Chapter 31 Plant response to the environment Answers to Study Questions 1 What is the difference between a negative and a positive tropic response tropism A negative tropic response would be growth away from a stimulus a positive tropic response would be growth toward a stimulus 2 What is the difference between a tropic response tropism and a nastic response nasty A tropic response is an irreversible plant growth movement response that is directed by a stimulus in either a positive or negative fashion A nastic response is a reversible plant movement the direction of which is not determined by the stimulus Nastic movements are usually the result of changes in turgor pressure in opposite sides of a plant organ which causes the movement 3 If you removed the root cap from a root would the root still respond to gravity Explain No the root would not respond to gravity because the gravity sensing cells are in the root cap 4 What is the falling or settling statolith hypothesis of root gravitropism How has this hypothesis recently been modified Statoliths are starch filled dense organelles found in root cap cells that allow roots to sense gravity As they resettle in response to a positional change they cause the redistribution of calcium and auxin which results in unequal elongation of the root and ultimately growth towards the center of the earth Starchless mutant plants are still positively gravitropic but they respond slower to a gravity stimulus It s possible that other dense organelles or the cytoplasm itself falls or pulls on cytoskeletal elements initiating a signal transduction pathway leading to differential growth and movement towards or away from the center of the earth This modified hypothesis would help explain gravitropic responses in shoots which do not possess statoliths