FOKMS OF PETIOLE. [BOOK i. 



exactly the same physiological value as the leaf itself. Its figure 

 is generally half cylindrical, frequently channelled on the 

 surface presented to the heavens ; but in some monocotyle- 

 donous plants it is perfectly cylindrical, and in others it is a 

 thin leafy expansion, called the sheath, or vagina, surrounding 

 the stem (fig. 57. 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 said to 

 be compound ; each of the stalks of the leaflets being called 

 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 leaflets, 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 Euellius, 

 and after him Link, called the pulvinus, and De Candolle 

 coussinet (fig* 57. 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 sensi- 

 tiva, and others : this is called the struma, or, by the French, 

 bourrelet, (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. 57. a). 

 When the lower part only of the petiole is sheathing, as in 

 Apiacese, 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 



