Biological Classification

Kingdom Plantae

All eukaryotes which have chloroplast are kept under this kingdom. Most of them are autotrophic; but some are heterotrophic. Cell wall is mainly composed of cellulose.

The life cycle of plants has two distinct phases; the diploid saprophytic and the haploid gametophytic phase. These phases alternate with each other. The lengths of the haploid and diploid phases vary among different groups of plants. This phenomenon is called Alternation of Generation.


All multicellular eukaryotes which lack cell wall and are heterotrophs are kept under this kingdom. The animals indirectly or directly depend for food on plants. They follow the holozoic mode of nutrition. Holozoic nutrition involves ingestion of food and use of internal cavity for digestion of food. Most of the animals are capable of locomotion. Sexual reproduction is by copulation of male and female which is followed by embryological development.


Virus is non-cellular organism. It is characterized by an inert crystalline structure outside the living cell. Once a virus infects a living cell, it begins to replicate and kills the host in the process. Virus is considered as a borderline case between living and non-living.

The name virus was given by Pasteur D. J. Ivanowsky (1892). M. W. Beijerinek (1898) demonstrated that the extract of infected tobacco plants could cause infection in healthy plants. He called the fluid from the infected plant as Contagium vivum fluidum (infectious living fluid). W. M. Stanley (1935) demonstrated that viruses could be crystallized and the crystals mainly consist of proteins.

Viruses also contain genetic material; which can be either RNA or DNA. Both RNA and DNA cannot be present in the same virus. Plant infecting viruses usually have single-stranded RNA and animal infecting viruses usually have double-stranded RNA or double-stranded DNA. Bacteriophages (bacterial viruses) usually have double-stranded DNA.

The protein coat on the virus is called capsid. It is made up of small subunits called capsomeres. The capsid protects the nucleic acid. The capsomeres are arranged in helical or polyhedral geometric forms.

Mumps, small pox, herpes, influenza and AIDS are examples of viral diseases in humans. In plants; viral infection leads to the symptoms; like mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

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