398 Mr Gordon, On the relation between the 



On the relation between the fossil Osmundaceae and the 

 Zygopterideae. By W. T. Gordon, M.A., B.Sc. (Edin.), Falconer 

 Fellow of Edinburgh University, and Advanced Student Exhi- 

 bitioner of Emmanuel College. (Communicated by Mr E. A. 

 Newell Arber.) 



[Read 7 February 1910.] 



The structure of the axis in the living Osmundaceae has given 

 rise to two main contentions as to the evolutionary position of the 

 group. The axis is regarded by Jeffrey* and Faullf as reduced 

 from a more complex dictyostelic ancestor, and by Boodle j, Seward 

 and Ford|, Chandler ||, TansleyH, and Kidston and Gwynne- 

 Vaughan** as the latest development in an ascending series from 

 a protostelic ancestor. 



This latter view is supported by the ontogenetic development 

 of living species and, now that Kidston and Gwynne-Vaughan 

 have completed their studies on the fossil representatives, by the 

 phyllogeny of the group as well. 



The present living genera may be placed in two categories. 

 Osmunda regalis may be taken as one extreme type where the 

 leaf gaps are long and a cross-section of the stem consequently 

 showing a ring of several xylem strands. In cases where the 

 leaf gap is short as in Todea barbara and T. superba a cross-section 

 shows the xylem as a much more continuous ring. This may be 

 looked upon as the other extreme. 



The stems of the most recent fossil representatives of the 

 Osmundaceae conform to these two extremes. Even Osmimdites 

 Skidegatensis, Penhallow, is of the Osmunda regalis type, though 

 it may be regarded as an even more extreme example. (This 

 species seems to lend colour to Jeffrey's reduction theory, but 

 the weight of the other evidence completely overrules this 

 single exception.) The tertiary species Osmundites schemnicensis 

 (Pettko) and 0. Dowkeri, Carruthers, resemble Osmunda regalis, 

 and so does the Jurassic form 0. Gibbiana, Kidston and Gwynne- 

 Vaughan. 0. Dunlopi, Kidston and Gwynne-Vaughan, another 

 species from the Jurassic, has a xylem ring which is almost later- 



* Jeffrey, Pliil. Trans. Roy. Soc. Vol. cxcv. 1902, p. 127. 



+ Faull, Bot. Gaz. Vol. xxxii. 1901, p. 418. 



t Boodle, Ann. Bot. Vol. xvii. 1903, p. 515. 



§ Seward and Ford, Trans. Linn. Soc. London, Series 2, Vol. vi. 1902, p. 254. 



II Chandler, Ibid. Vol. xix. 1905, p. 406. 



U Tansley, New Phytologist, Eeprint No. 2, 1908. 



** Kidston and Gwynne-Vaughan, Trans. Roy. Soc. Edinburgh, Vol. xlv. Pt. 3, 

 1907, p. 759; ibid. Vol. xlvi. Pt. 2, 1908, p. 213: ibid. Vol, xlvi. Pt. 3, 1909, 

 p. 651. 



