Two - Kingdom Classification
Carolus von Linnaeus the Swedish Naturalist in 1758 grouped living organisms into plants and animals. This is also known as Two Kingdom System of classification. He named these Kingdoms as -
Kingdom Planatae (including all plants) and
Kingdom Animalia (including animals).
Shortcomings of this Classification:
The Two Kingdom System groups together the organisms with true nucleus as eukaryotes and without true nucleus as prokaryotes. It also considers photosynthesizing green algae and non photosynthetic fungi as one group of plants. The Two Kingdom System considers unicellular and multicellular organisms like bacteria as plants. The placement of certain organisms into Two Kingdoms became difficult.
- Presence of cell wall is a characteristic feature of Plantae. It differs in constitution in different types of organisms.
- Plants as you are familiar with, are not mobile( although adult plants are capable of movement e.g Touch - me -not plant) but some of the gametes are capable of locomotion (movement from one place to another).
- They synthesise their own food This is possible due to energy capturing mechanism in plants and the presence of green pigment, chlorophyll. This is referred to as autotrophic mode of nutrition by a process known as photosynthesis.
- Reproduction takes place by the help of agents. Asexual and vegetative methods of reproduction are common.
- Reserve food material is starch.
Polytrichum commune - Hair - cap moss
- Animals are mobile; they have special organs for movement and locomotion.
- Holozoic mode of nutrition: They are not able to synthesise their food and depend on plants for their food.
- Lack of cell wall and presence of centriole.
- Reserve food material is glycogen.
- Growth is limited in animals.
Drawbacks of the Two Kingdom System of Classification
Mushrooms, bread moulds and other members of fungi were also included in plants though they are non-green and not able to synthesise their food.
Fungi as a group of organisms differ from plants not only in lack of green colour but also in their mode of obtaining food.
They have filamentous structures like green algae but differ from plants in the composition of cell wall.
The inclusion of fungi in plant kingdom has become debatable.
Let us take the example of Euglena. It is green in colour; single-celled and moves like animals. Some of the taxonomists considered it a plant or plant-like and included it in the plant kingdom based mainly on the presence of chlorophyll. While other protists are included in the animal kingdom along with the protozoans because of their locomotion by flagella. This also became a point of debate among the taxonomists.
You are familiar with bacteria, which are also unicellular organisms. The cell has nuclear material without well-defined nuclear membrane (incipient nucleus/nucleoid). It is a prokaryotic cell and resembles blue-green algae (cyanobacteria). Thus, bacteria were included in the plant kingdom. However, they differ from other plants and animals due to the absence of well-defined nuclear membrane and a distinct nucleus, an eukaryotic cell.
The Two Kingdom practices also needed reconsideration due to the discovery of viruses and bacteriophages (virus infecting a bacteria), which can neither be considered as prokaryotes nor as eukaryotes. They, in fact, are considered on the borderline of living and non-living objects.