18 PETIOLE [CH. 



channelled above, as in the Ash, and the transverse section 

 is then more or less crescentic. In the Aspen and some 

 other Poplars the petiole is flattened from side to side. 

 In some species of Lathyrus and others the petiole is 



Fig. 5. Simple leaf of Lilac, Syringa vulgaris, showing heart-shaped 

 leaf-blade (lamina) on its stalk (petiole). The margin is entire (D). 



winged, the mesophyll of the leaf being continued down 

 the sides as thin plates. 



There can be little doubt that long and flexible petioles 

 enable the lamina to take shocks of hail, rain, wind, &c., 



