Essay on the Living World!
Our first impression of the living world is apt to be of bewilderment, produced by its extraordinary diversity. Animals, flowering and non-flowering plants are all living organisms, but superficial examination does not suggest that they have a great deal in common.
The living world was separated by primitive man into two kingdoms the Plantae and the Animalia. This distinction between the two kingdoms was based on characters which are sharp and clear-cut; was for long time recognized by biologists as a scientific principle. Within the living world as a whole, the highest taxonomic rank usually recognized is the kingdom.
By tradition, living organisms are classified into two kingdoms, namely, the plant kingdom and the animal kingdom. Within each kingdom, several smaller groups are distinguished. Since discussions about the details of animal kingdom are beyond the scope of this book, more emphasis has been given on the details of plant kingdom.
Plant classification systems in particular, based on form or habit, were introduced by the Greeks and, for approximately 1000 years formed the basis of systems introduced by many subsequent workers and continued up to the time of Carl Linnaeus (1707- 1778).
The publication of ‘Genera Plantarum’ by Linnaeus in 1737 introduced a second era of plant classification, characterized by the introduction of artificial system, more commonly known as sexual system because Linnaeus concentrated on the number of sex organs and presence or absence of flowers. Linnaeus grouped plants into 24 convenient classes.
All flowering plants were subdivided among 23 classes. Under the twenty-fourth class, which he named as Cryptogamia, he included plants, having no flowers and designated them as Cryptogams. Linnaeus’s system was pioneer for it recognized for the first time the flowerless plants as a major taxonomic category.
During subsequent period of time there has been change in approach and a classification system of living world, established basically by Eichler (1886), was widely used for long time. This classification system is outlined below.
Kingdom Plantae:
Division Thallophyta —plant body not differentiated into stem, leaves and roots, i.e., thalloid
Subdivision Algae—plant body with chlorophyll
Subdivision Fungi—plant body without chlorophyll—true fungi, slime molds and bacteria
Division Bryophyta—plant body with some differentiation into root-like structures
Division Pteridophyta—plant body differentiated into stem, leaves and roots
Division Spermatophyta—higher plants bearing flowers and seeds
Kingdom Animalia:
The Thallophyta:
The term Thallophyta (Gk. thallos, a young shoot; phyton, a plant), was first introduced by Endlicher in 1836. It includes a vast and diverse assemblage of plants lowest in development with the least differentiation of vegetative bodies and having simple methods of reproduction.
Hence it forms the lowest division of the plant kingdom comprising of ancient members of the plant population. Some of them are the border line between primitive plants and animals. In general, the thallophytes are aquatic, semiaquatic,. and epigeal or hypogeal in habit.
The vegetative body of a thallophyte is known as a thallus (pi. thalli). Thallus is a relatively simple plant body which is not differentiated into roots, stem and leaves. A thallus may or may not be surrounded by a wall.
The thalli may range from microscopic unicellular forms, free-living or otherwise (Figs. 1A, B & B’, 2A & B; and 3A & B), cells aggregated together forming colony (Fig. 1C & D); filamentous (branched, un-branched or false-branched) with or without transverse partition wall known as septum (pi. septa) (Figs. 1G, 3G, IF, 3D, IE and 8H) to macroscopic forms (Fig. 1H), some of which may be even two hundred feet or more in length.
A filamentous branched thallus in which chlorophyll is absent is known as mycelium (pi. mycelia).
Mycelium may be septate. (Fig. 3C) or aseptate (Fig. 3D) with or without the presence of septa. It is designated as rhizomycelium when mycelium is poorly developed into a rhizoid-like structure (Fig. 3A). Thalli may also have considerable specialization of structure with corresponding specialization of function (Fig. 2C to E).
There are thallophytes whose vegetative bodies may have structures resembling superficially as roots, stem and leaves. These are not true roots, stem and leaves, because they lack vascular tissues.
