THE FILICALES 291 



the condition which the supporters of this hypothesis regard as 

 primitive that of the tubular stele without internal phloem and 

 endodermis. On the other hand, the Filicales, in -which the sipho- 

 nostele commonly incloses a pith strikingly resembling the tissues 

 of the cortex and quite generally lined inwardly with endodermis 

 and phloem, must be regarded as more modern. The improbability 

 of this general hypothesis seems very great. 



It may be assumed on the basis of the considerations advanced 

 in earlier paragraphs that the siphonostelic condition in the Filicales 

 is susceptible of complication through the development of medullary 

 strands on the one hand, and, on the other, of simplification 

 through the loss of internal phloem and endodermis and the 

 progressive narrowing of the leaf gaps resulting in sequestration 

 of the pith. The view that the latter simpler condition is more 

 primitive not only runs counter to the conditions shown in the 

 general sequence of types in geological time, but is also at variance 

 with the general principles of comparative anatomy detailed in a 

 former chapter of this work. The hypothesis that the simpler 

 condition of the tubular stele is more primitive than the more 

 complex marks, moreover, an evolutionary attitude which is 

 becoming generally obsolete as our knowledge of the actual organ- 

 ization of extinct forms, particularly of those types which mani- 

 fested their greatest luxuriance in the past, becomes fuller and 

 more complete. It is now clear from evidence of this kind that 

 the lower gymnosperms have come from the Filicales as a result 

 of the simplification and reduction of the primary structures 

 of the stele of the stem on the one hand, accompanied by the 

 marked development of secondary fibrovascular tissues on the 

 other. In the gymnospermous Pteropsida this anatomical progress 

 has been associated with the attainment of the seed habit. In 

 the Lycopsida, by contrast, a similar anatomical progress culmi- 

 nated in much earlier geological times and was not accompanied 

 by the evolution of true seeds. 



