Nastic and Epinastic Responses
Paratonic variation movements of plant parts in which direction of movement is independent of the direction of stimulus. Nastic responses are like closing of leaflets by touch (E.g. Mimosa pudica) and Epinastic responses are like bending downward of the petiole bringing the leaf to a hanging position. Ethylene causes epinasty of leaves by promoting elongation of cells on the upper side. If one half of a leaf of plant like xanthium is shaded, then the corresponding site of the petiole elongation so that the petiole bends displacing the leaf blade towards irradiated side. Secondly upward bending of the shaded side of the leaf (hyponasty) this mechanism helps the plant in avoiding overlapping of leaves and maximum exposure to Light. The canoppy forms leaf mosaics.