12.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 49 



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 hypopodiura on the one 

 side and the epi podium on the other. The most common form of 

 the petiole is somewhat cylindrical ; though, where the dorsiven- 

 trality 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. 32 6?; Nepenthes, Fig. 37; 

 Dionaea) 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 phyllude. In such cases, 

 the lamina, originally present, soon falls off. 



The Epipodium may be either winged or unwinged. 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 Glolmlus). 



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. 28 C) ; 

 it is filamentous in Chara and some other Algae ; cylindrical or 

 prismatic, as in Onion, Sedum, Mesembryanthemum, Aloe ; acicular 

 as in Pinus ; narrow, and flattened ante ro- posteriorly (ensiform) 

 so that the margins correspond 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- 

 tudinal axes of both the stem and the leaf, cuts the lamina into 

 two lateral halves ; in other words, it is so placed that its upper 

 (ventral) surface faces the apex of the stem, and its lower (dorsal) 

 surface is directed away from it. As a rule, the two lateral halves 

 of the lamina are symmetrical ; but in some cases (e.g. Elm, 

 Begonia) they are unsymmetrical, when the lamina is said to be 

 oblique. 



The ultimate form of the lamina mainly depends upon the 



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