20 T. G. HALLE, LOWER DEVONIAN PLANTS FROM RÖRAGEN IN NORWAY. 



stated to belong to the species P. princeps, though Solms-Laubach (1895, p. 74) thiiiks 

 that this was Dawson's opinion. The anatomical structure of this specimen, as of the 

 one figured in 1863, differs from that of the stems first referred to P. princeps through 

 the occurrence of an additional zone of »woody fibres» surrounding the »scalariform 

 axis». The outer layer, which is spöken of as »fibrous bark», is separated from the 

 cylinder of »woody fibres» by a thick zone which does not show any structure and 

 which probably was composed of parenchyma, though this is not actually stated. 

 The specimen which presents the structure now described does not show any leaves or 

 any surface-markings of any kind which could be regarded as a proof of specific identity 

 with the spine-bearing branches of Psilophyton princeps. If the specimen was really f ound 

 in situ in the Silurian limestone, it is very interesting, however, as proving the occurrence 

 on this low horizon of vascular plants with a stem-structure similar to that attributed 

 to Psilophyton. 



To sum up the evidence brought to light on the anatomical structure of Psilophyton, 

 it must be stated that there is a certain probability that at least one of the petrified spe- 

 cimens (Dawson 1863) belongs to this species. Not even in this case, however, is there 

 sufficient proof of specific identity with the impressions of spine-bearing stems. 



Among the Röragen specimens with the spines and the characteristic branching 

 of Psilophyton princeps, the tissue of the stem is sometimes preserved in a carbonized 

 state, but usually there is no trace of structure. The specimen in pl. 4, fig. 12, is an ex- 

 ception, as it shows in some places recognisable remains of vascular tissue. This is the 

 case especially in the place marked x in the figure. Under the microscope, the black 

 mäss in the centre is seen to consist of scalariform tracheids. The tracheids measure 

 about 0,05 mm. in diameter and have very dense horizontal thickenings. »Some of these 

 tracheids are shown in pl. 4, figs. 14 a and b. 



Though it is impossible to prepare a section of the stem, it seems to be fairly certain 

 that there is a comparatively narrow central zone composed of tracheids. This is especi- 

 ally evident in the branch given off to the right-hand side in the upper part of the spe- 

 cimen (fig. 13). Whether the central vascular zone formed a solid column or a pith was 

 developed is impossible to decide on the present material: if a pith existed, however. it 

 must have been very narrow. If compared with the central strand of Arthrostigma, 

 the vascular zone of Psilophyton princeps is comparatively thick. 



The specimen now described thus combines the external characters of Psilophyton 

 princeps with an internal structure which, as far as it can be studied, presents the same 

 main features as that of the petrified specimens referred by Daw t son to this species. 



Distribution and relation to other forms. 



Psilophyton princeps in the sense adopted above is not known from many different 

 localities. Even the specific identity of the Röragen form with the type-specimens from 

 Gaspé figured under the name of var. ornatum is not quite beyond doubt. The Röragen 

 specimens have not quite such long and thick spines as those seen in Da\vson's illustra- 

 tions; and White states that the latter are not exaggerated in this respect. Although 

 it is difficult to make any definite decision before the Canadian material has been re- 



