_Effect of age._--In the floral leaves of _Nymphæa_ the thermonastic movement is of positive sign; that is to say, an erection of the petal during rise, and a fall during the lowering of temperature. The corresponding movement of leaves would therefore be an erection of the leaf in day-time, and a fall of the leaf at night. The periodic curve of such leaves would be of opposite sign to the standard thermo-geotropic curves given above. The leaf of _Nicotina_ is adduced as an example of a leaf which exhibits a movement of fall at night. But the fully grown and horizontally spread leaf I find that gives the normal record. The very young growing leaves give a different and somewhat erratic curve. The difference between growing and fully grown leaves is explained by the fact that the former would be affected by thermotropism, and the latter by thermo-geotropism. Young leaves exhibit moreover a pronounced hyponasty or epinasty, which would naturally modify the diurnal curve. Certain interesting variation is met with in the diurnal record of sprouting leaves of _Mimosa_ in spring. The movements of leaves grown later in the season, as will be explained in a later chapter, are very definite and characteristic. But the young leaves in spring exhibit no definite diurnal curve, but a series of automatic pulsations, the unsuspected presence of which in all leaves of _Mimosa_ will be demonstrated in a subsequent chapter. Later in the season, the leaf becomes tuned, as it were, to the periodic variation of the environment; the automatic movements become suppressed, and the diurnal periodicity becomes deeply impressed on the organism. _Effect of season._--The diurnal curve may also be modified by the seasonal variation of any one of the effective factors. _Tropæolum majus_, for example, exhibits positive phototropic action in one season and a negative reaction in a different season. These seasonal variations must necessarily modify the diurnal curve. I shall now proceed to demonstrate the determining influence of thermal variation, and of stimulus of gravity on the thermo-geotropic movements. The striking similarity of the thermograph, and the record of movement of plants demonstrate the causal relation between temperature variation and diurnal movement, of which the two additional tests described below offer further confirmation. REVERSAL OF NORMAL RHYTHM. The normal diurnal movement is, as we have seen, a fall during rise of temperature from morning to afternoon, and a rise from afternoon till next morning. I succeeded in reversing the normal rhythm of _Basella_ by reversing the normal variation of temperature at the two turning points, in the morning and in the afternoon. The plant was subjected to falling temperature in the morning and to rising temperature in the afternoon. The normal movement now became reversed, _i.e._, an erection instead of fall in the forenoon and a fall instead of rise in the afternoon (p. 28).