122 PRACTICAL BOTANY 



LEAF 



i. The Common Bifacial type 



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 

 concave being the superior, while the convex is the 

 inferior surface : thus the petiole is dorsi- ventral, 

 whilst the stem is polysymmetrical. 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. In the presence of numerous stomata; beneath 

 each stoma the collenchyma is replaced by chloro- 



