400 Mi^ Gordon, On the relation between the fossil, etc. 



forming an inner and an outer zone. The thickenings on all the 

 elements are reticulate, except the protoxylem groups, where they 

 are scalariform. The leaf trace departs in a protostelic manner, 

 and has two immersed protoxylem groups. 



Among the Botryopterideae, B. cylindrica (Williamson), from 

 the Coal Measures, represents probably the highest known type 

 of development. Here the inner elements of the stem xylem 

 are smaller than those of the outer xylem. In B. antiqua, 

 Kidston, there is only one type of xylem element. In the last 

 species I have been able to show that the leaf trace is at first 

 mesarch. 



The thickenings on the xylem elements in these three groups 

 are interesting. In the Osmundaceae we get a series from 

 multiseriate pits through scalariform to reticulate thickenings. 

 Reticulate tracheides only occur in the archaic forms Zalesskya 

 and Thamnopteris and then only in the inner wood. The last- 

 mentioned type of thickening must therefore be considered the 

 most primitive. Among the Zygopterideae we find a similar 

 series from scalariform to reticulate (in D. Romeri) ; in the 

 Botryopterideae from scalariform to reticulate. 



Except for the multiseriate pits on the external xylem elements 

 Thamnopteris could not be distinguished from the zygopterid 

 D. Romeri on stem structure. The leaf trace has of course typi- 

 cally only one protoxylem group, but occasionally examples with 

 two are found. This feature may be considered as being probably 

 primitive since the double group ultimately functions as if it 

 were a single one. In D. Romeri the leaf trace has typically two 

 protoxylems and I have never observed either more or fewer than 

 that number. It is clear then that at first sight in their most 

 primitive representatives the two groups of the Osmundaceae and 

 the Zygopterideae cannot be distinguished in their anatomical 

 features. The two groups show exactly parallel development 

 from the two primitive types just cited, and it thus seems prob- 

 able that they have diverged from a common stock. The 

 geological position of the various representatives of these groups 

 points to the conclusion that the Zygopterideae diverged from 

 the parent stock before the Osmundaceae did; and the greater 

 suitability of the latter group for present conditions of growth 

 has allowed it to persist after the Zygopterideae had perished. 



I desire to express my indebtedness to Mr E. A. Newell Arber, 

 under whose supervision my work is being carried on, for his 

 criticism and advice. 



