256 



BOTANY 



points that the bundles are given off to the leaves. The bundles are 

 usually concentric in structure, but in the Ophioglossaceae and Osmun- 



FIG. 220. A, Pteridium aquilinum. Cross-section of vascular bundle from the 

 rhizome; en, endodermis ; s, sieve-tubes; (, scalariform vessels. B, part of two 

 large scalariform tracheae. C, sieve-tube of Struthiopteris Germanica (x 375). 

 (A, B, after ATKINSON.) 



daceae they are truly collateral. In some of the larger species of 

 Botrychium there is a genuine secondary growth, with a true cam- 

 bium, like that in the stem of normal 

 Dicotyledons or Conifers. In the 

 typical Ferns (Fig. 220) a section 

 of a stem-bundle appears circular 

 or oval. It is clearly separated 

 from the ground-tissue by a well- 

 marked bundle sheath or endo- 

 dermis, composed of cells with 

 radially folded walls. The endoder- 

 mis is the innermost layer of the 

 cortex. Within this are one or two 

 layers of cells forming the "Peri- 

 cycle." The tracheary tissue is 

 made up of large prismatic tracheids, with conspicuous narrow trans- 

 verse pits the "scalariform" elements which are typical of the 



FIG. 221. Aneimia hirsuta. Scleren- 

 chyma from the rhizome, showing 

 the lamination of the cell-walls and 

 pits (X 250). 



