lie 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 

 wliich the 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 

 coussinet {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 seiisitiva, and others : this 

 is called the struma, or, by the French, hourrelet {Jig. 56. 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 [Jig. 55. a). Wlien the 

 lower part only of the petiole is sheathing, as in Umbelliferse^ 

 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 {Jig. 55. h) {lauguctte, Fi'.; collare. Rich.) : for the na- 



