122 



PLANT I.J/'E. 



leaf bases may unite, so that the stem seems to pass through 

 the center of a leaf which extends equally on each side of 

 it. (See fig. 143- ) 



Fig. 139,— a young flowering shoot of dog-rose, showing \'arious forms of lca\-es and 

 transition from one to the other. ?!^-ir', scale leaves ; /*-/'^, foliage leaves ; /j'-//^, 

 bracts', the flower leaves not clearly shown. The scale leaf, ^?' , sho\\'s a leaf base, 

 winged by stipules /', with only a trace of stalk and blade a. Trace these parts into 

 foliage leaves, where the blade becomes compound, and subseciucnt reduction ihrough 

 the series of bracts. Natural size. — After Luerssen. 



153. 2. The leaf stalk. — The It-af stalk is also known 

 3.S the pe/io/e. Its form is more or less e^'lindrical, usually 

 with a ,L;r()0\ c or channel upon the u])|)er side. Sometimes 

 thr petiole is flattened in a vert leal })lane, as in aspen poplars. 



