THE LEAVES. 



lOI 



126. 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 



Fig. 90. — A young flowering shoot, of dog-rose, showing various forms of leaves and 

 transition from one to the other, w^-w*, scale leaves; .^-/^, foliage leaves; k^-h^, 

 hracts; the flower leaves not clearly shown. The scale leaf, w*, shows a leaf base, 

 winged by stipules 6, 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. 



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

 When this flattening is extensive, so that the petiole becomes 

 thin and leaf-like and the blade is wanting, it functions as a 

 foliage leaf (fig. 95). Not infrequently, the petiole is 



