130 MANUAL OF BOTANY 



continuous round the narrow ends of the wood masses (fig. 

 885). 



The monostehc arrangement persists in a few families 

 (OsmundacecB, Hymenophyllacece, &c.) ; tlie bundles in the stele 

 are then sometimes collateral. 



The pericycle is absent from the stele of some of the Poly- 

 podiacese, being then replaced by an inner layer of the endo- 

 dermis, as in the root of Equisetum. 



The stem of the fern is usually well supplied with scleren- 

 chyma, which is developed in various forms in the ground 



Fig. 8SG. 



Fig. 886. Conceulric buudle from Fern stem. en. Eudodermis. p. Peri- 

 cycle. ph. Phloem, px, Protoxylem. x. Xylem. 



tissue. It is often found as a strong hypodermal sheath, deep 

 brown in colour, and consisting of several layers of cells with 

 very greatly thickened walls. Isolated bands of greater or less 

 extent are common in the inner part of the ground substance 

 (fig. 884). 



The tissue of the wood and bast is generally characteristic. 

 The wood is chiefly made up of large tracheids thickened in a 

 scalariform manner. True vessels are rare, and there is not 

 much wood parenchyma. The sieve-tubes of the bast (fig. 887) 