A thallophyte having multinucleate unicellular (Figs. 2C and 3D) to multicellular (Fig. 45D) thallus is known as a coenocyte and the thallus is coenocytic. The thallophytes that are visible only under the high power magnification of a light microscope are commonly named as microbes (Fig. 4A to E).
Again, the vegetative body of certain thallophytes composed of multinucleate mass of protoplasm without any cell wall is known as a plasmodium (pi. plasmodia), which exhibits creeping movement (Fig. 5); or a pseudoplasmodium, a structure having resemblance with the Plasmodium being developed as a result of aggregation of naked, uninucleate structures (Fig. 4F); or a net Plasmodium characterized by naked, uninucleate spindle-shaped or oval cells which become interconnected by slime filaments along which the cells glide (Fig. 4G).
The thallophytes may be with or without having the normal green pigment, chlorophyll in their thalli. They are autophytes (autotrophic) when their thalli possess chlorophyll and can prepare their own food from carbon dioxide and water with the help, of chlorophyll.
Whereas, those which lack chlorophyll in their thalli arid live on food substances synthesized by other organisms are the heterophytes (heterotrophic). Some of the heterophytes thrive as parasites on living organic matters; others receive their nutrition from dead and decaying organic matters, they are the saprobes (saprophytes).
Again, there are other thallophytes which live in an intimate association with other living organisms deriving mutual benefit. They are known as symbionts and the phenomenon is symbiosis. But the thallophytes may also be endophytes or epiphytes when they live inside or outside plant parts respectively without receiving any nutrition from them. The endophytes are often designated as space parasites.
There are certain other thallophytes whose thalli possess a special kind of chlorophyll which is different from the chlorophyll that is present in green plants. With the help of this special kind of chlorophyll they can prepare their own food. Besides these, there are thallophytes which manufacture their food by the process of chemosynthesis.
The nature of reserve food is extremely variable in the thallophytes, which may be carbohydrate of various forms, fats, oils and similar other substances.
The methods of reproduction in thallophytes may be: vegetative, asexual and sexual. In vegetative reproduction the thallus breaks up into many pieces and each piece subsequently develops into a new individual. The most common method of reproduction is by the development of minute one to many-celled propagative units, the spores which on germination give rise to new plants.
The spores may be developed by processes other than sexual—asexual spores or as a result of sexual process followed by meiosis—sexual spores. They are usually developed in a structure known as a sporangium (pl. sporangia) which is normally a single-celled structure. Asexual reproduction takes place mainly in three ways: by fission, by budding, and by the development of spores.
Fission usually is, ‘splitting easily’ of the parent thallus into two daughter individuals which develop into new thalli. During budding the thallus produces one or more tiny outgrowths, the buds (Fig. 3B).
These buds increase in size, each one receives a daughter nucleus from the parent thallus. All of them are ultimately cut off from the parent individual by a process of constriction and then carry on separate existence as new independent thalli.
In cases where the asexual reproduction is by the development of spores, the spores may be flagellate or non-flagellate, that is, may or may not bear hair-like structures known as flagella (sing, flagellum). The flagellate spores are the zoospores.
Sexual reproduction takes place by the union of gametes. The gametes are minute uninucleate sex cells which by themselves cannot normally produce new individuals. Hence they fuse in pair and the fusion product— zygote undergoes various changes leading to the development of new individual without the development of embryo.
The gametes may or may not be developed in a gamete-producing structure known as a gametangium (pi. gametangia).
The gametangium is simple and is usually unicellular without being surrounded by any protective tissue. There are few exceptional cases in which the gametangium is multicellular, but without any surrounding layer of protective tissue.
Many of the thallophytes are extremely beneficial to human beings and plants. They are of great use to human society either as food, as medicine or as manures and in similar other ways.
Large number of thallophytes are used by fish as food. Again, many of the thallophytes are responsible for causing serious diseases of plants and animals including human beings. Thallophytes with beautiful markings on their cell wall are of great interest to the artists for their attractive designs (Fig. 2A and B).
