two kingdom classification of life

One of the earliest scheme of classification in the natural system is the two kingdom classification proposed by Corollus Linnaeus in 1758.

They are Kingdom Plantae and Animalia

Corollus Linnaeus placed the living organisms under two major kingdoms-Kingdom Plantae and for plants and kingdom Animalia for animals. This classification was quite reasonable at that period of times since plants and animals could be very clearly distinguished. Plants were stationary, fixed to the soil, absorbed water for growth and could prepare their own food. Animals, on the other hand were capable of movement, and were feeding on plants and animals for their growth and survival. Apart from this a few other significant differences, particularly at the level of cells, were established between plants and animals by scientists of the later period.

Kingdom: Plantae

The original plant kingdom proposed by Linnaeus and subsequent taxonomists of that period included the bacteria, fungi, algae, liverworts, mosses, ferns, conifers and flowering plants. Plants in general considered to exhibit the fallowing distinguishing characters.

  • Plants exhibit a great deal of variation in their form and size. Plant body is usually asymmetrical. However, in higher plants structures like leaves and flowers have a definite form and shape.
  • Plants are rooted mostly and as such are incapable of locomotion. However, certain localized movements may occur in a plant body.
  • Plants exhibit unlimited growth, almost throughout their life span.
  • Plants exhibit largely autotrophic nutrition or saprotrophic nutrition. Particulate matter cannot  be absorbed and only materials present in a state solution can be absorbed.
  • Plant body is composed of cells which have a distinct cell wall, a vacuole filled with sap and plastids of different kinds. The cells do not have centrioles and lysosomes.
  • Plant cells can synthesize all the amino acids, coenzymes and vitamins necessary for its functions.
Classification of Kingdom Plantae
On the basis of nonflowering and flowering plants Plant kingdom is classified into Cryptogamia and Phanerogamia respectively. 
Cryptogamia: Cryptogamia further classified into fallowing
a) Thallophyta: Which includes bacteria, algae, fungi and lichens.
b) Bryophyta: It includes liverworts
Pteridophyta: Includes ferns.

Phanerogamia: Phanerogamia further classified into fallowing
a) Gymnospermae: Includes Conifers
b) Angiospermae: Include fruit bearing plants they are monocots and dicots.

Kingdom: Animalia
The original Animal Kingdom proposed by Linnaeus included the protozoans, sponges, jelly fishes, worms, crabs, insects, spiders, snails, starfishes, sharks, bony fishes, frogs, lizards, birds and mammels. In general, animals exhibit the fallowing distinguishing characters.
  • The animal body mostly exhibits a definite from and shape and largely a symmetry.
  • Animals have the capacity to move from one place to the other in search of their necessities.
  • Growth in animals is determined and occurs propotionately in all parts of the body.
  • Animals are generally heterotrophic, obtaining their food from plants and other animals.
  • Animals have the property of irritability-the capacity to respond to a stimulus.
  • The cells, which form an animal's body do not have a cellwall. Plastids and vacuoles are generally absent. Centrioles and lysosomes are present.
  • Animals cells cannot synthesize all the necessary amino acids, vitamins and coenzymes and as such will have to obtain them from external sources.
Classification of Kingdom Animalia
The Animal kingdom was initially classified on the basis of number of cells as fallows:
1) Unicellular Animals
a) Protozoa

2) Multicellular Animals
a) Metazoa
i) Invertebrates: Sponges, Coelentrates, Worms, Arthropods, Molluscus, Echinoderms
ii) Vertebrates: Fishes, Amphibians, Reptiles, Birds and Mammals.

Merits and Demerits of Two Kingdom Classification

Merits: The two kingdom classification received considerable recognition from biologists and was in use of quite a long period of time. However, as more and and more information started emerging on the various groups of plants and animals, this system of classification was found to be inadequate. Studies made on the lower forms of life in particular, revealed an enormous variation of characteristics in those forms, not in agreement with the characteristics of the group in which they were placed.

Demerits:
  • There is no clear cut distinction in the lower forms of life, into plant and animals.
  • Slime molds are a type of fungi. They do not have a cell wall in the vegetative phase and ingest particulate matter like animals. However, they develop a cell wall in the reproductive phase which is similar to other fungi. Thus, they resemble animals in one phase and plants in the other.
  • Sponges are sessile, irregular animals which look like plants.
  • There are organisms like diatoms and protozoans which share a number of common characteristics and organization.
  • Fungi differ from plants in several aspects. They do not have chlorophyll and they are saprophytic in nature.
  • It is presumed that when life arose first on the earth it was neither in the 'plant form' or in the animal form. Definite plant or animals status is said to have been attained much later in the evolutionary history.


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