fossil Osmundaceae and the Zygopterideae. 399 



ally continuous, i.e. the leaf trace departs typically in a protostelic 

 manner. This also occurs occasionally in T. superba, so that 

 0. Dujdopi may be considered as pointing to Todea rather than 

 to Osmunda regalis. 



In the Jurassic period therefore forms existed which were 

 practically as divergent as the living ones are. The only differ- 

 ence is shown in 0. Dunlopi, where the leaf trace is typically 

 protostelic in its departure. The type of this last species is 

 still further accentuated in Upper Permian times in the case 

 of Zalesshya, Eichwald, and Thamnopteris, Eichwald. Both of the 

 last-named genera have not only the xylem ring continuous 

 laterally but they have no central pith. The central tissue of 

 the stem consists of short, reticulated tracheides. 



In the Permo-Carboniferous genus Grammatopteris Kidston 

 and Gwynne-Vaughan see the earliest example of a plant with 

 Osmundaceous affinities, and here we find a perfect protostele 

 with only one type of xylem tissue. This genus however is still 

 imperfectly known. 



The leaf trace in the fossil Osmundaceae also shows a very 

 distinct passage from exarchy to endarchy, and from protostelic 

 departure to cases where leaf gaps occur. The most interesting 

 example is probably Thamnopteris schlechtendalii, since in it we 

 occasionally find leaf traces with double mesarch protoxylem 

 groups. Even where such double protoxylems are found the 

 trace develops normally afterwards. 



Turning now to Zygopterid alliance, we find that the youngest 

 known representative is Asterochloena Corda ranging to the 

 Devonian. The stem xylem in this genus is stellate with exceed- 

 ingly long rays and a pith is always present. The trace has two 

 mesarch protoxylem groups. In the Permo-Carboniferous of Autun 

 Z. Brongniarti, Renault, occurs. The stele in this case has almost 

 circular xylem with a true pith and a system of internal tracheides 

 in that pith. The outer xylem ring is interrupted by the depart- 

 ing leaf traces. Z. Grayi, Williamson, from the Coal Measures, has 

 a stellate xylem but the rays are very short. A central pith and 

 system of internal tracheides are present. 



Corresponding to Z. Brongniarti in the Permo-Carboniferous we 

 have Z. Corrugata, Williamson, in the Coal Measures. The organisa- 

 tion of this stem is quite similar to Z. Grayi, but in outline it has 

 become more circular. The differences between the outlines in 

 these two species seems to be correlated with the greater length of 

 the internodes. The lately discovered stem of Diplolabio Romeri 

 Solms from Lower Carboniferous rocks exhibits the most primitive 

 type of zygopterid stem so far known. The stem as seen in trans- 

 verse section, in cases where no petioles are departing would be 

 classed as a Botryopterid. The wood is solid and of two kinds, 



26—2 