The common thallophytes are: the algae, the fungi, the slime molds, and the bacteria. Unger (1838) included algae, lichens, and fungi in the Thallophyta.
Validity of the Thallophyta:
With the advancement of human knowledge about plants—the lower plants in particular, and their natural relationships, it was realized that Thallophyta, a designation for the lower plants is an artificial collection of unrelated plants having extremely diversified characteristics, have attained the same level of development.
To accept the Thallophyta as a natural division of the plant kingdom implies acceptance of the view that both the Algae and the Fungi are each a more or less homogeneous phylogenetic series.
It has become increasingly clear during the past quarter century that the morphology and the physiology of the individual cells are the fundamental bases upon which the Algae must be classified. This evidence shows that there are several series among the Algae, each of which has cells with certain distinctive physiological and morphological traits.
Chief among the morphological characteristics is the structure of the motile cells, and for most of the groups among the Algae there is a striking constancy in its organization, especially with respect to the number, arrangement, and relative length of the flagella.
The physiological distinction is in the plastids and a constancy in the chemical nature of the food reserves accumulating through photosynthetic activity. All these suggest very strongly that the various major groups of Algae have little in common with one another and as such the Subdivision Algae is an artificial group. It is actually made up of a number of groups which are not closely related.
Acceptance of the view that the various groups of Algae are more or less independent of one another means that both the Thallophyta and its Subdivision Algae must be abandoned as natural units, Algae being a correspondingly higher rank.
Abandonment of the Algae as a Subdivision of the plant kingdom does not mean that the word alga must be abandoned. It is still of great service as a descriptive term for designating simple plants with an autotrophic mode of nutrition.
The taxonomic deposition of the Fungi, the other Subdivision of the Thallophyta depends upon the question of their origin. This is highly controversial and opinion is divided as to whether they had a monophyletic or polyphyletic origin.
If they arose from the protozoa, they should be put in one or more divisions co-ordinate in rank with the various algal divisions; if they arose from Algae, they should be placed as classes of one or more of the algal divisions.
Adoption by many botanists of the conclusion that Fungi did not evolve from the Algae invalidated the use of the term Thallophyta as a Division and fostered the more recent system of dividing the Algae into several Divisions and the Fungi into three classes; and separating slime molds and bacteria as independent taxa, previously included under Fungi, ranking as Fungi. This aspect will be elaborated further later on.
Therefore, the Division Thallophyta is now regarded as a highly unnatural assemblage of unrelated forms of plants.
Some of the contrasting features of Algae and Fungi are incorporated below:
i. The thallus of Algae is characterized by the presence of the pigment chlorophyll which, in some species, is masked by other pigment or pigments producing various colourations.
Chlorophyll is completely lacking in Fungi.
ii. Owing to the presence of chlorophyll the Algae are able to synthesize their own food from simple inorganic materials—autotrophic.
Most Fungi are incapable of manufacturing their own food and are dependent on food prepared from external sources—heterotrophic. They are usually either parasites on saprophytes. But there are Fungi which can synthesize protein and other organic substances when grown under cultural condition.
iii. Diffused or direct light is essential for the growth and nutrition of Algae. The Fungi can grow both under light and darkness.
iv. The reserve food in most Algae is a carbohydrate—starch, which turns blue with iodine solution.
The chief storage product in Fungi is a carbohydrate—glycogen, which may be distinguished from starch by a brown colouration produced by iodine solution.
v. The thallus of Algae ranges from unicellular to multicellular parenchymatous structure. Whereas, in Fungi, the thallus is unicellular-non filamentous or is made up of fine thread-like structure—hypha (pi. hyphae) interwoven in a mass — mycelium (pi. mycelia) which may be septate or aseptate.
vi. In Algae, the cell walls are composed of cellulose.
The cell walls of Fungi are not of cellulose usually, but of chitin, a complex nitrogenous compound, but simpler than animal chitin.
vii. In Algae, the rise in complexity of the vegetative body from a very simple form is correlated with the progressive evolution of sexuality from simple to complex condition.
