116 
ORGANOGRAPHY. 
BOOK I. 
by cellular substance. Its figure is generally half cylindrical, 
frequently channelled on the surface presented to the hea- 
vens ; but in some monocotyledonous plants it is perfectly 
cylindrical, and in others it is a thin leafy expansion, sur- 
rounding the stem [fig. 55. 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 leaflet (foliolum) : in such cases 
it is by some said to be compound ; each of the stalks of the 
leaflets being called petiolules or stalklets {ramastra^ Jungius). 
In all simple leaves the petiole is continuous with the axis of the 
lamina, from which it never separates ; in all truly compound 
leaves the petiole is articulated with each stalklet; so that, when 
the leaf perishes, it separates into as many portions as there 
are leaflets, as in the Sensitive Plant : hence, whenever an 
apparently simple leaf is found to be articulated with its 
petiole, as in the Orange, such a leaf is not to be considered a 
simple leaf, but the terminal leaflet of a pinnated leaf, of 
which tlie lateral leaflets are not developed. This is a most 
important difference, and must be borne constantly in mind 
by all persons who are engaged in the investigation of natural 
affinities. It is a secret sign which must never be neglected. 
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 
coiisslnet (fig. 56. a). At the opposite extremity of the petiole, 
where it is connected with the lamina, a similar swelling is often 
remarkable, as in Sterculia, Mimosa sensitiva, and others : this 
is called the struma^ or, by the French, hourrelet (fig. 56. h). 
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, 55. a). Wlien the 
lower part only of the petiole is sheathing, as in Umbelliferae, 
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 
ligida (fig. 55. h) (langnette, Fr.; collar Rich.) : for the na- 
