32 PART I. MORPHOLOGY. [12 



is said to be perfoKate ; when this occurs in two opposite leaves, 

 the leaves become connate (C ; see p. 22). 



There is, in some cases, a delicate membranous ventral outgrowth on 

 the leaf at the junction of epipodium and hypopodium, termed the ligule ; 

 it occurs in Grasses (Fig. 19 .4), in Selaginella and Isoetes, and in the 

 perianth-leaves of some flowers (Narcissus, Lychnis). 



The Mesopodium or Petiole is commonly, but not always, 

 present. When it is present the leaf is said to petiolate ; when 

 it is absent, sessile. It is developed by intercalary growth in a 

 portion of the primordial leaf lying between the hypopodium on 

 the one side and the epipodium on the other. The most common 

 form of the petiole is somewhat cylindrical ; though, where the 

 dorsiventrality of the leaf is well-marked, it is convex on the 

 lower (dorsal) surface, and flattened or grooved on the upper 

 (ventral) surface. In the Aspen (Populus tremula) it is flattened 

 laterally. 



Occasionally (e.g. Orange, Fig. 23 G ; Nepenthes, Fig. 28; 

 Dionsea) the petiole is winged. 



In some cases (e.g. Australian Acacias) the petiole has somewhat 

 the form of a lamina. Its flattened surfaces are directed laterally, 

 the edges upwards and downwards, so that the symmetry is isobi- 

 lateral. A petiole of this form is termed a phyllode. In such 

 cases, the lamina, originally present, soon falls off. 



Tlie Epipodium may be either icinged or unicinged. The 

 winged epipodium constitutes the lamina or blade of the leaf, and 

 is typically flattened and expanded in form and dorsiventral in 

 symmetry : but this is not always the case, for in some plants it 

 assumes the form of a sac or pitcher (e.g. Utricularia, Nepenthes, 

 etc.), and in others the symmetry is isobilateral (e.g. adult leaves 

 of Eucalyptus Globulus). 



The form of the unwinged epipodium presents great variety ; 

 thus, in Lathyrus Aphaca the epipodium branches into leaf- 

 tendrils, and this is partially the case also in the Sweet Pea (Fig. 

 19 C) ; it may be cylindrical or prismatic, as in Onion, Sedum, 

 Mesembryanthemum, Aloe; acicular as in Pinus ; narrow, and 

 flattened anteroposteriorly (ensiform] so that the margins corres- 

 pond to the dorsal and ventral surfaces of a dorsiventral leaf, 

 with isobilateral symmetry, as in Iris and Gladiolus. 



The flattened dorsiventral lamina is normally so placed with 

 regard to the parent stem that a plane, which includes the longi- 