Reverse is the condition in Fungi, where there is gradual degeneration of sexuality with the corresponding gradual progress in complexity of the vegetative body.
i. Origin of Life:
That the conditions under which life might be supposed to have arisen must have been quite different from those prevailing at the present time, that light need not be presumed to have been necessary, that amino acids can be produced from inorganic materials in an environment in which the attempt is made to approximate such conditions, that the earliest living material need not have possessed capacity for photosynthesis—these are the major viewpoints held by Oparin (1957), Cloud (1965), Ponnamperuma and Mack (1965), and many others.
These viewpoints have brought about a change in our traditional concepts of the origin of life. The increasing emphasis on the biochemical approach to the understanding of living organisms has quite naturally had an impact on theories of relationship.
The discovery of the remarkable similarity of the deoxyribonucleic acid molecule in widely different organisms is perhaps the most striking result of these studies. If this fundamental material is as similar in all organisms as present evidence indicates, then, it has been argued, all living things must be closely related.
This is often extended to imply that all organisms have been derived from a single fortuitous chemical event under suitable environmental conditions, such as may have existed on the earth at a remote time. Such happenings must inevitably have assumed by gradual stages, and certainly at a subcellular level, the characteristics which we now associate with life. The details of such a process are still speculative.
ii. Basic Similarities among Plants and Animals:
If one starts to examine the living world in terms of its chemical and microscopic organization, the initial impression of diversity among plants and animals gives way to a profound realization of its fundamental unity.
The great superficial diversity becomes understandable as the consequence of the operation of the evolutionary process on a kind of matter, the living cell, which probably had a single historical origin and which still preserves many common features.
What are the common features of living organisms? First, all of them share a common chemical composition. The most fundamental feature of this common chemical composition is the invariable presence of three types of complex organic macromolecules protein, deoxyribonucleic acid and ribonucleic acid. In addition, they all perform certain common chemical activities, known collectively as metabolism or metabolic activities.
There are, naturally, many differences in metabolic detail among the different kinds of organisms. However, all organisms are obliged to synthesize the universal constituents of living matter from external chemical building blocks and to generate the energy necessary for such synthetic activities by the formation of a characteristic kind of energy-rich bond at the expense of external energy sources.
With respect to these fundamental features of metabolism, resemblances, between the different kinds of organisms are much more marked than are the differences. Finally, all organisms share a common physical structure, being organized into microscopic subunits known as cells. As a consequence of their cellular organization, growth results in cell division, with an increase in the total number of cells.
All the properties described above are common to plants, animals, and microorganisms, despite their gross differences of size, shape, and internal structure. There is, however, one group of biological objects, the viruses, which does not conform to the list of common properties which have just been outlined.
This is why, according to some, viruses are not microorganisms but are biological objects with a different set of fundamental properties. Others believe that viruses are microorganisms which have lost some of the principal properties of organisms. Again, others are of opinion that viruses are organisms that share properties of both living and non-living objects and represent the connecting link between them.
iii. Present-Day Problem:
The distinction between the two kingdoms—plant and animal is, however, sharp and clear-cut as long as one considers only higher plants and animals. But it fails to accommodate lower groups of plants and animals which share characters of the members of both plant and animal kingdoms.
The intensive exploration of the microbial world in course of time has established the properties of the various microbial groups. It is now evident that some of them cannot be at all fitted into either of the traditional kingdoms—plant and animal.
Such groups appear to be intermediate forms, in which could be found all imaginable combinations of the classical differential properties that distinguish plants from animals.
Today, we can understand the significance of this fact in the light of general evolutionary ideas; the major microbial groups branched off prior to the emergence of the two great biological lines which eventually led to the plants and animals.
The point is that plants and animals, clearly so recognizable, were not in existence right from the beginning. Rather, some of the early organisms evolved in plant-like or animal-like directions slowly and gradually, and a definite, finalized ‘plant’ status or ‘animal’ status was attained only relatively late in evolutionary history.
