CH. V] STIPA. 115 



/. On the upper surface of P a graduated circle is 

 marked. When the Stipa-iiwn is dipped in water the 

 index moves in the direction of the hands of a watch, which 

 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 l-^^ — 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 temperature. 



Place the awn 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 effects 

 of temperature are not understood \ 



^ Francis Darwin, Transactions of Linnean Society, 1876. Is it 

 possible that they have something in common with the contraction 

 produced in violin strings by heat? See Engelmann Ursjirioi;) der 

 Muskelkraft, 1893. 



8—2 



