7 6 



BOTANY 



a one, being open on the upper side (Fig. 33). The 

 petiole is generally not cylindrical in shape, but flattened 

 on the upper face. It is continued with little change of 

 structure into the blade, where it appears as the mid-rib. 

 The blade of a stalked leaf is the ultimate portion of 

 the outgrowth with the flattened wing which has been 



" ep 



-co 



FIG. 33. Section of petiole of Primula sinensis. ep, epidermis; co, 

 cortex; en, endodermis ; pe, pericycle; ph, bast; x, wood. 



developed along its two edges. The part between the 

 two wings is generally known as the mid-rib. If we cut 

 a section through the blade (Fig. 20) we find its structure 

 adapted to the work for which the flat part was de- 

 veloped. On both surfaces we find an epidermis, the 

 lower one especially pierced with stomata. Under the 

 upper epidermis the cells are long and narrow and 

 arranged side by side much like the vertical railings 

 of a fence. They touch each other along nearly their 



