_Autonomous movements: Experiment 202._--The lateral organ, say the leaf or leaflet, may have an autonomous movement of its own. In some, the autonomous movement may be relatively quick; the complete pulsation in _Desmodium gyrans_ may be as short as a minute or so. I find that this autonomous movement becomes modified or even arrested by the paratonic effect of light. This is seen in figure 188, where light applied from above is seen to arrest the pulsation; the normal activity is, however, restored on the stoppage of light. [Illustration: FIG. 189.--Effect of unilateral light on hyponastic movement of the cotyledon of _Pepo_. Application of light indicated by arrows; light acting from below retards, acting from above accelerates the movement. The last part of the curve in each shows recovery on the stoppage of light.] _Epinasty and Hyponasty: Experiment 203._--There are other autonomous movements which are relatively slow. Even in an erect stem there may be a to and fro oscillation. In such a case the effect of an external stimulus, say of light, is one of algebraical summation. The following is the summary of results of unilateral action of light on the nutating hypocotyl of a pea seedling: +--------------------------------------------------------------------+ | Natural movement. | Effect of light applied on the right side.| +------------------------+-------------------------------------------+ |Movement to the right |Acceleration of existing movement. | |Movement to the left |Retardation, arrest or reversal of natural | | | movement. | +--------------------------------------------------------------------+ Figure 189 exhibits the effect of light applied alternately above or below the cotyledon of _Cucurbita Pepo_. On account of the more rapid growth of the lower side, the cotyledon was exhibiting a hyponastic up-movement. Application of light from above enhanced the existing rate of movement, whereas light acting from below retarded the movement. Here we have instances of photo-hyponastic modification of natural movement. Similarly epinastic organs will, normally speaking, have their natural down movement retarded by light from above, and accelerated by light from below. If the periodicity of the autonomous movements coincides with the periodicity of the external stimulus, the resulting movement will be determined by algebraical summation; it will be very pronounced when the two effects are concordant. If the two periodicities do not agree, the interference effects will become extremely complicated. _Positive thermonasty._--Rise of temperature inducing differential growth brings about the _closure_ of the flower. Fall of temperature on the other hand induces a movement of opening. Example of this has already been given in the responsive movement of _Nymphæa_.