CH. VIII] 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 1 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 

 Pfeffer's 2 account of the experiments, in which he showed 



1 Physiologische Untersuchungen, 1873, p. 183. 



2 Ibid., p. 167. 



