338 FILICALES [CH. 



bearing a superficial resemblance to that oWsmundaClaytoniana, 

 which may be the foliage borne by Osmundites skidegatensis. 

 The xylem cylinder is broken by the exit of leaf-traces into 50 

 or more strands varying in size and shape, and it is noteworthy 

 that the phloem is also interrupted as each leaf-trace is given 

 off. In recent species the xylem cylinder is almost always 

 interrupted, but the phloem retains its continuity. In the 

 Canadian fossil an internal band of phloem occurs between the 

 xylem and the pith, and this joins the external phloem at each 

 leaf-gap. This internal phloem finds an interesting parallel in 

 certain recent species^, but in these the internal and external 

 phloem do not meet at the foliar gaps as they do in the extinct 

 type. In Osmunda cinnamomea the internal phloem occurs 

 only at the regions of branching of the stem stele; in the fossil 

 it is always present. 



It is clear that Osmundites skidegatensis represents the most 

 complex type of stem so far recognised in the Osmundaceae; it 

 illustrates a stage in elaboration of the primitive protostele in 

 advance of that reached by any existing species. 



The primitive Osmundaceous stele was composed of solid 

 xylem surrounded by phloem (Thamnopteris and Zalesskya); 

 at a later stage the xylem cylinder lost its inner zone of 

 wide and short tracheae and assumed the form seen in 

 Osmundites Kolhei, in which the centre of the stele consists of 

 -parenchyma with some tracheae. Another type is represented 

 by 0. Dowkeri in which the pith is composed wholly of paren- 

 chyma and the xylem ring is continuous. From this type, 

 by expansion of the xylem ring and by the formation of over- 

 lapping leaf-gaps, the form represented by Osmunda regalis was 

 reached. Osmunda cinnamomea, with internal phloem in the 

 regions of stelar branching, probably represents a further stage, 

 as Kidston and Gwynne-Vaughan believe, in increasing com- 

 plexity due to the introduction of phloem from without through 

 gaps produced by the branching of the stele. In Osmundites 

 skidegatensis the leaf-gaps became wider and the external phloem 

 projected deeper into the stele until a continuous internal 



1 See p. 314. Also Jeffrey (03) ; Faull (01) ; Seward and Ford (03). 



