378 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM 



fig. 214. The successive positions of the tip of the circum- 

 nutating plant are read at different hours of the day against 

 a circular scale placed below, in order to determine the 

 changing rates of movement due to external variations of 

 intensity of light and of temperature. The rates during 

 the hours of the day are given in the table. 



As regards external conditions, the plant was in the 

 shadow of surrounding trees up to 9 A.M., when sunlight 

 fell on it directly and continuously till 3 p.m. ; after this 

 the rays of the sun were obstructed by trees on the other 

 side. The temperature rose from 12° C. at 5 A.M. to the 

 maximum of 20° C. at 2 p.m. Very strong sunlight and 

 high temperature at 2 p.m. brought about a condition of 

 drought which caused a reversal from anti- clockwise to 

 clockwise movement. The incidental effect of drought will 

 be described later in detail. The rise of temperature in the 

 morning enhanced the rate of circumnutation up to 9 a.m., 

 but sunlight caused increasing retardation ; the effect of 

 drought referred to above caused a reversal of movement 

 after 2 p.m. As the sun disappeared behind the trees there 

 was a recovery to 25° between 3 and 4 p.m. This increased 

 to 40° between 4 and 5 p.m. After 5 p.m. the temperature 

 fell rapidly, causing diminution in the rate of movement. 

 The above example shows in a general way how the con- 

 joint effects of changes of intensity of hght, of temperature. 



