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. '-;/ 5 , scale leaves ; / ! -/ 3 , foliage leaves ; /z 1 -/* 3 , 

 bracts; the flower leaves not clearly shown. The scale leaf, J , shows a leaf base, 

 winged by stipules b, 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 the 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. 



