OEGANS OF NUTRITION. 



169 



appendage the name of lignle has been given. This appendage 

 is supposed by most authorities to be analogous to the stipides. 

 In the Aspen {PopiiJv.s treiuida) the petiole is flattened in a line 

 at right angles to the blade ; this is the cause of the peculiar 



Fig. 349. 



Fig. 350. 



->^^& 



Fiy.M9. A portion of the stem with some leaves of Tenus's Fly-trap 

 {Dioncea muscipula). I. Lamina fringed with hairs, hence it is said to 



be ciliated, p. Winged petiole. Fig. 350. Decurrent leaves of the 



Comfrey (Symphytum). 



mobility of such leaves ; in other plants it is flattened in a 

 horizontal direction. In Water Plants the petiole is frequently 

 more or less dilated from the presence of a number of air 

 cavities, as in Pontederia and Trapa; such petioles by di- 

 minishing the specific gravity of the plants enable them to 

 float readily in the water. At other times the petiole becomes 

 dilated at its base, and embraces the stem, in which case the 

 leaf is said to be amplexicaul{fig. 255); this commonly occurs 

 in Umbelliferous Plants. Frequently the petiole presents at its 

 two edges a leaf-like border called a wing, when it is said to 

 be winged or bordered; examples of such a petiole occur in 

 the Orange {fig. 294, p), Venus's Ply-trap {fig. 349,^), Sweet 

 Pea {fig. 359), and many other plants. In some plants the 

 winged expansion does not terminate at the base of the petiole, 

 but it is continued downwards along the stem; in which 

 case the stem is also termed winged, and the leaf is said to 

 be decurrent {figs. 257 and 350). Besides the above forms of 

 petiole, others still more remarkable occur, which will be al- 

 luded to hereafter, under the head of Anomalous Forms of 

 Leaves. 



