Nature

===Plants and animals===
{{Main|Plant|Animal}}
[[File:Diversity of plants (Streptophyta) version 2.png|thumb|left|A selection of diverse [[plant species]]]]
[[File:Animal diversity.png|thumb|A selection of diverse [[animal species]]]]
Originally [[Aristotle]] divided all living things between plants, which generally do not move fast enough for humans to notice, and animals. In [[Carl Linnaeus|Linnaeus]]' system, these became the [[kingdom (biology)|kingdoms]] [[Vegetabilia]] (later [[Plant]]ae) and [[Animal]]ia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the [[fungus|fungi]] and several groups of [[alga]]e were removed to new kingdoms. However, these are still often considered plants in many contexts. Bacterial life is sometimes included in flora,<ref>{{cite web |title=flora |url=http://webster.com/cgi-bin/dictionary?va=flora |work=Merriam-Webster Online Dictionary |publisher=Merriam-Webster |access-date=September 27, 2006 |archive-url=https://web.archive.org/web/20060430072626/http://webster.com/cgi-bin/dictionary?va=flora |archive-date=April 30, 2006 |url-status=dead |df=mdy-all }}</ref><ref>{{cite book |date=1998 |title=Status and Trends of the Nation's Biological Resources |chapter=Glossary |chapter-url=http://biology.usgs.gov/s+t/SNT/noframe/zy198.htm |publisher=Department of the Interior, Geological Survey |location=Reston, VA |id=SuDocs No. I 19.202:ST 1/V.1-2 |url-status=dead |archive-url=https://web.archive.org/web/20070715060359/http://biology.usgs.gov/s+t/SNT/noframe/zy198.htm |archive-date=July 15, 2007  }}</ref> and some classifications use the term ''bacterial flora'' separately from ''plant flora''.

Among the many ways of classifying plants are by regional [[flora]]s, which, depending on the purpose of study, can also include ''fossil flora'', remnants<br /> of plant life from a previous era. People in many regions and countries take great pride in their individual arrays of characteristic flora, which can vary widely across the globe due to differences in climate and [[terrain]].

Regional floras commonly are divided into categories such as ''native flora'' and ''agricultural and garden flora'', the lastly mentioned of which are intentionally grown and cultivated. Some types of "native flora" actually have been introduced centuries ago by people migrating from one region or continent to another, and become an integral part of the native, or natural flora of the place to which they were introduced. This is an example of how human interaction with nature can blur the boundary of what is considered nature.

Another category of plant has historically been carved out for ''weeds''. Though the term has fallen into disfavor among [[Botany|botanists]] as a formal way to categorize "useless" plants, the informal use of the word "weeds" to describe those plants that are deemed worthy of elimination is illustrative of the general tendency of people and societies to seek to alter or shape the course of nature. Similarly, animals are often categorized in ways such as ''domestic'', ''farm animals'', ''wild animals'', ''pests'', etc. according to their relationship to human life.

Animals as a category have several characteristics that generally set them apart from other living things. Animals are [[eukaryote|eukaryotic]] and usually [[multicellular]] (although see [[Myxozoa]]), which separates them from bacteria, [[archaea]], and most [[protist]]s. They are [[heterotroph]]ic, generally digesting food in an internal chamber, which separates them from plants and [[alga]]e. They are also distinguished from plants, algae, and [[fungus|fungi]] by lacking [[cell wall]]s.

With a few exceptions—most notably the two [[Phylum|phyla]] consisting of [[sponge]]s and [[placozoa]]ns—animals have bodies that are differentiated into [[biological tissue|tissues]]. These include [[muscle]]s, which are able to contract and control locomotion, and a [[nervous system]], which sends and processes signals. There is also typically an internal [[digestion|digestive]] chamber. The eukaryotic cells possessed by all animals are surrounded by a characteristic extracellular matrix composed of [[collagen]] and elastic [[glycoprotein]]s. This may be calcified to form structures like [[Animal shell|shells]], [[bone]]s, and [[spicule (sponge)|spicules]], a framework upon which cells can move about and be reorganized during development and maturation, and which supports the complex anatomy required for mobility.