GROWTH AND WORK OF PLANTS 



132. Position of the leaves of "compass" plants. There 

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

 the "compass" plant (Silphium laciniatum) 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 be- 

 cause of any magnetic 

 influence. The sun- 

 light at midday is too 

 strong for the leaves 

 and the leaf is stimu- 

 lated 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 exposure of the surface at this time of day. 



When these plants grow in the shade the leaves do not point 



north and south and the blade is horizontal. 



133. Movement of leaves in response to light. When 

 leaves are young and their position has not become fixed, they 

 often show marked movements in response to light stimulus. 

 While many plants manifest this peculiarity, it is more marked in 

 some than in others. Seedlings of many plants when placed 

 where they receive a one-sided illumination, as near a window, 



Fig. 70. 



Prickly lettuce (Lactuca scariola), a compass plant, 

 showing side and edge view. 



