Editing Coleoptile (section)

==Tropisms==
Early experiments on [[phototropism]] using coleoptiles suggested that plants grow towards light because [[plant cell]]s on the darker side elongate more than those on the lighter side. In 1880 [[Charles Darwin]] and his son [[Francis Darwin|Francis]] found that coleoptiles only bend towards the light when their tips are exposed.<ref>Darwin, C. R. (1880). ''The Power of Movement in Plants''. London: Murray.</ref> Therefore, the tips must contain the [[photoreceptor cell]]s although the bending takes place lower down on the shoot. A chemical messenger or [[hormone]] called [[auxin]] moves down the dark side of the shoot and stimulates growth on that side. The natural [[plant hormone]] responsible for phototropism is now known to be [[indoleacetic acid]] (IAA).

The [[Cholodny–Went model]] is named after [[Frits Warmolt Went]] of the [[California Institute of Technology]] and the Ukrainian scientist [[Nikolai Cholodny]], who reached the same conclusion independently in 1927. It describes the [[phototropism|phototropic]] and [[gravitropism|gravitropic]] properties of emerging shoots of [[monocotyledon]]s. The model proposes that auxin, a plant growth hormone, is synthesized in the [[Meristem|coleoptile tip]], which senses light or gravity and will send the auxin down the appropriate side of the shoot. This causes [[symmetry|asymmetric]] growth of one side of the plant. As a result, the plant shoot will begin to bend toward a light source or toward the surface.<ref name="basipetal">{{cite journal| journal=Plant Physiology| title=Basipetal Auxin Transport Is Required for Gravitropism in Roots of Arabidopsis| author=Rashotte|date=February 2000| volume=122| number=2| pages=481–490| doi=10.1104/pp.122.2.481|display-authors=etal| pmc=58885| pmid=10677441}}</ref>

Coleoptiles also exhibit strong [[geotropism|geotropic]] reaction, always growing upward and correcting direction after reorientation. Geotropic reaction is regulated by light (more exactly by [[phytochrome]] action).