What is Phototropism?
[[File:Arabidopsis thaliana.jpg|thumb|The Thale Cress (''[[Arabidopsis thaliana]]'') is regulated by blue to UV light ([http://www.plantphys.net/article.php?id=266 plantphys.net] {{Webarchive|url=https://web.archive.org/web/20060512073313/http://www.plantphys.net/article.php?id=266 |date=2006-05-12 }})]] [[File:Azuki Bean phototropism.ogv|thumb|Example on [[Azuki bean]]s]] '''Phototropism''' is [[growth]] in the direction of light. Phototropism is common in plants, but can also occur in other organisms such as [[fungi]]. The [[cell]]s on the plant that are farthest from the light have a plant growth [[hormone]] called [[auxin]], which causes phototropism to occur. It causes the plant to have '''elongated cells''' on the farthest side from the light. Phototropism is one of the many plant [[tropism]]s or movements which respond to external stimuli. Growth towards a light source is a ''positive phototropism'', while growth away from light is called ''negative phototropism''. Most [[plant shoot]]s have positive phototropism. [[Root]]s usually have negative phototropism, though [[gravitropism]] may play a larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them.<ref>Salisbury, Frank B. & Ross, Cleon W. 1992. ''Plant physiology''. 4th ed, Belmont, California: Wadsworth Publishing. {{ISBN|0-534-15162-0}}</ref> Auxin collects mainly on the darker side of the stem and stimulates cell elongation. Thus, the cells on the side not directly exposed to light will grow faster than the opposing side, and the stem will curve towards the light source. Phototropism in plants such as ''[[Arabidopsis thaliana]]'' is directed by blue light receptors called [[phototropin]]s.<ref>{{cite web|url=http://abstracts.aspb.org/pb2004/public/S01/9179.html|title=American Society of Plant Biologists|publisher=|access-date=2012-03-29|archive-date=2015-11-18|archive-url=https://web.archive.org/web/20151118210936/http://abstracts.aspb.org/pb2