COMPARATIVE ANATOMY OF GYMNOSPERMS 303 



nostelic like Adiantum pedatum and Pteris aquilina; and Lygi- 

 nodendron siphonostelic, without internal phloem, as is gener- 

 ally the case in Osmunda, but resembling this genus in the occa- 

 sional occurrence of internal sieve-tissue. The only striking 

 anatomical difference between the cycadofilicinean forms de- 

 scribed above and the parallel cases from the ferns lies in the 

 absence of secondary growth in the latter. This feature is now 

 known to be of minor importance, although great weight was 

 attached to it by the Brongniartian school of paleobotanists. 



In regard to the particular type of the Cycadofiliees which 

 gave rise to the Gymnosperms there is some difference of opin- 

 ion. Potonie, 9, n Worsdell, 16, 17 and Jeffrey 10 consider the 

 Cycads to be derived from Medullosa-like ancestors through a 

 Lyginodcndron-like phase, by the gradual disappearance of the 

 internal secondary wood, and the final suppression of the cen- 

 tripetal primary wood. Scott, 6, 15 on the other hand, regards 

 Lyginodendron as the ancestral type, and as derived directly 

 from Heterangium by the formation of an intrastelar pith, and 

 not from medullosan ancestors by reduction. He further con- 

 siders the Medullosae to constitute merely a side branch of the 

 phylogenetic tree, and expresses the opinion that " we should 

 involve ourselves in unnecessary complications if we endeav- 

 ored to derive the simple primary structure of the cycadean 

 stem from the more elaborate organization of a Medullosa." 

 However, examples of phylogenetic progression from the com- 

 plex to the simple are not at all uncommon. Striking illustra- 

 tions of this principle are afforded by the derivation of the 

 simple hyoid bone of the mammals from the complex hyoid 

 apparatus of the lower vertebrates, and the evolution of the 

 monodactyl horses of the present day from their four-toed an- 

 cestors of the Eocene. The histological structure of the medulla 

 in Lyginodendron strikingly resembles that of the cortex in the 

 presence of sclerotic nests, and this feature indicates strongly 

 community of origin of the medullary and cortical tissues. 

 Further, the occasional occurrence of internal phloem and in- 

 ternal secondary wood in Lyginodendron can most easily be ex- 

 plained as a vestigial relic of a siphonostelic condition, in which 

 internal phloem was normally present — i. e., a Medullosa with 

 a single series of bundles. 



In regard to the special pteridophytic ancestry of the Cyca- 



