ORGANS OF NUTRITIOX. 



175 



Grasses terminates above in a membranous appendage {fig. 

 351, lig), which is either entire, incised in various ways* or 

 divided into two symmetrical portions ; to this appendage 

 the name of ligule has been given. It is supposed to be 

 analogous to the stipules ; the researches of De Mercklin on 

 the development of leaves have not however enabled him 

 to decide, whether the formation of the ligule is absolutely 

 identical with that of stipules. In the Aspen (Populus tre- 

 inula) the petiole is flattened in a line at right angles to 

 the blade ; this is the cause of the peculiar 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 diminishing the specific gravity 

 of the plants enable them to float readily in the water. At 

 other times it becomes dilated at its base, and embraces the 

 stem, in which case the leaf is said to be amplexicaul {fi<]. 258); 

 this commonly occurs in Umbelliferous Plants. Frequently it 



Fig. 352. 



Fig. 353. 



>>H:1;C^ 



Fig. 352. A portion of the stem with some leaves of Venus's Fly-trap 

 {Dkmceamuscipula). I. Lamina fringed with hairs, hence it is said to be 



ciliated, p. Winged petiole. Fig. 353. Decurrent leaves of the Com- 



frey (^Symphytum). 



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. 300, />), Venus's Ply-trap {fig. 352, p). 

 Sweet Pea {fig. 362), and many other plants. Iii the latter 

 plant the winged expansion does not terminate at the base of 



