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



Structure of the stem. 



A feature of some importance in regard to the internal structure of the axis is shown 

 in some of the specimens described above, under C. This is the occurrence of a very dis- 

 tinct strand running up the centre of the stem. On the impressions these strands appear 

 as longitudinal median furrows averaging 1,6 — 2,5 mm. in breadth. They appear in 

 varying distinctness in most of the specimens of this kind and are shown in pl. 1, figs. 

 14 — 17, and in pl. 4, fig. 5. In the branching specimens shown in pl. 1, fig. 14, and pl. 

 4, fig. 5, it can be noted that the strand of the main axis gives off, at a nearly right angle, 

 another strand, apparently of about the same size, which runs out into the branch. 



Sometimes the strand itself is preserved in a carbonized sta te; but the tissue is then 

 generally so altered and penetrated with some ferruginous matter that it cannot be exam- 

 ined microscopically — not even after treatment with fluoric acid to remove the mi- 

 neral substance. In the specimen in pl. 1, fig. 17, however, the carbonized remains are 

 fairly unaltered and can be examined under the microscope, though it is very difficult 

 to make satisfactory preparations. The carbonized material can be treated with Schultze's 

 mixture; but if ammonia is added as a clearing agent the whole is dissol ved. It was 

 possible, however, to get an idea of the general structure. The whole preserved portion 

 of the strand was found to consist of a mäss of tracheids. As far as could be seen, these 

 are all of the scalariform type. They are rather long: in no case was it possible to observe 

 their ends; their diameter is about 0,05 mm. It was found difficult to photograph the 

 tracheids because in thick masses they are not sufficiently translucent, and if the macer- 

 ation is continued long enough to separate them, they are usually destroyed. Fig. 19, 

 pl. 1, shows a part of an isolated tracheid. Fig. 18 of the same plate is an enlarged photo- 

 graph of a part of the surface of the specimen from which this tracheid was taken. The 

 impression of the vascular strand shows 2 or 3 distinct furrows corresponding to project- 

 ing ridges on the strand itself. In the furrows on the impression a part of the carbonized 

 vascular tissue of these ridges still remains. Fig. 20, pl. 1, is a photograph in higher 

 magnification of the impression of the central strand in another stem. Here some re- 

 mains of the carbonized tracheids still adhere to the impression and are seen fairly dis- 

 tinctly in black tones against the light surface of the rock. 



It was naturally impossible to prepare sections of the strand. As far as it could 

 be studied by means of dissecting the carbonized mäss of tracheids under the micro- 

 scope, it would appear that the vascular tissue formed a solid column without any pith. 

 There is naturally no trace of any phloém. With the exception of the central strands, 

 the stems have evidently been of very soft texture, as the strands could cause impres- 

 sions in the matrix on either side, and they were therefore probably composed of paren- 

 chyma. 



The central strand now described has been noted or figured by some writers from 

 other specimens of the species, though it has not hitherto been possible to ascertain its 

 nature by means of a microscopical examination of the tissue. Dawson, in his first de- 

 scription of the species (1871, p. 43), says: »The flattened stems of Arthrostigma gracile 

 often show distinct indications of a slender central axis, probably of scalariform vessels, 

 though the structure is obscure ». Some of Dawson' s figures show traces of these strands, 



