CH. VIIl] PERIODICITY. 233 



(263) Periodicity : Bellis {light and darkness). 



Three or four among a patch of daisies on a lawn are 

 to be darkened by covering them with an inverted flower- 

 pot : the hole in the pot must be plugged and a layer of 

 earth placed over the plug to make sure that no light 

 enters. For the same reason a ring of earth should be 

 placed round the junction of the rim of the pot with the 

 ground. If the daisy is kept darkened for two days it will 

 cease or almost cease to open or shut, the flower-heads 

 taking on a permanently half-shut condition. This shows 

 that the alternation of light and darkness is necessary for 

 continuance of " sleep " movement. 



If, however, the plants are covered in the evening the 

 flowers will open the following morning, showing that a 

 certain inherent periodicity exists. 



If the plants are covered in the morning the 

 periodic movement will probably begin to be irregular 

 by the next day. For instance the waking movement 

 will only occur late and the closure at night will also 

 be irregular. 



(264) Periodicity : Bellis {temperatvre). 



The flower-heads of the daisy are to some extent 

 influenced by changes of temperature, but they behave 

 very differently from the crocus or tulip. When they are 

 naturally closed in the evening a rise of 15' C. in tempera- 

 ture does not open them ; nor does a corresponding foil 

 close them in the morning. But, according to Pfeffer, if 

 they are warmed in the morning or cooled in the evening 



