In the five kingdom classification of Whittaker, there is no mention of some acellular organisms like viruses and viroids, and lichens.

Viruses:

• Viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell.

  • Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.

• The name virus (venom or poisonous fluid) was given by Pasteur. D.J. Ivanowsky.

• The Tobacco Mosaic Virus was found to be smaller than bacteria because they passed through bacteria-proof filters.

  • M.W. Beijerinek demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and is called the fluid Contagium vivum fluidum.

• W.M. Stanley showed that viruses could be crystallized and crystals consist largely of proteins****.

  • They are inert outside their specific host cell.

  • They are obligate parasites.

  • Viruses also contain genetic material, that could be either RNA or DNA but never both.

  • Viruses are nucleoproteins and the genetic material is infectious.

  • Viruses that infect plants have single-stranded RNA and viruses that infect animals have either single or double-stranded RNA or double-stranded DNA.

  • Bacterial viruses or bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses.

• The protein coat called capsid made of small subunits called capsomeres protects the nucleic acid.

  • These capsomeres are arranged in helical or polyhedral geometric forms.

• Viruses cause diseases like mumps, smallpox, herpes, AIDS, and influenza.

• In plants, the symptoms can be:

  • The mosaic formation

  • leaf rolling and curling

  • yellowing and vein clearing

  • dwarfing

  • stunted growth.

Viroids:

• T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease.

  • It was found to be a free RNA.

  • It lacked the protein coat that is found in viruses.

  • The RNA of the viroid was of low molecular weight.

Lichens:

• They are symbiotic associations i.e. mutually useful associations, between algae and fungi.

• The algal component is known as phycobiont and the fungal component is mycobiont, which is autotrophic and heterotrophic, respectively.

  • Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for their partner.

  • Lichens are very good pollution indicators – they do not grow in polluted areas.