DAILY MOVEMENT DUE TO THERMO-GEOTROPISM 563 



day-time, aud a fall of the leaf at night. The periodic 

 curve of such leaves would be of opposite sign to the stan- 

 dard thermo-geotropic carves 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 some- 

 what erratic curve. The difference between growing and 

 fully grown leaves is explained by the fact that the former 

 woukl be affected by thermotropism, and the latter by 

 thermo-geotropism. Young leaves exhibit moreover a pro- 

 nounced hyponasfy 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 move- 

 ments of leaves grown later in the season, as will be ex- 

 plained in a later chapter, are very definite and charac- 

 teristic. But the young leaves in spring exhibit no definite 

 diurnal curve, but a series of automatic pulsations, the un- 

 suspected 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 modi- 

 fied by the seasonal variation of any one of the effective 

 factors, TropcBolum 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 



