CHAPTER II. 



FORM AND COMPOSITION OF LEAVES. 



Petiole and lamina Venation Simple and compound leaves 

 Petiolate and sessile leaves Sheath Stipules Outline of the 

 lamina Types of form Margin Apex Types of simple 

 leaves. 



ON regarding a typical foliage-leaf, such as that of the Ivy, 

 Aspen, Lime, Pear, or Plane, for example, the student at 

 once distinguishes two parts, the leaf-stalk or petiole, and 

 the blade or lamina. The former may be long or short, 

 and the latter may be large or small and variously shaped, 

 but it is characterised chiefly by being thin and expanded, 

 and is joined to the leaf-insertion by the petiole. 



The lamina is the essential part of such a leaf, and the 

 petiole is merely a support and connection for it. Some- 

 what closer inspection of the lamina discloses the existence 

 of a more or less copiously branched system of ribs or 

 veins, buried in the softer green structure (mesophyll) of 

 the lamina, and evidently of the nature of strands or cords 

 of a tough consistency that resists tearing, which run 

 down the petiole to the stem on which the leaf is inserted : 

 this venation may usually be seen more clearly in thin 

 leaves by holding the leaf up to the light, and its com- 

 parative toughness is detected by tearing the lamina 



