CHAPTER XXI 

 THE FILICALES 



This group of vascular plants presents the features of the 

 Pteropsida in their most primitive and least modified condition. 

 Large leaves are consequently the rule, and these normally, when 

 functioning as sporophylls, bear numerous sporangia on the lower 

 or abaxial (dorsal) surface. When the central cylinder of the 

 stem is siphonostelic, as is most frequently the case, the traces of 

 the leaves take their departure from the wall of the stelar tube 

 with the formation of foliar gaps subtending the departing strands. 

 In the Filicales reproduction is always by means of spores, which 

 are in general isosporous, but which in a few instances represent the 

 heterosporous condition. The Filicales constitute a remarkably 

 clearly denned group, at least so far as their modern representatives 

 are concerned; and the only family which has had its affinities 

 with the Pteropsida brought into question is the Ophioglossaceae, 

 regarded in some quarters as derived from lycopsid ancestry. 

 This attribution of affinity, however, is not now considered justi- 

 fied. The Filicales constitute the largest element composed of 

 vascular cryptogams in the existing flora of our earth and are 

 on that account of great importance from the evolutionary stand- 

 point. Anatomical problems which, in the Lycopsida, are difficult 

 of elucidation by reason of the large degree to which the group has 

 suffered extinction in the existing flora are much more advanta- 

 geously approached, in the group under consideration, as a conse- 

 quence of the large number of forms which are offered for study 

 by the existing pknt population of the earth. The value of 

 the filicinean Pteropsida is particularly great in respect to the 

 fibrovascular structures, and an attempt will be made in the 

 present connection to utilize these to the full. Naturally, in a 

 group surviving in relatively large numbers only the more salient 

 and significant facts can be brought into prominence in an 

 elementary treatise like this. 



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