496 



MOSSES AND FERNS 



CHAP. 



in most species occupied by elongated sclerenchyma cells, with 

 no intercellular spaces. 



The central vascular cylinder contains, as is well known, 

 several, usually transversely placed, tracheary plates, alter- 

 nating with phloem masses, and surrounding these a varying 

 amount of parenchyma. In upright species the tracheary 

 plates are often more or less completely confluent, and in cross- 

 section have a somewhat star-shaped outline. In the dorsi- 

 ventral stems the tracheary plates are quite separate and per- 

 fectly transverse in position. Their outer angles are occupied 



FIG. 28%. A-D, Lycopodium volubile; A, transverse section of the stem, Xi8; /, leaf- 

 base; B, tissues of the central part of the stem, X about 200; C, sieve-tube show- 

 ing lateral sieve-plates, X about 600; D, section of the wall of a sieve-tube; E, 

 section of the leaf of L. lucidulum, X35. 



by the small primary spiral or annular tracheids, from which 

 the centripetal formation of the large scalariform elements 

 proceeds exactly as in the leptosporangiate Ferns. The mass 

 of tracheary tissue is compact, and contains no parenchyma- 

 tous elements. According to Strasburger the oblique end 

 walls of the large tracheids show the same elongated pits as the 

 lateral walls, but in no cases could any communication between 

 'adjacent tracheids be demonstrated. Each tracheary mass is 



