64 



PLANT-ANIMALS 



[CH. 



one of a single, twelve-hour spell of " upness " with a 

 like twelve-hour spell of " downness." In other words 

 they phototrope themselves up to the light as day 

 breaks and sink down with the sun. 



Whence comes the power whereby C. roscoffensis 

 acts as a tide-indicator? What orders its rhythmic 

 coming and going? 



Onth* 

 Short 



Ligkt 



Fig. 13. The rhythmic tidal movements of C. roscoffensis. The 

 curves represent the rise and fall of the tide. The horizontal 

 lines included within the tidal curve indicate the "up" or 

 "down" positions assumed by the animals. "In Laby. light" 

 shows that, with animals kept in the laboratory and exposed to 

 light during the day, the rhythm is lost after seven or eight 

 periodic tidal movements up or down. "Light agitated" shows 

 that animals exposed to constant vibration lose their periodicity 

 more quickly. " Dark " that in constant darkness periodicity does 

 not manifest itself. 



A French biologist, Dr Bohn (1903), who has 

 also observed this periodicity of upward and down- 

 ward movement, rejects the view which is put forward 