If we go even further back in time, the very first organisms on earth probably possessed neither plant-like nor animal-like features at all. Therefore, a division of the living world merely into plant and animal kingdoms is too simple.
It does not take into account this gradual evolutionary development, and it allows no place for primitive organisms which still are neither plant nor animal or share the characters of both plants and animals.
iv. Present-Day Approach:
With increase in knowledge about the living world, the traditional division of living world into the plant and animal kingdoms is thus no longer a reasonable approach to the living world. During the last century various solutions, involving the establishment of one or more new kingdoms have been offered.
In 1866, one of Darwin’s disciples, the German Zoologist Haeckel, proposed the establishment of a third kingdom, the Protista, to include both photosynthetic and non-photosynthetic organisms, some clearly ‘plant-like’, and some sharing many properties common to both kingdoms—plant and animal.
What distinguishes all members of the Protista from higher plants and animals is their relatively simple biological organization.
The protists are a large and very heterogeneous group of organisms, distinguished by their preservation, throughout the course of biological evolution, of a relatively simple and undifferentiated structure. Many protists are unicellular and -even those that are multicellular do not show the differentiation into separate tissue regions characteristic of plants and animals.
Neither Haeckel’s proposal, as first put forward or as later modified (1878, 1894), nor those of his contemporaries were generally adopted. In recent years renewed interest has been shown in this problem. It is now widely accepted that the two-kingdom system allows no place for organisms of simple structure which are neither ‘plant’ nor ‘animal’, or which possess characters of both.
The living world can thus be subdivided most logically into three kingdoms: Protista, Plants, and Animals.
The component groups of the three kingdoms of living organisms are:
The perfection of the electron microscope as an instrument for studying the fine details of cellular structure has revolutionized our knowledge about the internal organization of cells. One product of this revolution has been the demonstration that there are two quite different kinds of cells among living organisms.
The more highly evolved type, the eucaryotic cell, is the unit of structure in all plants and animals and in several large groups of protists fungi, slime molds, protozoa, and most algae.
The eucaryotic cell shows innumerable specialization of form and of function having a cell wall composed of a variety of carbohydrate materials. Its nucleus has a well-defined nuclear membrane and the hereditary information of the eucaryotic cell, coded in the DNA (deoxyribonucleic acid) of the nucleus, is carried on a number of different structural subunits, the chromosomes.
In all eucaryotic organisms gametic fusion is followed by nuclear fusion.
A eucaryotic organism whose life cycle contains a sexual stage may exist almost exclusively in the diploid state, as do all higher animals and flowering plants. With a few exception, all eucaryotic cells contain mitochondria, and chloroplasts occur in eucaryotic photosynthetic cells. Nuclear division is both mitotic and meiotic.
A much simpler kind of cell, the procaryotic cell, is the unit of structure in all bacteria and in blue-green algae.
The procaryotic cell is small possessing a cell wall which contains a specific mucopeptide as its strengthening component. No mitochondria or chloroplasts have been detected in a procaryotic cell. Its nucleus is never separated from the cytoplasm by a nuclear membrane. The nuclear region appears to be closely filled with a system of very fine fibrils which consist (at least in large part) of DNA.
There is never any sign of organization of this material into individual chromosomes. Nuclear division does not take place by mitosis as in eucaryotic cell. Protists which are constructed of eucaryotic cells are higher protists and those which are constructed of procaryotic cells are lower protists.
The living world, based on the nature of internal organization of cells, can thus be subdivided into Kingdoms, Subkingdoms and Divisions in the following manner:
Again one school of thought suggests that bacteria and blue-green algae should be termed the Monera (Dougherty and Allen, 1960, Whittaker, 1969); while another school terms them the Procaryota. This latter school includes all other plants in one major subdivision, the Eucaryota. Within this subdivision the simplest photosynthetic members are those plants known as the Algae.
