104 STIPA. [CH. V 



we may call the "wet" direction; when it is removed 

 it will, after a time, reverse itself and move in the " dry " 

 direction. 



(132) Stipa : effects of temperature. 



As in the case of experiment 129, so here, it may 

 be shown that warmth increases the action of water very 

 greatly. Prepare 2 beakers of water, one at 14° — 15° C, 

 the other at 40°^45° C. ; place the awn in the cold water, 

 and when the index has clearly begun its slow movement, 

 plunge it in the warm water, when the untwisting is 

 at once accelerated. 



(133) Stipa: effects of temperatwre. 



Place the Stipa in water at about 15° C. and allow 

 it to come to rest : then transfer it to water at about 

 40° C, there will be a sudden deflection in the ''wet" 

 direction and a return to a position slightly on the " dry " 

 side of the original position of rest. 



A similar result may be obtained with a dry awn, by 

 holding it high above a spirit lamp or a small gas flame, 

 taking care not to scorch it ; the first sudden move will be 

 in the " wet " direction, the heat will then dry the awn 

 and a steady " dry " movement will follow. These eflects 

 of temperature are not understood*. 



(134) Stipa: salt-solution. 



A Stipa which has come to rest in water can be made 

 to twist in the dry direction by transferring it to 10 per 

 cent. NaCl solution. 



1 Francis Darwin, Transactions of Linnean Society, 1876. 



