FORMS OF THE PETIOLE. 181 
respective plants frequently for months, which thus form a strik- 
ing contrast in their appearance to the surrounding trees, which 
have lost their leaves in consequence of these being articulated. 
On the lower surface of the petiole at its base, the parenchyma 
commonly forms a more or less evident swelling (figs. 372, c, and 
373), to which the name of pulvinus has been given. A some- 
what similar swelling may be also seen in many compound leaves 
at the base of each partial petiole ; each of which is then termed 
a struma. The compound pinnaie leaves of the Sensitive Plant 
afford a good illustration of the presence of both pulvinus and 
strume. 
Forms of the Petiole.—The form of the petiole varies in 
different leaves. It is usually rounded below, and flattened, or 
more or less grooved above ; but in other cases it is cylindrical, 
Fic. 376. 
Fie. 375. 
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Fig. 375. A portion of the stem with some leaves of Venus’s Fly-trap 
(Dioncea muscipula ye q. Lamina fringed with hairs, and hence said to be 
ciliated. p. Winged petiole.— Fig. 376. Decurrent leaves of the Comfrey 
(Symphytum officinale), 
especially in the leaves of Monocotyledons ; while in other 
plants of the same class, as in Grasses, it becomes widened 
at its base, and surrounds the stem in the form of a sheath 
(fig. 374, 7). This sheath in all true Grasses terminates above 
in a membranous appendage (fig. 374, lig), which is entire, 
or divided into two symmetrical portions, or incised in various 
ways ; to this the name of ligule has been given, and is now 
supposed by most authorities to be analogous to the stipules. 
In the Aspen (Populus tremula), the petiole is flattened in a 
line at right angles to the blade, and is thus one of the causes of 
the peculiar mobility of such leaves ; while 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 
