136 



OSMUNDACEAE 



[CH. 



at its periphery: then follows a band of phloem which is continuous, but 

 may be uneven in width, extending inwards at the xylic gaps. Within 

 this are the xylem-strands, which vary greatly in number. O. claytonimia 

 may have as many as 40, O. regalis about 15 (in Fig. 426, A, there are 14), 

 Todea barbara 8 or less (in Fig. 426, B, there are 8, in C there are only 3), 

 while in T. stiperba the xylem may sometimes form an unbroken cylinder. 

 The position of the protoxylem also varies: in Osvnmda it is nearly on the 

 outer edge of the metaxylem, but in Todea the xylem is mesarch, or in 

 T. hymenophylloides the strands are almost exarch. Centrally lies the pith: 

 sometimes an internal endodermis is present ((9. cmnamomea, T. hymeno- 

 phylloides), while in the former species some internal phloem has been found 

 locally (see Vol. I, p. 133 ; also Jeffrey, Phil. Trans, cxcv, p. 1 19, etc.; Faull, 

 Bot. Gaz. 1901, p. 381 (Fig. 427). 



Fig. 427. Stele of a full-grown stem of Osinunda 

 <:m;;aw(?wm, from a photograph by Gwynne-A^aughan. 

 For details see Vol. I, p. 133. (X25.) 



If the course of the several xylem-strands be followed further, they are 

 found to fuse downwards according to a regular scheme, so that they form 

 a cylindrical network of which the meshes are very long and narrow. The 

 number and proportions of these vary, but in all the intercommunication of 

 the whole dictyoxylic cylinder is close and effective. The scheme is repre- 

 sented for Osinimda in Fig. 428, A, as flattened into a single plane; and for 

 Todea where the number of the strands is less in Fig. 428, B. In the latter, the 

 xylic gaps are shorter than they are in Osinunda. The structure thus de- 

 scribed is not far removed from the protostelic state: moreover, the leaf-trace, 

 being undivided and containing at its insertion on the stele only a single 

 group of protoxylem, indicates a relatively simple vascular structure. There 



