The biological classification

Aristotle proposed the first scientific biological classification of plants and animals on the basis of simple morphological characters and later Linnaeus classified all living organisms into two kingdoms:

1. Plantae

2. Animalia.

R.H. Whittaker (1969) suggested five kingdom classification

1. Monera,

2. Protista,

3. Fungi,

4. Plantae

5. Animalia.

The main criteria of classification were cell structure, body organisation, mode of nutrition and reproduction, and phylogenetic relationships.

In the five kingdom classification, bacteria are included in Kingdom Monera.

Bacteria are cosmopolitan in distribution. These organisms show the most extensive metabolic diversity.The bacteria may be autotrophic or heterotrophic in their mode of nutrition.

The kingdom Protista includes all single-celled eukaryotes such as Chrysophytes, Dinoflagellates, Euglenoids, Slime-moulds and Protozoans. Protists have defined nucleus and other membrane bound organelles. They reproduce both asexually and sexually.

The members of kingdom fungi show a great diversity in structures and habitat. The most of fungi are saprophytic in their mode of nutrition and they show asexual and sexual reproduction. Phycomycetes, Ascomycetes,Basidiomycetes and Deuteromycetes are the four classes under this kingdom.

The plantae includes all eukaryotic chlorophyll- containing organisms. Algae, bryophytes, pteridophytes, gymnosperms and angiosperms are included in this group. The life cycle of plants exhibit alternation of generations (gametophytic and sporophytic generations).

The heterotrophic eukaryotic, multicellular organisms lacking a cell wall are included in the kingdom Animalia and mode of nutrition of these organisms is holozoic.

They reproduce mostly by the sexual mode. Some acellular organisms like viruses and viroids as well as the lichens are not included in the five kingdom system of classification.