Those who recognize the Monera, they group the Algae, Protozoa, and Fungi as the Ptotista and sometime segregate the fungi into a separate subdivision the Mycota. The terrestrial plants and aquatic higher plants with more complex organization are classed as the Metaphyta.
Hence, an alternative classification system of the living world has been suggested to reflect our present knowledge of evolution based on the biochemical and ultra-structural studies of the living organisms.
Such a classification system recognizes four basic taxonomic Categories of organisms. Each of these four categories has a taxonomic rank roughly equivalent to a kingdom, although it may not be desirable to use this rank so long as it is technically still reserved for ‘plants’ and ‘animals’. The four basic categories are the Monera, the Protista, the Metaphyta, and Metazoa.
The Monera and Protista go back farthest in evolutionary history. Both groups are believed to be descended, independently, from the very first cells on earth. Moneia include all those organisms in which the cells do not possess true nuclei. Bacteria and blue-green algae are the modern representatives of this group. Protista possess true nuclei.
Ancient protists were unicellular, and primitive-modern ones still are. Other modern protists are multicellular. They possess the ability to acquire food by both plant-like and animal-like methods. The major groups among modern protists are the Algae, the Fungi, the Slime molds, and the Protozoa.
The Metaphyta and Metazoa are both believed to have evolved from ancient protists. Certain ancient, green alga-like stocks were probably the ancestors of the Metaphyta, and other ancient algae or protozoa, or both may have been ancestors of the Metazoa. The derivation is far less certain in the latter case than in the former.
The Metaphyta and Metazoa all are exclusively multicellular, their structural complexity reached the level of complicated organs and combinations of organs. Moreover, both Metaphyta and Metazoa have life cycles which include more or less distinct embryos.
The Metaphyta are almost exclusively photosynthetic and are unmistakably plants’. They include the Bryophyta and Tracheophyta (Vascular plants). The Metazoa are exclusively non-photosynthetic and they are unmistakably ‘animals’.
This classification system is schematically presented below:
In this four-part classification, every living creature has a proper place. The Monera and Protista include organisms which are plant-like, animal-like, or both, as well as some organisms traditionally regarded as ‘true’ plants (e.g., advanced algae) and ‘true’ animals (e.g., most protozoa). The Metaphyta and Metazoa include the remainder of the ‘true’ plants and ‘true’ animals.
A simplified classification system has been outlined in the following manner keeping the viewpoints of both traditional and modern approaches to the living world.
Kingdom Plantae:
Division Cyanophyta or Blue-green Algae:
Include unicellular to filamentous organisms; procaryotic cell; true nucleus and chromatophores lacking; pigmentation includes chlorophyll a, β-carotene, various xanthophylls, and two c-phycobilins; storage food cyanophycean starch, proteins; flagellation absent; reproduction by accidental breaking and by production of specialized cells; sexual system most inefficient.
Division Chlorophyta or Green Algae:
Include both motile and non-motile organisms of diverse organization; eucaryotic cell; mainly chlorophylls a and b, a- and -carotenes present; cell walls of cellulose; storage food starch; reproduction vegetative, sporulative and gametic.
Division Euglenophyta:
One to three anterior tinsel-type flagella; eucaryotic cell; storage food paramylum and lipids.
Division Xanthophyta or Yellow-green Algae:
Eucaryotic cell; chlorophylls a and e present, carotinoids predominant; do not form starch; storage food oils; flagella unequal length.
Division Bacillariophyta or Diatoms:
Largely unicellular; eucaryotic cell; chlorophylls a and c, fucoxanthin and special pigment diatomin present; walls conspicuously in two halves with silica deposition; storage food volutin, sexuality common; life cycles diplontic.
Division Pyrrophyta or Fire Algae:
Usually biflagellate; eucaryotic cell; chlorophylls a and c present; storage food lipids and carbohydrates including starch; cell walls when present of cellulose; sexuality rarely known.
