90 PRACTICAL BOTANY. 



LEAF. 

 A. PETIOLE. 



External Characters. Note in the leaf of the Sun- 

 flower the channelled upper surface, and the insertion 

 on the stem by a broad Pulvinus; in the axil may 

 usually be observed an axillary bud. 



I. Cut transverse sections of the petiole and mount 

 in glycerine. The details of structure resemble in many 

 respects those of the young stem, from which the petiole 

 differs in the following points : 



1. The general outline of the section is semilunar, the 

 concave being the superior (ventral), while the convex 

 is the inferior (dorsal) surface : thus the petiole is dorsi- 

 ventral whilst the stem is polysymmetrical. (This pro- 

 perty extends also to the vascular bundles, of which the 

 xylem is as a rule directed towards the upper surface.) 



2. In the presence of numerous Stomata (two guard 

 cells, cf. infra) ; beneath each stoma the collenchyma 

 is replaced by chlorophyll-containing parenchyma with 

 intercellular spaces. Note beneath each stoma the 

 large respiratory cavity. 



3. In the number and arrangement of the vascular 

 bundles. In the petiole there are three main bundles, 

 besides several smaller ones (cf. observation of stem with 

 the naked eye, p. 47). 



4. In absence of interfascicular cambium, the larger 

 bundles are, for a time at least, open bundles, [i.e. have 

 an active cambium,] while the smaller ones are closed 

 [i.e. have no secondary thickening by cambium.]. 



