LEAVES, THEIR MOVEMENT AND FORM 85 



are certain plants like the prickly lettuce (Lactuca scariola) and 

 the "compass" plant (Silphium laciniatuni) whose leaves often 

 stand so that they point north and south, no matter on which 

 side of the stem they are attached. The leaves at the same time 

 are turned on edge, so that the surfaces face east and west. 

 This position is taken by the leaves in response to light and not 



Fig. 73- 

 Prickly lettuce (Lactuca scariola), a compass plant, showing side and edge view. 



because of any magnetic influence. The sunlight at midday is 

 too strong for the leaves and the leaf is stimulated to turn its edge 

 to the incidence of the strongest rays so that they glide by with 

 no injury to the leaf. The light during the morning and after- 

 noon hours is not so strong so that no injury comes from the ex- 

 posure of the surface at this time of day. When these plants 



