and sexually. Various methods involve in reproduction are discussed in the following account.

(i) Vegetative reproduction : The bryophytes reproduce vegetatively by following methods :

(a) Death and Decay : Most of these plants reproduce vegetatively by gradual death and decay of the older part of the plant body.

(b) Adventitious branches : Many plants like Riccia fluitans, Reboulia, Asterella, Pellia etc. reproduce by adventitious branches. They separate and produce new plants.

(c) Tubers : Several species of Riccia, Anthoceros, Sewardiella, Asterella etc. produce tubers which give rise to new plants on the arrival of favourable conditions.

(d) Gemmae : Several members, reproduce vegetatively by forming multicelled gemmae. In Marchantia, Lunularia, the gemmae are produced in gemma cups. In some liverworts, 1–3 celled gemmae are prodcued on the axis or on the 'leaves' or on thalli. Gemmae are also produced on the thallus of Anthoceros. Several mosses also produce gemmae on the 'leaves' (Bryum), or axis or rhizoids or on the protonema (Funaria).

(e) Leafy propagules : Some liverworts also reproduce vegetatively by forming leafy propagules.

(f) Primary protonema : The mosses generally reproduce vegetatively by breaking of the primary protonema. New gametophores now arise from the buds differentiated on it.

(g) Secondary protonema : In several mossess a secondary protonema may arise from the rhizoids or primary protonema or even from the injured sporophyte. It may produce buds which give rise to new gametophores.

(h) Rhizoids : Mosses may also reproduce vegetatively from the rhizoids e.g., Leucobryum.

(ii) Sexual reproduction : The male sex organs is called as antheridium and the female as archegonium. The antheridial stalk is very distinct whereas the archegonial stalk is generally short. They may be of embedded type e.g., Riccia, Anthoceros or of projecting type e.g., Marchantia, mosses.

(a) Antheridia : They are generally borne on the dorsal surface of the thallus. While in Riccia the antheridial chambers are open, in Anthoceros they are closed. The antheridia lie embedded within the thallus in both the plants. In Jungermanniales the antheridia are borne in one or more rows. In Marchantiaceae they are present on a special branches, the antheridiophores or the male receptacles which may be stalked or sessile. While in most of the members the antheridia are superficial in origin, in Anthoceros they are endogenous. Each antheridium is distinguishable into a stalk and the body. The antheridial body consists of a mass of androgonial cells covered by a 1-cell thick sterile jacket. The terminal cell of the jacket, when distinct, is called as operculum. Each androgonial cell finally behaves as androcyte mother cell. The androcyte mother cell then forms two androcytes (antherozoid mother cell), each of which is metamorphosed into a biflagellate antherozoid.

(b) Archegonia : These are also borne on the dorsal surface of the thallus. In Riccia and Anthoceros they lie embedded in the thallus. In many members of Marchantiaceae they are borne on special branches called archegoniophores or the female receptacles, that may be stalked or sessile. The archegoniophore or carpocephalum has rows of archegonia protected by involucre or perichaetium. The archegonia are flask shaped structures distinguishable into a long neck and a globular, swollen venter. A multicelled stalk is also present in mosses but in others it is very short. The neck is one-cell thick. It is generally made up of six vertical rows of cells but in Jungermamnniales it is compose of 4 or 5 vertical rows only. The neck is capped by four cover cells and contain varying number of neck canal cells inside. While in Riccia there are only 4 neck canal cells, the mosses however, possess more than six of them. The venter is also 1-cell thick in most of the plant but in Jungermanniales it is 2–3 layered. In mosses it is double layered. The venter contains an egg and a ventral canal cell.

(7) Importance of water in bryophytes : The bryophytes are fundamentally terrestrial plants but require presence of water to complete their life cycle. The water is needed for dehiscence of antheridia, liberation of antherozoids, transfer of antherozoids from antheridia to archegonia, opening of archegonial neck, and the movement of antherozoids into the archegonial neck.

