THE FILICALES 285' 



only be included, but in some instances may be entirely shut off 

 from the similar tissues outside. Gr>od illustrations of this con- 

 dition are provided by species of the Gleicheniaceae, Polypodiaceae 

 (notably the common bracken fern), and Marattiaceae. The 

 possibility, then, of the inclusion of the pith within the stele must 

 apparently be granted. 



The second argument against the cortical origin of the medulla 

 is directed against the evidence based on the similar histological 

 nature of pith and cortex in many cases, a resemblance which 

 is the more marked the more the medullary tissues are in histo- 

 logical continuity with those lying outside the tubular stele. It 

 is assumed by the advocates of the tracheary origin of the pith 

 that the striking histological similarity which often exists 

 between the medulla and the cortex is merely "physiological." 

 This argument would be worth more if its exponents did not admit 

 all histological evidence in favor of the tracheary origin of the 

 medullary tissues while denying that indicating their derivation 

 from the fundamental system. Such argument is clearly fallacious, 

 and the logical procedure is to admit to equal consideration evi- 

 dence for the tracheary hypothesis of the appearance of the medulla 

 on the one hand and that for its derivation by the inclusion of 

 cortical or fundamental tissues on the other. The advocates of 

 the cortical origin of the central parenchyma of the tubular stele 

 are apparently advantageously situated in this respect, for they 

 can equally well emphasize the extremely abundant data for 

 the origin of the pith from the fundamental system and at the 

 same time explain away the meager evidence for the tracheary 

 derivation of the medullary tissues. So far as the plentiful evi- 

 dence in the case of the Filicales is concerned, it seems beyond 

 reasonable doubt that the median parenchyma of the tubular 

 or siphonostelic central cylinder has come from the outside and 

 is not the result of internal differentiation within the stele. 



The question of the origin of the pith is a very important one 

 from the phylogenetic standpoint, since correct views in regard 

 to this matter are necessary for the interpretation of the evolution 

 of the tubular cylinder from lower to higher forms. It would 

 be going beyond the range of the present somewhat elementary 



