Plants, animals, viruses and lichens

Plantae: many-celled eukaryotes with chlorophyll and cellulose cell walls, from algae to flowering plants. Their life cycle swings between a diploid stage (sporophyte) and a haploid stage (gametophyte): this is alternation of generations. A few are partly heterotrophic: insect-eating bladderwort and Venus fly trap, and the parasite Cuscuta.

Animalia: many-celled eukaryotes with no cell walls. They take in food and digest it (holozoic nutrition), store food as glycogen or fat, and grow in a definite pattern.

Viruses have no cells at all. Outside a host cell they are inert, like a chemical; W.M. Stanley (1935) even crystallised one. Inside a cell they take over its machinery to copy themselves. Ivanowsky (1892) found the agent of tobacco mosaic disease; Beijerinck (1898) called it Contagium vivum fluidum (infectious living fluid). A virus is nucleic acid, either RNA or DNA, never both, inside a protein coat (capsid) made of units called capsomeres. Viruses cause mumps, smallpox, herpes, influenza and AIDS.

Two even simpler agents. Viroids (T.O. Diener, 1971) are free RNA with no protein coat; one causes potato spindle tuber disease. Prions are abnormally folded proteins; they cause mad cow disease (BSE) in cattle and CJD in humans.

A lichen is a partnership between an alga (the phycobiont, which makes food) and a fungus (the mycobiont, which gives shelter and absorbs water and minerals). They are so closely joined that they look like one organism. Lichens do not grow in polluted air, so they are good pollution indicators.