Biological classification class 11 ncert
♣ Since the dawn of civilisation, there have been many attempts to classify living organisms. Aristotle was the earliest to attempt a more scientific basis of classification. He used simple morphological characters to classify plants into trees, shrubs and herbs. He also divided animals into two groups, those which had red blood and those that did not.
♣ The art of identifying distinctions among organisms and placing them into groups that reflect their most significant features and the relationship is called biological classification. It is a method by which biologists group and categorise species of organism. The purpose of biological classification is to organise the vast number of known plants and animals into categories that could be named, remembered and studied. As a result, the study of one organism of a group gives us the idea about the rest of the members of that particular group.
NUMBER OF KINGDOMS
♣ Various schemes dividing the organisms into two, three, four, five and six kingdoms have been proposed from time to time.
TWO-KINGDOM SYSTEM OF CLASSIFICATION
♣ This system of classification is the oldest. It was proposed by Carolus Linnaeus in 1758. He divided the living organisms into two kingdoms: Plantae and Animalia. Each kingdom was split up into phyla or divisions. Each phylum or division was divided into classes. A class was subdivided into orders. An order was broken up into still smaller groups, the families. Each family comprised of many genera and in each genus were included one or numerous species.
Drawbacks of Two-kingdom Classification
♣ The two-kingdom system of classification worked well for a long time. Now, however, it seems inadequate and unsatisfactory in view of the new information that has come to light about the organisms, particularly the lower forms. Following are some of the shortcomings of the two-kingdom system:
♦ Higher organisms can easily be differentiated as plants and animals, but lower organisms can’t be easily placed either in plant or animal kingdom because these possess dual characters of both kingdoms, For example, Euglena resembles plants in having an autotrophic mode of nutrition as it contains chlorophyll pigment. However, like animals, it is motile bearing flagellum and lacks a cell wall.Sponges resemble plants in being fixed, having an irregularly branched body. They have the holozoic mode of nutrition and excrete nitrogenous waste materials like animals.
♦ The two-kingdom system takes unicellular and multicellular organisms together. Even unicellular organisms like bacteria were considered as plants.
♦ Unicellular plants (diatoms, dinoflagellates) and animals (protozoans) resemble each other in level of organisation and reproduction by fission but placed in two separate kingdoms.
♦ Fungi are included in kingdom Plantae in spite of the fact they lack chlorophyll, cellulosic cell wall and are either saprophyte or parasite, unlike typical plants.
♦ Some of the organisms like viruses and lichens can’t be placed in either of these two kingdoms because of peculiar characteristics.
♦ It puts together eukaryotes with prokaryotes.
♦This system does not indicate the gradual evolution of early organisms into plants and animals.