386 <A TEXTBOOK OF GENERAL BOTANY 
solid stele, in which there is no pith within the vascular sys- 
tem. A more advanced type is the siphonostele (Fig. 427), in 
which the vascular system is in the form of a cylinder that in- 
closes a central pith. Where a bundle that supplies a leaf leaves 






iD 
RSH IL 
\ < )- 
y 
oY | ON 
d As ‘ ey s 
4) @, die, om ] S ON 
Le A y oO a\ 
ex \ 
‘\ 







Fie. 427. Section of a stem of a maidenhair fern (Adiantum), showing a 
siphonostele 
Note that there is a central pith composed largely of thick-walled sclerenchyma 
cells, and that outside the stele is a wide cortex. The xylem is bordered on 
both sides by phloém. (x 35) 
the siphonostele, an opening, or leaf gap (Fig. 428), occurs in 
the cylinder above the place of origin of the leaf bundle. When 
these gaps are greatly elongated and extend through several 
nodes, the cylinder consists of a coarse mesh, and in cross sec- 
tion the stele appears to be composed of a number of separate 
bundles (Fig. 429). Such a type of stele is merely a modification 
