388 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM 



Effect of Variation of Temperature 



For studying the effect of variation of temperature, 

 either fall or rise, it is necessary that the variation should 

 be gradual and not abrupt : for sudden variation acts as a 

 shock, causing excitatory reaction. 



Method of gradual variation of temperature. — This is 

 accomplished by enclosing the plant within a double- 

 walled cylindrical chamber made of highly conducting 

 copper. Warm or cold water from a reservoir enters the 

 hollow cylinder, and leaves it by a pipe provided with a stop- 

 cock. By careful manipulation of the stopcock it is possible 

 to change the temperature of the enclosure very gradually, 

 the rate of variation being approximately i° C. per i J minutes. 



Effect of lowering the temperature. — The following results 

 were obtained with tw^o different species of plants, Porana 

 and Thunbergia. 



Experiment 207. — With falling temperature, the normal 

 rate gradually diminished in both till the torsional move- 

 ment was practically arrested at or about 9° C. There 

 was a revival of growth on raising the temperature above 

 this critical point. My other investigations have shown that 

 various physiological activities are also arrested at a critical 

 temperature. For example, the photosynthetic activity of 

 HydriUa is arrested at 9*5° C. 



Table XLII. — Effect of 

 Lowering the Tem- 

 perature (Porana 

 Paniculata). 



Table XLIII. — Effect of Cyclic 

 Variation of TExMPErature 

 (Thunbergia). 



Table XLII gives the diminution of the rate of torsional 

 movement in Porana by falling temperature. Table XLIII 



