UIURNAL MOVEMENTS IN PL4NTS 531} 



The temperature remains constant for nearly an hour 

 (luring the period of transition from falling to rising 

 temperature, and vice versa. The average period of minimum 

 temperature may be taken at <> a.m., which I shall 

 distinguish as the thermal-dawyi. The average period for 

 maximum temperature, the IJievraal-noon, id at 2 p.m. Varia- 

 tions from these average periods at different seasons do 

 not amount to more than an hour. 



The light-dawn and thei'mal-drawn are more or lest5 

 coincident, while the thermal- noon is two hours later than the 

 light-noon. A change in the diurnal curve of movement due 

 to thermal variation will thus be detected at about 2 p.m. 

 If the curve of daily movement of the plant-organ closely 

 resemble the diurnal thermographic curve, there can then 

 be no doubt of the causal relation of variation of tem- 

 perature in the production of the periodic movement. Two 

 different classes of phenomena, as already stated, arise 

 however from the variation of temperature, thernionasty 

 and thermo-geotropism. In the former, the movement is 

 autonomous, and determined in relation to the plant ; in the 

 latter, the movement is related to the direction of external 

 stimulus of gravity. Further tests will be given later, to 

 distinguish the phenomenon of Thernionasty from that of 

 Thermo-geotropism. 



I shall in the succeeding papers describe the principal 

 types of diurnal movements as sketched above. The success 

 of the investigation greatly depends on the elaboration of 

 automatic apparatus of precision, which gives a continuous 

 record of the diurnal movement of different plant organs. 

 The description of this Nyctitropic Recorder will be given 

 in the next paper, 



SUMMARY. 



The obscurities in the nyctitropic movement of plants 

 arise from the presence of numerous complicating factors. 



