292 PLANT STRUCTURES 



In Equisetum and Isoetes the vascular bundles may be 

 said to be collateral, as in the Seed-plants, but the charac- 

 teristic Pteridophyte type is very different. In fact, the 

 vascular masses can hardly be compared with the bundles 

 of the Seed-plants, although they are called bundles for 

 convenience. In the stele one or more of these bundles 

 are organized (Fig. 272), the tracheary vessels (xylem) being 

 in the center and completely invested by the sieve vessels 



X 



FIG. 273. Cross-section of concentric vascular bundle of a fern (Pteris): the single 

 row of shaded cells investing the others is the bundle sheath; the large and heavy- 

 walled cells within constitute the xylem; and between the xylem and the bundle 

 ehealh is the phloem. CHAMBERLAIN. 



(phloem). This is called the concentric bundle (Fig. 273), 

 as distinguished from the collateral bundles of Seed-plants, 

 and is characteristic of Pteridophyte stems. 



