The main principle of classification is to arrange all plants of a similar character (one type of character), in a group. Previously the entire living creatures were placed either in Plant or in the Animal Kingdom. But as more and more information regarding the characteristics of various living beings, there develops a controversy regarding these two kingdoms theory. There established a concept of five kingdoms. According to this concept:
(1) The entire Prokaryotic creature is placed in the Kingdom Monera. As a result Virus and Bacteria are separated from the Plant Kingdom.
(2) There were some controversies regarding some uniCellular Eukaryotic organisms (e.g., Euglena), that whether they are plants or animals? Because they simultaneously possess some characters of plant and some characters of the animal. To solve this controversy now all the unicellular prokaryotes are placed in the Kingdom of Protista or Protoctista. All the unicellular Algae and Protozoa are placed in this Kingdom.
(3) Rest of the multicellular eukaryotic organisms are arranged in there different kingdoms depending on their nutrition and food habit. They are,
(A) Plantae – Autotrophic, photosynthetic green creatures.
(B) Fungi -Saprophytic, those which intake (absorb) liquid food from organic bodies and
(C) Animalia – Rest of the heterotropic creatures. As a result of this classification the range of Plant Kingdom. become reduced, as Algae, Fungi, Bacteria and Viruses are separated from the Plant Kingdom.