<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> A tropism (from Greek τρόπος, tropos, "a turning") is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus . In tropisms, this response is dependent on the direction of the stimulus (as opposed to nastic movements which are non-directional responses). Viruses and other pathogens also affect what is called "host tropism", "tissue tropism", or "cell tropism", or in which case tropism refers to the way in which different viruses / pathogens have evolved to preferentially target specific host species, specific tissue, or specific cell types within those species . Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to sunlight) and may be either positive (towards the stimulus) or negative (away from the stimulus). </P> <P> Tropisms occur in four sequential steps . First, there is a perception to a stimulus, which is usually beneficiary to the plant . Next, signal transduction occurs . This leads to auxin redistribution at the cellular level and finally, the growth response occurs . </P>

What is significance of tropic movement in plants