Division Phaeophyta or Brown Algae:
All sessile and multicellular; eucaryotic cell; chlorophylls a and c, carotene, xanthophyll and special pigment fucoxanthin present; storage food laminarin and mannitol; cell walls of cellulose and algin; exposed but multicellular reproductive structures; biflagellate reproductive cells.
Division Rhodophyta or Red Algae:
Eucaryotic cell; pigments chlorophylls a and d, α- and β-carotenes, r-phycoerythrin, r-phycocyanin; storage food flori- dean starch; cell walls of cellulose and pectin; flagella entirely absent.
Division Eumycota or Fungi:
Body unicellular uninucleate to filamentous branched mycelium; eucaryotic cell; cell walls of cellulose, chitin, or both; storage food glycogen; reproduction vegetative, sporulative, and gametic.
Division Myxomycota or Slime Molds:
Diplohaplontic life cycles, with haploid phase often as independent solitary cells, and diploid phase coenocytic Plasmodium to uninucleate aggregated amoeboid mass; haploid phase flagellate cells; eucaryotic cell; reproduction sexual.
Division Schizophyta or Bacteria:
Chlorophylls where present unique; pro- carytic cell; flagella where present of unique structure; cells walls of polysaccharides, protein and lipid; storage food glycogen; reproduction usually by fission, sexual in some.
Division Bryophyta or Mosses and Liverworts:
Vascular tissues absent; gametophyte dominant, sporophyte attached to gametophyte; homosporous.
Division Tracheophyta or Vascular Plants:
Vascular tissues present in sporophyte; gametophyte independent or attached to sporophyte; homo- and hetero-sporous.
Kingdom Animalia:
Even now some systematists prefer to place fungi and slime molds in a Division designating as Mycota or Fungi and subdivide Mycota as follows:
Division Mycota:
These are the fungi having somatic structure ranging from a microscopic unicell to an extensive mycelium. Nuclei are eucaryotic. Reproduction is both asexual and sexual.
Subdivision Myxomycotina:
These are slime molds. The somatic structure is a free-living Plasmodium.
Subdivision Eumycotina:
These are the true fungi. The, somatic structure ranges from unicellular uninucleate to an extensive mycelium.
Glass Chytridiomycetes:
Fungi possessing variety of thalli whose motile cells bear a single posterior whiplash flagellum.
Glass Hyphochytridiomycetes:
A group of aquatic fungi whose motile cells possess a single anterior tinsel flagellum.
Glass Oomycetes:
Fungi possessing a usually well-developed coenocytic mycelium whose motile cells bear two flagella, one whiplash, the other tinsel. Product of sexual reproduction is an oospore.
Class Plasmodiophoromycetes:
Parasitic fungi having plasmodial thalli, motile cells possess two anterior whiplash flagella of unequal size.
Class Zygomycetes:
Saprobic or parasitic fungi with a well-developed coenocytic mycelium, no motile cells are produced. Product of sexual reproduction is a zygospore.
Class Trichomycetes:
These are fungi with a simple or filamentous branched coenocytic mycelium attached to the digestive tract or the external cuticle of living arthropods. Asexual reproduction is by a variety of spores. Sexual reproduction is as in the Zygomycetes.
Class Ascomycetes:
This group includes fungi which form sexual spores by karyogamy and meiosis in a sac-like structure called ascus.
Class Basidiomycetes:
These are fungi which form sexual spores by karyogamy and meiosis on the surface of a special structure known as basidium
Form-Class Deuteromycetes (Fungi Imperfecti):
This is a group to fungi in which sexual stages have not been discovered.
Living organisms do not classify themselves. They are classified by systematists for convenience of reference. An ideal scheme of classification should reflect natural relationships among living organisms. But in considering natural relationships systematists differ in their approach by not attaching same weight to the criteria available resulting different schemes of classification of Jiving organisms.
