*** Appendix - What is a plant?

Back to: Which organisms are capable of associative learning? *** SUMMARY of Conclusions reached References

There is currently some disagreement on what defines a plant. Many lay-people would say "photosynthesis", but there are bacteria (cyanobacteria) which photosynthesise and there are some plants (beech drops) which do not. The old saw, "That which is green is a plant", falls foul of counter-instances, too - a few plants have lost their green pigment during their evolution, and the status of green algae as plants is in dispute. However, plants do have certain common features: they are eukaryotes, and they develop from embryos - multicellular structures enclosed in maternal tissue. Because all plants develop from embryos, they are all multicellular. The cell walls of plants are made of cellulose. Different authorities define plants on the basis of either chemistry (restricting plants to the groups where the cells contain a membrane-bounded chloroplast, where the products of photosynthesis are stored, with a particular type of chlorophyll) or the structure of the wall of the chloroplast. For this reason, the status of certain algae (e.g. red algae) as plants remains controversial (Hoverkamp, 2002; McCourt, Chapman, Buchheim and Mishler, 1995; World Biodiversity Project, 2000).