Editing Plant physiology (section)

===Tropisms and nastic movements===
{{main|Tropism|Nastic movement}}

Plants may respond both to directional and non-directional [[Stimulus (physiology)|stimuli]].  A response to a directional stimulus, such as [[gravity]] or [[light|sun light]], is called a tropism.  A response to a nondirectional stimulus, such as [[temperature]] or [[humidity]], is a nastic movement.

[[Tropism]]s in plants are the result of differential [[cell (biology)|cell]] growth, in which the cells on one side of the plant elongates more than those on the other side, causing the part to bend toward the side with less growth.  Among the common tropisms seen in plants is [[phototropism]], the bending of the plant toward a source of light.  Phototropism allows the plant to maximize light exposure in plants which require additional light for photosynthesis, or to minimize it in plants subjected to intense light and heat. [[Geotropism]] allows the roots of a plant to determine the direction of gravity and grow downwards.  Tropisms generally result from an interaction between the environment and production of one or more plant hormones.

[[Nastic movement]]s results from differential cell growth (e.g. epinasty and hiponasty), or from changes in [[turgor pressure]] within plant tissues (e.g., [[nyctinasty]]), which may occur rapidly.  A familiar example is [[thigmonasty]] (response to touch) in the [[Venus fly trap]], a [[carnivorous plant]].  The traps consist of modified leaf blades which bear sensitive trigger hairs. When the hairs are touched by an insect or other animal, the leaf folds shut.  This mechanism allows the plant to trap and digest small insects for additional nutrients.  Although the trap is rapidly shut by changes in internal cell pressures, the leaf must grow slowly to reset for a second opportunity to trap insects.<ref>{{cite book| author = Adrian Charles Slack|author2=Jane Gate | title = Carnivorous Plants| year = 1980| publisher = Cambridge, Massachusetts : MIT Press| isbn = 978-0-262-19186-9| page = 160 }}</ref>