_Experiment 141._--I took four creeping stems of _Mimosa_ in vigorous condition and tied them in such a manner that their free ends should be vertical. The shaded sides of the four specimens were so turned that each faced a different point of the compass--east, west, north and south. Subjected thus to diffuse stimulus of light from the sky, they all executed curvatures. The specimen whose under side faced the east, became bent towards the east; the same happened to those which faced north, south, and west, that is to say they curved towards the north, south, and west respectively (Fig. 140). The fundamental action by which all these were determined was the induced concavity of the under or normally shaded side, which was the more excitable. I obtained similar results with various other creeping stems. [Illustration: FIG. 140.--Photonastic curvature of creeping stem of _Mimosa pudica_: in the central figure the stem is seen to be vertical: action of diffuse light induced appropriate curvatures by greater contraction and concavity of the more excitable lower or shaded side, as seen in figures to the right (_b_) and left (_c_).] It has been shown that under prolonged unilateral stimulation, excitation becomes internally diffused; this gives rise to an effect similar to that of external diffuse stimulus. Under strong light the shaded side becomes concave, and thus press against the ground or the support; this will be the characteristic response of creeping stems in which the shaded side is the more excitable. The facts given above will probably explain the response of midribs of leaves, of the creeping stem of _Lysimachia_, all of which, in response to the action of strong light acting from above, exhibit concavity of the shaded and more excitable side. PARA-HELIOTROPISM. Under strong sunlight, the leaflets of various plants move sometimes upwards, at other times downwards, so as to place the blades of leaflets parallel to incident light. This 'midday sleep' has been termed _para-heliotropism_ by Darwin. It has been thought that para-heliotropic action has nothing to do with the directive action of light, since many leaflets either fold upwards or downwards, irrespective of the direction of incident light. I shall for convenience distinguish the leaflets which fold upwards under light as _positively_ para-heliotropic, and those which fold downwards as _negatively_ para-heliotropic. This is merely for convenience of description. There is no specific irritability which distinguishes one from the other. POSITIVE PARA-HELIOTROPISM.