SUMMARY 205 



temperature is reached at about 3 p.m. in summer, and 

 about I P.M. in winter. The range of daily variation in 

 summer may be taken to be between 23° C. and 38° C. ; in 

 winter between 16° C. and about 29° C. These are the 

 normal variations and not the sudden fluctuations that 

 occur during uncertain weather conditions. 



The temperature remains constant for nearly an hour 

 during the period of transition from falling to rising tempera- 

 ture, and vice versa. The average time of minimum tem- 

 perature may be taken to be at 6 a.m., which I distinguish 

 as the thermal dawn ; the maximum temperature, the 

 thermal noon, is attained at about 2 p.m. Variation from 

 these average times at different seasons does not amount 

 to more than an hour. 



Light dawn and thermal dawn are more or less 

 coincident, while thermal noon is two hours later than 

 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 resembles the diurnal thermographic curve, there 

 can then be no doubt of the causal relation of variation of 

 temperature in the production of the periodic movement. 



The effect of geotropism can be detected, as will be 

 explained later, by taking the record of the plant in normal 

 and in inverted positions. 



Summary 



The phenomena of the diurnal movements of plants are 

 greatly complicated by the algebraical summation of the 

 effects of numerous factors. The most important of these 

 are the effects of light and darkness, of variation of tem- 

 perature, and of thermal variation on organs subjected to 

 the action of gravity. 



In order to trace the effects of the more important indi- 

 vidual factors, it is necessary to obtain continuous record of 

 variation of temperature by the Thermograph, of variation 