Besides this, biological objects, like the viruses, which, according to some, do not conform to the common properties of living organisms are difficult to categorize. So also the Rickettsiales which bear a morphologic resemblance to bacteria but are biologically related to viruses. The situation becomes even more difficult with the discovery of very tiny organisms which have similarities both to viruses and bacteria.
Though they have been grouped, by some, as Chlamydiales and Mycoplasmatales still they have to get a proper position in the scheme of classification of living organisms based on natural relationship.
The above facts indicate that it is extremely difficult to draw up an outline of classification of living organisms which will be acceptable to all systematists.
Nomenclature refers to the correct naming of living organisms. The naming of living organisms started on the basis of their utility. Before the middle of the eighteenth century the names of plants commonly were polynomials, that is, being composed of several words in a series, constituting more or less the description of a plant. In most cases it was rather arbitrary.
The polynomial system of naming often became inconvenient to handle and it was felt that a better system of naming should be introduced.
A remarkable change came in the field of nomenclature when Carl Linnaeus in 1753 in his Species Plautarum put to practice the binomial system of nomenclature which was first introduced by G. Bauhin in 1623. The binomial system of nomenclature is naming of any plant or animal by two words usually Greek, Latin or of Latin form.
The first word being- that of the genus to which the organism belongs generic name—and the second of the two words making up the name is called the specific epithet. The two words in combination form a binomial, and the binomial is a binary epithet.
A particular specific epithet may be used only once within a given genus, but it may be used repeatedly in different genera. The first word is a noun or substantive designating the genus to which the organism belongs. The specific epithet is usually a Latin or Latinized adjective, which takes its grammatical gender from that of the genus, but it is sometimes a noun in apposition to the generic name.
When the present system of naming plants was adopted, in the middle of the eighteenth century, Latin was the written language of scholars. Although Latin has diminished in importance, its use in scientific names is retained. Since it is not a spoken language it does not change, and it is intelligible to scientific workers of all nationalities.
An important asset of the scientific name is its relative stability. Once a plant is named, the name remains, or if it is changed, the change is made according to established botanical rules. The scientific name is the name wherever the plant is found.
But common names often vary with each locality, country, or other geographic subdivision. They differ, of course, from one language to another. It is customary to capitalize the initial letter to the generic name and not to capitalize specific epithets, although the rules of botanical nomenclature permit the optional capitalization of certain specific epithets.
The binomials are, in general, descriptions of certain characters of the organisms to be named and are usually derived from internationally known language, Greek or Latin. When written they always should be underlined and printed in italics.
The name of a plant is thus given on the basis of characters of a specimen, the type specimen. Again the name (short names usually appear in full, e.g., Thom; Peck) or the abbreviated name (e.g., Fr. for Fries; L or Linn, for Linnaeus) of author who first described the name of the organism usually follows the binomial and is known as author citation.
If the binomial is changed in part or whole in subsequent time, the name of the author who first described the particular organism should be put in parenthesis after which will follow the name of the author who consequently changed the binomial.
The author citation is very helpful to find out the original and subsequent descriptions of the plant/ when found necessary. To make the situation even more clear, sometimes the year in which the organism was described in written after the author citation following the binomial.
With the gradual increase in human interest in plants there started to arise problems of nomenclature. For the settlement of these problems, first organized effort was made at the First International Botanical Congress, meeting in Paris in August 1867.
There, attempts were made to standardize and to formulate certain principles of nomenclatural practices, the laws of Botanical nomenclature which in course of time with additions and alterations took the shape of International Rules of Botanical Nomenclature.
The International Rules of Botanical Nomenclature has Principles, Rules and Recommendations which are printed as a series of Articles aiming at; fixity of plant names keeping out the use of forms and names that might lead to error or ambiguity and avoidance of useless creation of names.
A legitimate or valid name is the one that is published in accordance with the International Rules of Botanical Nomenclature. In general, the earliest name legitimately applied to a particular plant is the correct name. This is the principle of priority in nomenclature.
The post Essay on the Living World appeared first on Biology Discussion.