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| Two Kingdom Classification |
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| The natural system of classification has been subjected to periodic reviewing as a result of the continuous input of information and new scientific tools being available on various aspects of living organisms. Various schemes of classification are now available, each having certain merits and demerits. One of the earliest scheme of natural system classification is the two-kingdom classification proposed by Carolus Linnaeus in 1758. |
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| Carolus Linnaeus placed all the living organisms in two major kingdoms Kingdom Plantae for plants and Kingdom Animalia for animals. This classification was quite reasonable at that period of time since plants and animals could be very clearly distinguished. Plants were stationary, fixed to the soil, absorbed water for growth and could prepare their own food. Animals, on the other hand were capable of movement, and were feeding on plants and other animals for their growth and survival. Apart from this a few other significant differences, particularly at the level of cells, were established between plants and animals by scientists of the later period. |
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| The original plant kingdom proposed by Linnaeus and subsequent taxonomists of that period included the bacteria, fungi, algae, liverworts, mosses, ferns, conifers and flowering plants. Plants in general are considered to exhibit the following distinguishing characters. |
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 Plants exhibit a great deal of variation in their form and size. Plant body is usually asymmetrical. However, in higher plants structures like leaves and flowers have a definite form and shape. |
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 Plants are rooted mostly and as such are incapable of locomotion. However, certain localized movements may occur in a plant body. |
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 Plants exhibit unlimited growth, almost throughout their life span. |
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 Absence of locomotion. |
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 Plants exhibit largely autotrophic nutrition or saprotrophic nutrition. Particulate matter cannot be absorbed and only materials present in a solution state can be absorbed. |
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 Plant body is composed of cells which have a distinct cell wall, a vacuole filled with sap and plastids of different kinds. The cells do not have centrioles and lysosomes. |
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 Plant cells can synthesize all the amino acids, coenzymes and vitamins necessary for its functions. |
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 Plants have reserve food as starch. |
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| Classification of Kingdom Plantae |
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| The Plant kingdom was initially classified as follows |
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| fig. 7.6 - Classification of Kingdom Plantae |
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| The original Animal Kingdom proposed by Linnaeus included the protozoans, sponges, jelly fishes, worms, crabs, insects, spiders, snails, starfishes, sharks, bony fishes, frogs, lizards, birds and mammals. In general, animals exhibit the following distinguishing characters. |
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 The animal body generally exhibits a definite symmetry, form and shape. |
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 Animals have the capacity to move from place to place in search of their necessities. |
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 Growth in animals is determined and occurs proportionately in all parts of the body. |
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 Animals are generally heterotrophic, obtaining their food from plants and other animals. |
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 Animals have the property of irritability - the capacity to respond to a stimulus. |
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 The cells, which form an animal's body do not have a cell wall. Plastids and vacuoles are generally absent and centrioles & lysosomes are present. |
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 Animal cells cannot synthesize all the necessary amino acids, vitamins and coenzymes and as such will have to obtain them from external sources. |
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 Reserve food is glycogen. |
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| Classification of Kingdom Animalia |
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| The animal kingdom was initially classified as follows: |
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| fig. 7.8 - Classification of Kingdom Animalia |
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| The two-kingdom classification received considerable recognition from biologists and was in use for quite a long period of time. However, as more and more information started emerging on the various groups of plants and animals, this system of classification was found to be inadequate. Studies made on the lower forms of life in particular, revealed an enormous variation of characteristics in those forms, not in agreement with the characteristics of the group in which they were placed. |
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| The two kingdom classification has certain demerits such as: |
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 There is no clear-cut distinction in the lower forms of life, into plants and animals. |
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| Euglena for example is a unicellular organism having certain features of animals and certain features of plants. It has a flagellum which is used for locomotion and food capturing. However, it has chloroplasts like plant cells. |
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| Chlamydomonos is a unicellular alga. It is purely autotrophic but has a locomotor structures called flagella, like protozoans. |
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 Slime moulds are a type of fungi. They do not have a cell wall in the vegetative phase and ingest particulate matter like animals. However, they develop a cell wall in the reproductive phase which is similar to other fungi. |
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| Thus, they resemble animals in one phase and plants in the other. |
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 Sponges are sessile, irregular animals which look like plants. |
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 There are organisms like diatoms (plants) and protozoans (animals) which share a number of common characteristics and organization. |
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 Fungi differ from plants in several features. They do not have chlorophyll and they are saprotrophic in nature. |
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 It is presumed that when life first evolved on the earth it was neither in the plant form or in the animal form. Definite plant or animal status is said to have been attained much later in the evolutionary history. |
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 Lichens are not given an appropriate place. It represents an example of symbiotic relationship between algae and fungi. |
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| In view of all these considerations, the two kingdom classification had to be revised. One of the earliest attempt in this direction was that of Earnst Haeckel in 1868. He proposed a three-kingdom classification in which he suggested the creation of a new kingdom Protista to include all the unicellular organisms, both from plants and animals. |
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| The advent of electron microscope opened up newer situations. The discovery of prokaryotic and eukaryotic conditions in the cell made the scientists to list the organisms that showed these kinds of cells. The blue-green algae among algae and the bacteria which were earlier placed in the group Thallophyta, were found to contain prokaryotic cells. All other algae, fungi, protozoans and multicellular animals were found to be composed of eukaryotic cells. Copeland suggested in 1966, the erection of a new kingdom called Monera to include all the prokaryotic organisms. This led to the practice of a four-kingdom classification - Kingdoms Monera, Protista, Metaphyta (all advanced eukaryotic plants) and Metazoa (all advanced eukaryotic animals). |
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| fig. 7.8 - Evolution of Classification |
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| In the year 1969, Robert H. Whittaker proposed a five kingdom classification in which he suggested the creation of a new kingdom called Mycota to include exclusively the fungi, since they differ from any other group of plants in their mode of nutrition and reproduction. This five kingdom classification is in agreement with the phylogenetic relationships among organisms. Hence, it has found wide acceptance among taxonomists all over the world. |
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