Limitations of Two Kingdom System of Classification:
The two kingdom system of categorization proposed by Linnaeus has been in utilization for a long time. However later it is verified to be insufficient and unsatisfactory in view of latest information and discoveries concerning the lower forms of organisms. The shortcomings of the two kingdom system of classification are as follows:
Figure: Diagrammatic representation of Two Kingdom System
1. Some organisms share the characteristics of both animals and plants. Example: Euglena and Sponges. In Euglena, certain species contain chlorophyll and are autotrophic similar to plants. Though similar to animals they are dependent on an exterior supply of vitamins B, and B12 that they can’t synthesize themselves. Some species of Euglena lack chloroplasts and are thus colorless and non-photosynthetic (i.e., heterotrophic). They contain saprotrophic mode of nutrition, taking out extra-cellular digestion. The other colorless forms ingest little food particles and carry out intracellular digestion (i.e., holozoic nutrition). When green species of Euglena are kept in dark place they lose their chloroplasts and become colorless and survive saprotrophically. Chloroplasts return whenever the organisms are returned to light. Euglena is too characterized by the existence of an animal pigment astaxanthin in an eye spot.2. Fungi are a class of organisms that contain features of their own. They lack chlorophyll. They are heterotrophic similar to animals. They are placed all along with green plants.
3. Most of the primitive organisms like bacteria did not fit into either group and organisms similar to slime moulds are amoeboid however form fruiting bodies alike to fungi.
4. The status of virus whether they are non-living or living is a point of debate even today.
For all such reasons the 250 years old Linnaeus system of categorizing organisms into two rigid groups plants and animals and is considered highly artificial and random.
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