ii8 COMPASS PLANT. [chap, v. 



Stahl and Pick have pointed out that even in the 

 same species, leaves growing in shade differ somewhat 

 in these respects from those which are exposed to 

 bright hght. 



The leaves of the Prickly Lettuce (Lactuca 

 scariola) have also, when growing in sunny situations, 

 a tendency to point north and south. Under such 

 circumstances also they have a layer of pallisade 

 cells on each side. Perfoliate leaves are perhaps 

 advantageous in stopping the ascent of creeping 

 insects. 



