CHAP. IT. 
LEAVES. 
139 
Its figure is generally half cylindrical, frequently channelled 
on the surface presented to the heavens ; but in some mono- 
cotyledonous plants it is perfectly cylindrical, and in others it 
is a thin leafy expansion, called the sheath, or vagina, sur- 
rounding the stem [fig. hi. a). If the petiole is entirely 
absent, which is often the case, the leaf is then said to be 
sessile. Generally the petiole is simple, and continuous with 
the axis of the leaf; sometimes it is divided into several parts, 
each bearing a separate leaf or leafiet [foliolum) : in such cases 
it is said to be compound ; each of the stalks of the leaflets 
being Q^Wedi petiolules or stalklets [ramastra, Jungius). In 
some leaves the petiole is continuous with the axis of the 
lamina, from which it never separates ; in others the petiole 
is articulated with each stalklet ; so that, when the leaf 
perishes, it separates into as many portions as there are leaf- 
lets, as in the Sensitive Plant. When an apparently simple 
leaf is found to be articulated with its petiole, as in the 
Orange, such a leaf is not to be considered simple, but as the 
terminal leaflet of a pinnated leaf, of which the lateral leaflets 
are not developed. This is an important difference, and 
must be borne constantly in mind by those engaged in the 
investigation of natural affinities. 
At the base of the petiole, where it joins the stem, and 
upon its lower surface, the cellular tissue increases in quan- 
tity, and produces a protuberance or gibbosity, which Ruellius, 
and after him Link, called the pulvinus, and De Candolle 
coussinet [fig. hi. a). At the opposite extremity of the pe- 
tiole, where it is connected with the lamina, a similar swell- 
ing is often remarkable, as in Sterculia, Mimosa sensitiva, and 
others : this is called the struma, or, by the French, hourrelet, 
(fig. 57. b). 
Occasionally the petiole embraces the branch from which 
it springs, and in such case is said to be sheathing ; and is 
even called a sheath, or vagina, as in grasses [fig. hi. a). 
When the lower part only of the petiole is sheathing, as in 
Apiaceae, that part is sometimes called the pericladium. 
In grasses there is a peculiar membranous process at 
the top of the sheath, between it and the blade, which has 
received the name of ligula [fig. hi . h) ; for the nature of 
