82 



FORMS OF PETIOLES OK LEAF-STALKS. 



(fig. 186 p). In some Australian Acacias, and in some species of 

 Oxalis, Bupleurum, &c., the petiole is flattened in a vertical direc- 

 tion, the vascular bundles separating immediately after quitting the 



stem, and running nearly parallel from base to apex. This kind 

 of petiole (fig. 188 p), has been called Phyllodium (<p^xxo, a leaf, 

 and ?8o? , form). In these plants the lamina? or blades of the leaves 

 are pinnate, bipmnate, or ternate, and are produced at the extre- 

 mities of the phyllodia in a horizontal direction (fig. 188 I) ; but in 

 many instances they are not developed, and the phyllodium serves 

 the purpose of a leaf. Hence, some Acacias are called leafless. These 

 phyllodia, by their vertical position, and their peculiar form, give a 

 remarkable aspect to vegetation. On the same Acacia, there occur 

 leaves with the petiole and lamina perfect ; others having the petiole 

 slightly broadened or winged, and the lamina imperfectly developed ; 

 and others in which there is no lamina, and the petiole becomes large 



Fig, 186. Leaf of Dioiuea muscipnla, or Venus' Fly-trap, p. Dilated or winged petiole, 

 e, Jointed blade, the two fringed halves of which fold on each other, when certain hairs on the 

 upper surface are touched. 



Fig. 187. Ascidium, or Pitcher of Sarracenia, formed by the petiole of the leaf. The lid is not 

 articulated to the pitcher as hi Nepenthes (fig. 184). 



Fig. 188. Leaf of Acacia heterophylla. p, Phyllodium or enlarged petiole, with straight 

 venation. / /, Lamina or blade which is bipinnate. The blade is frequently awanting, and the 

 phyllodium is the only part produced. 