(8) Fertilization : Before fertilization the walls of androgonial cells disorganise to form a mucilagenous mass. The opercular cell is removed and the antherozoids are liberated. The neck canal cells and the ventral canal cell also disorganise. The cover cells split apart giving a free passage to incoming antherozoids. The antherozoids are attracted towards the egg by chemotactic stimulus, which in bryophytes, is provided in the form of sugars. Antherozoids enter in to archegonia and fertilized the egg.

(9) Sporophyte

(i) The diploid fertilized egg (zygote) is the first cell of sporophytic generation. It divides and develops into a sporophytic plant body, called sporogonium.

(ii) The wall of venter forms calyptra, which provides a protective covering to the developing sporogonium.

(iii) The sporogonium, in most of the cases, is differentiated into foot, seta and capsule.

(iv) The sporogonium is completely dependent on the gametophyte for water and mineral supply and, in most of the cases, partly or wholly for organic nutrition. The sporogonium remains attached to the gametophytic plant body throughout its life.

(v) The sporogonium is mainly concerned with the production of asexually formed haploid spores (or meiospores). The spores are produced inside the capsule of sporogonia as a result of meiosis in the spore mother cells.

(vi) The spores are the first cells of gametophytic generation. They germinate to produce the gametophytic plant body either directly or through a juvenile filamentous stage, called protonema.

(10) Alternation of generation : Bryophytes exhibit a distinct and heteromorophic alternation of generations in which two phase gametophytic and sporophytic follow each other in regular sequence. The sporophytic plant body (2 N) of bryophytes is dependent on the gametophyte (N).

(11) Classification : Eichler (1883), Engler (1892), Bower (1935) divided Bryophyta into two classes – Hepaticae and Musci. On the other hand, Campbell (1940), Smith (1955), Takhtajan (1953) divided into three classes namely Hepaticae, Anthocerotae and Musci. Proskauer (1957) changed the names of these classes in accordance with the recommendations of the code, into Hepaticopsida, Anthocerotopsida and Bryopsida.

(12) Salient features of classes

(i) Hepaticopsida : The latin word Hepatica means liver. Thus the members of hepticopsida are popularly known as liverworts. The important characters of hepaticopsida are :

(a) The gametophytic plant body is small, dorsiventral, thallose or leaf axis (foliose).

(b) Chlorophyllous cells contain many chloroplasts and one to several oil bodies.

(c) Pyrenoids are absent.

(d) Rhizoids are unicellular.

(e) Sex organs develop from single superficial cells.

(f) Sporogonium has little or no chlorophyllous tissue and stomata.

(g) The capsule is not linear. It lacks columella and intercalary meristem.

(h) Capsule dehisces by drying of capsule-wall, usually by more than two valves.

(ii) Anthocerotopsida : This class is characterised by the following characters –

(a) Gametophyte is thalloid. Thalli are lobed, dorsiventral, internally homogenous without any differentiation of tissues.

(b) Air chambers and air pores are absent but mucilage cavities may be present.

(c) Rhizoids are only smooth walled.

(d) Scales are absent.

(e) Each cell possesses single (some times more) large chloroplast with central pyrenoid.

(f) Oil bodies are absent.

(g) Antheridia are endogenous in origin, borne singly or in groups inside the closed cavities.

(h) Sporogonium is differentiated into foot, meristematic zone and capsule (the seta is absent).

(i) Capsule has central sterile columella.

(j) The capsule dehisces basipetally by two valves and shows hygroscopic twisting.

(iii) Bryopsida : The members of bryopsida are commonly known as mosses. The class is characterised by the following characters –

(a) Gametophyte is differentiated into two stages – prostrate protonema and erect radial leafy shoot.

(b) Leaf-like appandages are spirally arranged on stem – like axis.

(c) Rhizoids are multicellular with oblique septa.

(d) Sex organs develop from superficial cells.

(e) Sporogonium is differentiated into foot, seta and capsule.

(f) Wall of capsule is several layered with stomata on epidermis.

(g) The capsule has central columella.

(h) Elaters are absent.

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