CH. VIIl] TULIP, WARMTH. 219 



in a room free from sunshine, and where the temperature 

 is not above 15° C, — a temperature of 11° or 12° better 

 still. 



The flower having been left to itself for 15 minutes is 

 placed in a temperature of about 20° C. In 5 or 10 

 minutes a clear increase in the reading on the scale shows 

 that the flower is opening. 



It may now be replaced in a temperature of 10° — 12° C. 

 Notice that the flower continues to open for some time 

 and then begins to close. The same phenomenon mutatis 

 mutandis is to be seen on changing a low into a high 

 temperature. It is easy to make a tulip open, close, and 

 open again within one hour. 



(251) Tulip: sensitiveness to small change of temperature. 



Pfeffer^ has seen a crocus flower open slightly in 

 15 minutes during which the temperature rose by less 

 than 1° C. The change of temperature was produced by 

 opening the door between a cold and a warm room. For 

 class-work it is perhaps best to try rather larger changes 

 of temperature. A tulip, fitted with two indices as 

 described above, shows distinct opening in half-an-hour 

 when moved from a temperature of 13*5C. to a temperature 

 of 15*5°, closing slightly again on being replaced in a 

 temperature of 13° C. 



(252) Crocus: mechanism of the movement. 



The following instructions are based entirely on 

 Pfefifer's'^ account of the experiments, in which he showed 



1 Physiologische Untersuchungen, 1873, p. 183. 



2 Ibid., p. 167. 



