THE HIERARCHY OF CLASSIFICATION

The main purpose of taxonomic study is appropriate placing of an organism in a systematic frame work of classification. This framework is called taxonomic hierarchy. By this the taxonomic groups are arranged in definite order, from higher to lower categories. A category is called taxon (pl. taxa).

There are seven categories.

Hierarchy of taxonomic categories or taxa

• Species: It is the basic unit of classification. It consists of individuals which breed among themselves and are morphologically similar.

• Genus:A group of closely related species form a genus. They have common ancestry and show similarities in their organization (reproductive organs).

• Family: It is a larger group of related genera. For example, the genus Felis (cat) and genus Panthera (lion) are in one family felidae as both are characterized by retractile claws.

• Order:An order is a group of closely related families. For example, the family felidae (cat, lion) and family canidae (dogs) are together in the order carnivore.

• Class:It is a group of related orders.

• Phylum/Division: It is a group of related classes.

• Kingdom: It is the highest category in biological classification. It is a group of phyla (in animals) and divisions (in plants).

Classification of Organisms:

The major area or basis of classification are

• Mode of nutrition

• Level of organization

• Plant kingdom  was divided in two sub kingdoms by Eichler

Two kingdom system: 

This system of classification is the oldest. It was proposed by Carolus Linnaeus in 1758. 

He divided the living organisms into two kingdoms: Plantae (Plant kingdom) and Animalia (Animal kingdom). 

In plant kingdom, chlorophyll containing green plants, mosses, moulds, fungi (including mushrooms etc.), lichens, bacteria, many colourless and coloured unicellular organisms and multicellular sea weeds were included.

Still other unicellular protozoa and multicellular organisms were included in kingdom Animalia e.g., sponges, jelly fishes, worms, insects, crabs, centipedes, millepedes, spiders, snails, starfishes, sharks, frog, lizards, birds and mammals.

  Each kingdom was split up into phyla or divisions. Each phylum or division was divided into classes. A class is sub divided into orders. An order is 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. Followings are some of the shortcomings of 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 autotrophic mode of nutrition as it contains chlorophyll pigment. However, like animals it is motile bearing flagellum and lacks cell wall. 
• Sponges resemble plants in being fixed, having irregularly branched body. They have 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 organization 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. 
• Viruses lack protoplasm and exhibit characters of living organisms only inside a living cell.
• Lichens are peculiar in being association of an alga and a fungus having neither distinct plant nor animal characters.
• It puts together eukaryotes with prokaryotes.

Three kingdom system:Certain organisms did not strictly fit either plant or animal kingdom.

• Earnst Haeckel (1866), a German zoologist suggested that a third kingdom, Protista be created to include those unicellular microorganisms that are typically neither plants nor animals. He included bacteria, algae, fungi and protozoa under Protista.
• Three kingdoms according to Haeckel are Protista, Plantae and Animalia.
• This solves the problem of assigning suitable kingdom to the organisms which have similarities with both plants and animals. However, certain drawbacks of two kingdom system, persist in this system also. 

Acellular and multicellular organisms are kept together in Protista.

Bacteria and fungi have been grouped with unrelated organisms.

Four kingdom system: It was given by Copeland. Kingdom Monera was also included in this system of classification.

Five kingdom system: It was given by Robert Whittaker. According to him organisms were 

Divided into five kingdoms.

• Kingdom Monera : Unicellular, prokaryotic, microscopic, most ancient, can live in deep  oceans, hot spring, deserts, high salt concentration etc. they include bacteria, filamentous and  photosynthetic blue green algae etc.

• Kingdom Protista: Unicellular, colonial, eukaryotic. They include photosynthetic algae,  decomposers (slime moulds) and protozoa (predators) etc.

• Kingdom Fungl: Unicellular or multicellular eukaryotic organisms, they are heterotrophic,  parasitic or saprotrophic.

• Kingdom Plantae: They are multicellular, eukaryotic, autotrophic (photosynthetic), some are  heterotrophic and parasitic. They include photosynthetic algae, green plants etc.

• Kingdome Animal: Multicellular, eukaryotic, heterotrophic.

SIGNIFICANCE OF FIVE KINGDOM CLASSIFICATION

• This system seems more natural and indicates gradual evolution of early organisms into plants and animals.
• Kingdom Animalia has become more homogenous with the exclusion of protozoa.
• Kingdom plantae has become more coherent after exclusion of bacteria, fungi and some unicellular algal forms. 
• Creation of kingdom Monera from prokaryotes is fully justified. 
• Some organisms like Euglena showing mixotrophic mode of nutrition could be placed either in plant or animal kingdom easily. The creation of kingdom protista including all unicellular eukaryotes, irrespective of the mode of nutrition, has resolved this problem.
• The fungi, included as sub division of division Thallophyta of two kingdom classification is raised to the rank of a kingdom as they differ morphologically and physiologically from plants with whom they are grouped in old two kingdom classification.

KINGDOM: PLANTAE

A. Sub kingdom Cryptogamae : (Cryptos = hidden gamous = marriage) :

• These are also called as lower plants, flowerless or seedless plants.
• Their reproductive organs are hidden till they reproduce.

Division Thallophyta :

• Thallus: Undifferentiated plant body i.e. absence of root, stem & leaves.
• There is no vascular system.
• Reproductive organ are single-celled and there is no embroyo formation after fertilization.
• Dominant gametophyte.

Three classes of thallophytaare :

(A)Class Algae:

Characters:

• These are aquatic or terrestrial, fresh water or marine. Autotrophic, photosynthetic containing various pigments like chlorophyll, carotenoids, xanthophylls etc.
• Unicellular, colonial, filamentous.
• Cll wall of cellulose, e.g. blue green algae (Nostoc) , Green algae (Ulothrix, Spirogyra)

Brown algae, red algae etc.

