76 PRACTICAL BOTANY 



LEAF 



A. PETIOLE 



Observe that the mature leaf of the Sunflower consists of an 

 upper, flat, expanded portion the lamina, and a lower, narrow 

 stalk the petiole, by which it is inserted on the stem. Note 

 the channelled upper surface of the petiole, and the broad 

 insertion on the stem : in the angle between the petiole and the 

 stem may usually be observed an axillary bud, or shoot. 



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 con- 

 cave being the superior, while the convex is the inferior surface : 

 thus the petiole is dorsi-ventral whilst the stem is polysym- 

 metricai. This property extends also to the arrangement of 

 the vascular bundles, of which the xylem is as a rule directed 

 towards the upper surface of the petiole. 



2. The presence of numerous stomata ; beneath each stoma 

 the collenchyma is replaced by chlorophyll, containing paren- 

 chyma with intercellular spaces. Note beneath each stoma an 

 enlarged intercellular space the air-chamber. 



3. The number and arrangement of the vascular bundles. 

 In the petiole there are three main bundles, besides several 

 smaller ones. 



4. The absence of interfascicular cambium : the larger 

 bundles are for a short time at least, open bundles, having an 

 active cambium, while the smaller ones are closed, having no 

 secondary thickening by cambium. 



