182 FORMS OF THE PETIOLE. 
of air cavities, as in Pontederia; such petioles by diminishing the 
specific gravity of the plants in which they are found, enable them 
to float readily in the water. At other times the petiole becomes 
flattened at its base, and embraces the stem, in which case the leaf 
is said to be amplexicaul or clasping (fig. 281); 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 ; as in the Orange (fig. 320, p), Venus’s Flytrap (fig. 
375, p), Sweet Pea (fig. 385), 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 decwrrent (fiys. 283 and 376). Besides the above forms 
of petiole, others still more 
Fic, 377. remarkable occur, which will 
‘> be alluded to hereafter, under 
ae 2, the head of Anomalous Forms 
ne YP of Leaves (page 185). 
Generally speaking, the 
petiole is less developed than 
VEZ) the lamina; it is also com- 
“ monly shorter than it, and 
iQ) f is of sufficient thickness to 
Le support it without bending. 
When the petiole is very long 
or thin, or when the lamina 
is very heavy, and in other 
cases, it becomes more or 
less bent downwards towards 
the earth, and no longer 
supports the blade in a hori- 
zontal direction. 
6. STIPULES. 
Stipules are small leafy 
bodies situated at the base, 
and usually on each side of 
the petiole of simple (fig. 274, 
| 9 
Fig. 377. A portion of the flowering stem of os 8), ae compound (fig. 377), 
the common Pea, with a pinnate leaf ter- leaves. They have the same - 
minated by a tendril, and having two gtructure as the blades of 
large stipules at its base, the lower mar- ] d li 1bl eager 
gins of which are dentate. eaves, and are Hable to Simit- 
lar modifications as regards 
venation, apex, incision, outline, margins, surface, &c. The 
stipules are often wanting, and the leaves are then said to be 
exstipulate; when present the leaves are stipulate. They are 
often overlooked from their small size; while in other cases 
they are very large, as in the Pansy (fig. 379), and in the common 
