I have already referred to the great difficulty of explanation of nyctitropism from the fact that the diurnal movements may be brought about by different agencies independent of each other. It is, moreover, not easy to discriminate the effect of one agency from that of the other. The combined effects of different factors will evidently be very numerous. This will be understood from consideration of the number of possible combinations with only two variables, geotropism and phototropism. The effect of geotropism may be strong _G_, or feeble, _g_. Similarly we may have strong effect of light _L_, or feeble effect of light _l_. Light may exert positive phototropic action +_L_ or negative action -_L_. Thus from two variables we obtain the following eight combinations: _G_ + _L_; _G_ - _L_; _G_ + _l_; _G_ - _l_; _g_ + _L_; _g_ - _L_; _g_ + _l_; _g_ - _l_. The number of possible variables are, however, far more numerous as will be seen from the following: _Geotropism._--The effect of geotropic stimulus on horizontally placed organs is one of erection. But this stimulus, which is constant, cannot by itself give rise to periodic movements. It has however been shown that variation of temperature has a modifying influence on geotropic curvature (p. 519). _Phototropism._--The action of unilateral light is to induce a tropic curvature, which in some cases is positive, in others negative (p. 386). In addition to these effects induced during the incidence of light, we have to take account of the after-effects on the cessation of light. _After-effects of light._--I find two very different effects, depending on the intensity and duration of previous illumination. Of these the most important is the phenomenon of 'overshooting' which occurs on the cessation of light of long duration. This particular reaction, to be fully described, will be found to offer an explanation of certain anomalous effects in diurnal movement. _Periodic variation of turgor._--I have shown (p. 39) that artificial enhancement of turgor in the plant induces an erectile movement of the leaf of _Mimosa_, diminution of turgor inducing the opposite movement of fall. Kraus and Millardet have shown that a diurnal variation of tension takes place in the shoot of all plants, which is presumably indicative of variation of turgor. This variation of turgor in the shoot must have some effect on the lateral leaves. But the leaves are subjected to conditions which are absent in the stem. The erect stem is, for example, free from geotropic action, whereas the lateral leaf is subject to it. The effect of turgor variation in the shoot on the movement of leaves may be, and often is, overpowered by the predominant geotropic action. I shall, later on, refer to this question in greater detail. [Illustration: FIG. 188.--Arrest of pulsatory movement of leaflet of _Desmodium gyrans_ by light from above and gradual restoration on cessation of light. Up-movement represented by up-curve.]