OSMUNDACKAK 



537 



central pith : as it does so its endodermis opens, and becomes continuous 

 with that which completely surrounds the stelar system (Fig. 298 A, B, C}. 

 The latter, as seen in transverse section, is composed of several layers 

 of parenchyma at the periphery : then follows a band of phloem which 

 is continuous, ' but may be uneven in width, extending inwards at the 

 medullary rays. Within this are the xylem-strands, which vary greatly in 

 number. O. claytoniaua may have as many as 40, O. regalis about 1 5 

 (in Fig. 298 A there are 14). Todea barbara 8 or less (in Fig. 2986 there 

 are 8, in c there are only 3), while in T. superba the xylem may form an 

 unbroken cylinder. The position of the protoxylem also varies : in Osmunda 

 it is nearly on the inner edge of the metaxylem, but in Todea the xylem 

 is mesarch, or in T. hymenophylloides the strands are almost exarch. 



l-'lc,. 298. 



.J =diagram showing the arrangement of the vascular tissue in the axis of Osiuuinia 

 regalia (after Zenetti). B and C = transverse sections of the stele of Todea barbara, with 

 leaf-trace (after Seward and Ford), showing the greater continuity of the xylem than in 

 Ostunda. s = phloem. s/=sclerotic tissue. 



Centrally lies the pith : in some cases an internal endodermis is present 

 (O. cinnanwmea, T. hymenophylloides), while in the former species some 

 internal phloem has been found locally in several specimens examined 

 by Jeffrey 1 and by Faull.'-' 



If the course of the several strands be followed 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 in different cases, but in all the inter-communication 

 of the whole system is exceedingly close and effective. The scheme is 

 represented for Osmunda in Fig. 299 A, as flattened into a single plane, 

 and for Todea, where the number of the strands is less, in Fig. 299 B. 

 A comparison of these diagrams with the figures of transverse section 

 will explain the main features of the system of the axis. 



There are two possible views as to the nature of this system of the 

 axis, as seen in the living Osmundaceae : either that it is a result of reduction 



1 Phil. Trans., vol. cxcv., p. 119, etc. 



-"Anatomy of the Osmundaceae," Bot. Gaz., 1901, p. _}Si. 



