122 PLANT LIFE. 



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 various forms of leaves and 

 transition from one to the other. «'-«», scale leaves; Z 1 -/ 3 , foliage leaves; h l -h 3 , 

 bracts; the flower leaves not clearly shown. The scale leaf, «', shows 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 subsequent reduction through 

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



153. 2. The leaf stalk. — The leaf stalk is also known 

 as tint petiole. Its form is more or less cylindrical, usually 

 with a groove or channel upon the upper side. Sometimes 

 the petiole is flattened in a vertical plane, as in aspen poplars. 



