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; A'-A*, 



- hracts ; the flower leaves not clearly shown. The scale leaf, «', shows a leaf base, 



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



foliage leaves, where the blade becomes compound, and subsequent reduction through 

 the series oi bracts. Natural size. — Alter I.uerssen. 



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, a.s in aspen poplars. 



