Source:http://linkedlifedata.com/resource/umls/id/C0242732
MSH: The directional growth of an organism in response to an external stimulus such as light, touch, or gravity. Growth towards the stimulus is a positive tropism; growth away from the stimulus is a negative tropism. (From Concise Dictionary of Biology, 1990),GO: The movement of an organism, or part of an organism, in response to an external source of stimulus, usually toward or away from it. [GOC:curators, ISBN:0877795088]