14 Hick, on Rachiopteris cylindrica, Will. 



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 Lycopodia- 

 ceous 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 Rachiopteris 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. Williamson tells us that " the 

 entire series of [his] sections of this plant displays 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 xylem 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 investigation. 



