472 MOSSES AND FERNS chap. 



recognisable are spiral tracheids, both in the stem and leaves^ 

 and these are followed in the former by the much wider 

 scalariform tracheids that occupy the central part of the 

 tracheary plates in the fully-developed bundles. 



The fully- developed central cylinder of the stem -^ is un- 

 doubtedly to be considered as a group of confluent vascular 

 bundles or as gamostelic. The oval or nearly circular cross- 

 section (Fig. 243, C) is sharply separated from the surrounding 

 ground tissue by a clearly-marked endodermis, within which is 

 a pericycle which may be only one cell thick, but is usually 

 several-layered. According to Strasburger " this pericycle does 

 not properly belong to the central cylinder, but is of cortical 

 origin. The cutinised band (" radial folding ") of the endo- 

 dermal cells is only observable in the younger stages, as later the 

 whole wall of the endodermal cells becomes cutinised.^ This 

 cutinisation extends also through a number of the succeeding 

 cortical layers. The rest of the cortical region is in most 

 species occupied by elongated sclerenchyma cells, with no 

 intercellular spaces. 



The central vascular cylinder contains, as is well known, 

 alternating, 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 

 dorsiventral stems the tracheary plates are quite separate 

 and perfectly transverse in position. Their outer angles are 

 occupied 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 

 parenchymatous 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 surrounded by a single layer of 

 parenchyma, whose inner cell walls show bordered pits, like 

 those of the adjacent tracheids. 



^ Russow (i), p. 128 ; De Bary (3), p. 281 ; Strasburger (11), vol. iii. p. 458. 

 2 Strasburger, I.e. p. 460. ^ Strasburger, I.e. ; Van Tieghem (5), p. 553. 



'^ Strasburger, I.e. p. 459. 



