(a) Positive tropism is the MOVEMENT or growth of plants toward the direction of the stimulus. For example, the roots of a plant always grow towards the pull of gravity. Negative tropism is the movement or growth of plants away from the direction of the stimulus. For example, the shoot of a plant always grows in the opposite direction of the gravitational pull.(B) Thigmotropism is the response of a plant to the physical contact. Thigmotropism is directional which means the plant response towards or away from the direction of the physical contact. For example, the growth of ivy on a wall DUE to the physical contact with the wall.(c) Thigmotropism is the directional response of the plant to the physical contact whereas thigmonasty is the non-directional response of the plant to the physical contact. An example of thigmotropism is tendrils of grapevine. The tendrils grow towards the OBJECT with which it experiences a physical contact. This is a directional response of the tendrils toward the physical contact with the object. An example of thigmonasty is MIMOSA pudica. On physical contact, the leaves of Mimosa pudica shrinks and folds. It does not depend on the direction of the physical contact.