130 MANUAL OF BOTANY 



continuous round the narrow ends of the wood masses ifig. 

 885). 



The monostehc arrangement persists in a few families 

 {OsmundacecB, HymenojjhijUacece, &c.) ; the bundles in the stele 

 are then sometimes collateral. 



The pericycle is absent from the stele of some of the P0I3'- 

 podiaceae, 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- 

 chj-ma, which is developed in various forms in the ground 



Fig. 880. 



Fi'j. 886. Coucenlric buudle from Fern stem. en. Eudodermis. p. Peri- 

 cycle. ^>/(. Phloem, px. Protoxylem. x. Xylem. 



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

 brown in colour, and consisting of several la^^ers of cells with 

 very greatly thickened walls. Isolated bands of greater or less 

 extent are common in the inner part of the ground substance 

 (^^.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) 



