RACHfOPTERIS CYLINDRICA. 65 



help us but little towards a knowledge of the position which Rachiop- 

 teris cylindrica should occupy in the Vegetable Kingdom. In the 

 Memoir referred to at the outset, Williamson found it extremely 

 difficult to form a reasonable conjecture on this point, and ultimately 

 remarks, " it may be a Fern Stem, though I know no recent type of 

 Fern which it resembles ; it may be the root of some type of Fern, 

 an idea suggested by the tendency to a concentric arrangement of the 

 cortical cells ; or it may belong to some dwarf type of Lycopodiaceous 

 plants." Whether in the last sentence Williamson meant that it 

 might be the root or the stem is not certain. 



From what has been said of the histology of the stele, botanists will 

 allow that its characters do not support the view that the axis of 

 Rackiopteris cylindrica is a root. Nor is it otherwise with the mode of 

 branching. We may conclude then with some confidence that it is 

 either a stem structure — a caulome, in fact — of some kind, or the 

 phyllopodium of a foliar organ. 



The further question as to whether it should be referred to the 

 Lycopodiacece, or to the Filices, cannot be answered definitely. William- 

 son tells us that " the entire series of \p.ls] sections of this plant dis- 

 plays a considerable resemblance " to " sections of the aerial and 

 subterranean stems of Psilotum triquetrum," but those I have examined 

 hardly confirm this. Indeed, if the small elements within the strand 

 of xylern are, as I presume, protoxylem elements, that fact of itself 

 would be evidence against Lycopodiaceous affinities. On the other 

 hand, it would not be inconsistent with what we know of the xylem 

 strands of Ferns. If, therefore, our choice is to be restricted to the 

 Lycopodiacece and Filices, the latter seem entitled to the preference. 

 As, however, there are other types of Carboniferous plants in addition 

 to those mentioned, it will be well to leave this point for future in- 

 vestigation, 



