Two Kingdom Classification of Organisms

In this article we will discuss about the Two Kingdom Classification of Organisms (Before 1969).

In the past all the organisms had been divided into two groups, plants and animals, mainly on the basis of presence or absence of cell wall. This was also done by Linnaeus (the father of taxonomy). He founded two kingdoms— Kingdom Plantae and Kingdom Animalia. The members of kingdom plantae are plants.

They are distinguished by:

(i) Presence of cell wall,

(ii) Occurrence of inorganic crystals in the cells,

(iii) Presence of central vacuole in the cell,

(iv) Absorption of inorganic nutrients from outside,

(v) Well defined growing points with unlimited growth,

(vi) Absence of excretory organs, sense organs and nervous system,

(vii) Ability to manufacture food due to presence of chlorophyll— holophytic or autotrophic nutrition,

(viii) Reserve food as starch,

(ix) Occurrence of branches and less definite shape,

(x) Absence of locomotion,

(xi) Absence of muscular tissue,

(xii) Presence of external organs and

(xiii) Slow response to external stimuli.

Members of kingdom animalia possess the opposite characters as:

(i) Absence of cell wall,

(ii) Inorganic crystals are not present in their cells,

(iii) Absence of central vacuole in the cell,

(iv) Ingestive (holozoic) type of nutrition,

(v) Well defined growing points absent, growth limited,

(vi) Presence of excretory organs, sense organs and nervous system,

(vii) Inability to manufacture food due to absence of chlorophyll,

(viii) Reserve food as glycogen,

(ix) Absence of branches and having definite shape,

(x) Presence of locomotion,

(xi) Mus­cular tissue present,

(xii) Organs internal and

(xiii) Quick response to external stimuli.

Several objections have been raised against this two kingdom classification.

They are:

(i) First formed organisms were neither plants nor animals.

(ii) Fungi differ in the structure, physiology and reproductive details from plants.

(iii) At the lower level of organisation there are several instances where the distinction of plant and animal disappears. For example, Euglena has both holophytic and saprobic nutri­tion (mixotrophic nutrition).

Its relatives have both holophytic and holozoic nutrition. Sponges are branched, fixed and irregular in outline just like plants. Unicellular algae like Chlamydomo­nas are motile, possess regular shape, definite growth and photosensitive organelles.

(iv) Slime Moulds, a group of fungi, are wall-less in the vegetative phase when nutrition is holozoic. They develop cell walls in the reproductive phase. Slime moulds can neither be placed in fungi, nor plants. This group is popularly called an animal group which is studied by mycologists.

(v) Lichens constitute a peculiar group of dual organisms which are formed by an association between an alga and a fungus. They have no plant character, neither any animal character.

(vi) Viruses have a volume of 10^-6 to 10^-3 µm³. Prokaryotes have a volume range of 0.2 to 10 cubic µm³ while the average eukaryotic cell has volume between 1000-10000 µm³. All of them cannot occur in a single kingdom.

(vii) Prokaryotes have a naked genetic material without being organised into a nucleus, a single envelope organisation, absence of spindle apparatus, meiosis and sexual reproduction.

Viruses have no protoplasm and metabolic machinery of their own. On the other hand, eukaryotes have a well defined nucleus, a double envelope organisation, spindle apparatus, meiosis and sexual reproduction. Therefore, the three must be separated from one another.

(viii) Groups of unicellular algae (euglenoids, diatoms and dinoflagellates) and protozoa show sufficient degree of resemblance.