(B) Class Fungi:

Characters:

• These are heterotrophic.
• They lack chlorophyll but have cell wall of chitin (fungus cellulose)..
• They can be parasitic or saprotrophic
• Their body is filamentous called as mycelium.
• Reserve food material consists of glycogen.  E.g. moulds (Rhizopus) , Yeasts (Saccharomyces)

Mushroom, (Agarics)..

(C) Lichen:

Characters:

• It is symbiotic relationship between algae and fungi.
• Algal part is Phycobiont and fungal part is mycobiont.

• They grow on rocks, tree trunks, grounds etc.e.g. parmellis, Alectoria etc.

Division Bryophyta :

Characters:

• Bryophytes are called terrestrial amphibians (amphibians of plant kingdom) because they require moist soil surface for awimming of their sperm & supply of water to all parts.
• They are the first amongst land plants which occur in damp & shady habital.
• Plant body is of primitive from i.e. differentiated only in stem & leaves.
• Vascular tissue & mechanical tissue are absent in them.
• Male gamete is flagellated.

• Sex organs are jacketed & multicellular.
• Fertilization produces embryo.

• They show heteromorphic type of alternation in generation. E.g. liverworts (Riccia,Marchantia), hornworts (Anthoceros) and mosses (Funaria).

Division Pteridophyta :

Characters:

• They are seedless vascular plants, primitive tracheophytes or vascular cryptogarr
• Plant body is differentiated into true stem, leaves & roots.
• Vascular tissues are present.
• Sperms are flagellated.
• Embryo stage is present.
• Gametophytes are small, exosporic or endosporic.
• Sex organs are jacketed & multicellular, e.g. fems (Dryopteris, Pteris), club moss (Lycopodium),Horsetail (Equisetum).

(B) Sub kingdom: Phanerogamae: (phaneros = visible: gamous = marriage.)

Characters:

• These are higher plants having flowers and seeds both.
• Body differentiated into true stem, leaves and root.
• Vascular system is well developed.
• Sex organ are multicellular.
• Embryo develops from fertilized egg.

It is divided into two divisions:

Division Gymnospermae:

• The word Gymnosperm was first used by Theophrastus (300 BC) in his book Inquiry into Plants to embrace the plants with unprotected seeds. 
• Gymnosperms (Gk. gymnos : naked; sperma : seed) are naked seeded plants with their ovules freely exposed on open megasporophylls. Goebel (1887) has rightly described them as phanerogams without ovary.
• Gymnosperms are the most ancient seed plants that originated during the late Paleozoic era (265 million years ago), but flourished well during the Mesozoic era. The Jurassic period perhaps was the best time for gymnosperms.
• 200 million years ago the earth was dominated by the flora of gymnosperms. Gradually the members of gymnosperms disappeared and replaced by angiosperms with changing time and climatic conditions. 
• Living gymnosperms are spread over 70 genera and 725 species. Bold (1963) has reported only 722 species of present day gymnosperms. Of these, some 50 genera and 500 species belong to conifers. About 16 genera and 53 species of living gymnosperms have been reported from India. 

CHARACTERISTICS OF GYMNOSPERMAE

• The seeds are formed which are naked or exposed.
• Life cycle is heterosporous.
• Better adaptation for land habit.
• Fertilization is siphonogamous i.e., by pollen tube.
• Due to development of cambium, they show secondary growth.

Gymnospermae includes the following two groups : 

• Cycadae e.g., Cycas etc.
• Coniferae e.g., Pinus, Cedrus, Ginkgo etc.
• 

CHARACTERISTICS OF ANGIOSPERMS

• They produce flowers.
• Seeds are covered by fruit wall or pericarp.
• Process of double fertilization is present.
• The ovary ripens into fruit which contains the seeds.
• The phenomenon of pollination, i.e., transfer of pollen-grains from stamen to the stigma of carpel, is a unique mechanism in angiosperms. It is facilitated by wind, water, insects, birds and other animals including man. Presence of nectar, bright colour of petels, odour and edible pollens are contrivances to attract pollinators. 
• Pollens germinate to produce male gametophyte which forms two male gametes. The megaspore germinates to female gametophyte, also called embryo-sac. Embryo-sac is a 7– celled and 8–nucleate structure having three antipodals, one binucleate polar nucleus (or a diploid secondary nucleus) and an egg apparatus. The egg apparatus has a large egg or oosphere (the female  gametes) and two synergids.
• Both the non–motile male gametes, brought about by pollen tubes are functional in the same embryo–sac. One fuses with the egg to produce zygote while the second fuses with the secondary nucleus (central cell)  to form a triploid (3n) primary endosperm cell. Since fertilization takes place twice, this phenomenon is called double fertilization (S.G. Nawaschin, 1898), a unique feature of angiosperm. The zygote develops into embryo. Prior to embryo development, the primary endosperm cell divides and redivides mitotically to form a nutritive tissue, endosperm. The endosperm provides nourishment to growing embryo during its development, during formation of seeds and in many cases afterwards during germination of seeds.

Class: Dicotyledonae:

• Their seeds have 2 cotyledons in the embryo.

• Leaves are dorsiventral and show reticulate venation.
• Tap root is present.  E.g. neem, Peepal,
• Mango, pea, Mustard.

Class:Monocotyledonae:

• Their seeds have one cotyledon in embryo.
• Leaves are isobilateral, with parallel venation.
• Fibrous root system is present, e.g. wheat, Maize, Onion.