ORGANS OF NUTRITIOK. 



173 



Fig. 349. 



vascular bundles of the petiole in 

 the lamina or blade constitute the 

 ribs or veins of the leaves, which 

 have been already described under 

 the head of venation. 



The petiole is either simple or 

 undivided, as in all simple leaves, 

 and in those of a compound cha- 

 racter in which the leaflets are 

 sessile ; or it is said to be com- 

 pound when it divides into two 

 or more portions, each of which 

 bears a leaflet. The divisions of 

 the petiole or the stalks of the 

 leaflets are then called petiolules, 

 stalklets, or partial petioles, while 

 the main petiole is called the rachis 

 or common petiole. 



The petiole is frequently more j^,v,. 349. section of a stem and 



the base of a leaf, showing 

 the passage of a fibro-vas- 

 cuhir bundle /r into the pe- 

 tiole. pCypc. Parenchyma- 

 tous tissue of the stem and 

 petiole, c. Pulvinus. /. Ar- 

 ticulation between the leaf 

 and stem. h. Leaf bud in the 

 axil of the leaf. 



,^e a scar, called a cicatrix 



or less contracted at the base 

 where it joins the stem owing 

 to the presence of an articulation 

 or joint {fig. 349, /). Leaves 

 thus furnished with an articu- 

 lated petiole fall away from the 

 stem after they have performed 

 their functions ; in doing so they lej 



{fig. 193), which frequently exhibits on its surface several little 

 points, which are produced by the rupture of the vascular 

 bundles of the petiole {fig. 1 93). The outline of the cicatrix 

 and the arrangement of the vascular bundles \sa'j much in 

 different species, and thus frequently form characters by which 

 we may distinguish plants after the leaves have fallen. Lesti- 

 baudais has also endeavoured to show, that the number and 

 distribution of the vascular bundles of the petiole influence, to 

 some extent at least, the arrangement of 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 also frequently articulated to the rachis, so that 

 each leaflet becomes detached separately from the common 

 petiole when the leaf begins to decay, as in the Sensitive Plant. 

 By many botanists, indeed, no leaf is considered truly com- 

 pound unless it presents 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 practice, and when we consider that the presence of 

 an articulation is by no means constant even in simple leaves, I 



