OEGAXS OF XUTEITIOX. 



16/ 



The ramification of the 

 Fig. 346. 



elements essentially as the \rood itself, 

 fibro-vascular bundles of the petiole 

 in the lamina constitute the ribs 

 or yeins of the leaves, which have 

 been already described under the head 

 of venation. 



The petiole is either sira'ple or im- 

 divided, as in all simple leaves, and 

 in those of a compound character in 

 Tvhich the leaflets are sessile ; or it 

 is cora'pound -when it divides into two 

 or more portions, each of which bears 

 a leaflet {fig. 352). The divisions of 

 the petiole or the stalks of the leaflets 

 are then c-Ale^ i^ctiohdes, stalklets, or 

 jpariial petioles, while the main petiole 

 is called the rachis oTcornmon petiole. 



The petiole is frequently more or 

 less contracted at the base where it 

 joins the stem owing to the presence 

 of an articulation ot joint (fig. 346, 

 f). Leaves thus furnished with an 

 articulated petiole fall away from the 

 stem after they have performed their 

 functions ; in doing so they leave a 

 scar, called a cicatrix (fig. 185, b,b.). 

 This cicatrix frequently exhibits on 

 its surface several little points, which are produced by the rupture 

 of the vascular bundles of the petiole. The outline of the cicatrix 

 and the arrangement of its ruptured vascular bundles vary miich 

 in different species of plants, and thus frequently form charac- 

 ters by which we may distinguish one plant from another after 

 the leaves have fallen. It is probable that the number and dis- 

 tribution of the vascular bundles of the petiole influence, to some 

 extent at least, the arrangement of the leaves upon the stem, and 

 also their varying forms. In compound leaves the petiole is not 

 only generally articulated to the stem, but the partial petioles 

 are frequently articulated to the common petiole, so that each 

 leaflet becomes detached separately from the rachis when the 

 leaf begins to decay, as in the Sensitive Plant. By many botan- 

 ists, indeed, no leaf is considered truly compound unless it pre- 

 sents this characteristic ; consequently all leaves however much 

 divided, and apparently compound, but which have not their 

 separate portions articulated, are considered simple. Such a 

 distinctive character cannot, however, be well carried out in prac- 

 tice, and when we consider that the presence of an articulation 

 is by no means constant even in simple leaves, I can see no 

 sufficient grounds for insisting upon this character in the sepa- 



Fig. 346. Section of the stem 

 and the base of a leaf, show- 

 ing the passage of a flbro- 

 vascular bundle fv into the 

 petiole. ;3C,pc. Parenchyma- 

 tous tissue of the stem and 

 petiole, c. Pulvinus. /. Ar- 

 ticulation between the leaf 

 and stem. b. Leaf bud in the 

 axil of the leaf. m. pith. 



