186 PKACTICAL BOTANY 



dons : it is especially well seen in the aerial roots of Pandanus 

 or in the larger roots of Dracaena. 



Cut tangential sections through the velamen, and observe the 

 irregular form of the tracheides, and the uneven thickening of the 

 walls, the extreme outer wall being beautifully marked : where 

 the sections have traversed the exodermis note the wavy appear- 

 ance of the radial walls (see p. 76), and the somewhat regular 

 alternation of long and short cells : the walls of the longer 

 cells are distinctly cuticularized. 



Longitudinal median sections should be prepared through the 

 apex of the root : in these the root-cap will be seen covering the 

 tip, externally to the velamen. 



