COMPARATIVE DISCUSSION 



487 



"these two great stocks appear to have been separate back to the 

 beginning of the period when the palaeontological record begins." If 

 this were so, the anatomical difference would, in all cases, indicate a true 

 phyletic distinction. It is necessary to obtain a clear idea whether or 

 not this will hold good. 



The structural difference is based upon the greater or less dominance 

 of the leaf in the whole shoot ; the phyllosiphonic type going, as a rule, 

 with a megaphyllous state. But megaphylly may have been attained along 

 more than one line of descent. If it arose in more than one phyletic 



tr 



FIG. 268. 



is tannensis. Transverse section of the sterile region, high up. The proto- 

 xylem (/?-. xy.~) is mesarch. The xylem of the stele is fading out, and being replaced 

 by parenchyma ; three of the tracheides (/. tr.) show incomplete development ; there 

 is no longer a complete ring, and the leaf-trace bundles (/. t.) enter the gaps which result, 

 in much the same way as in a phyllosiphonic type. There is no definite endodermis. 

 X 150. 



line, then the phyllosiphonic state, which is its internal structural expres- 

 sion, will also have originated more than once. If this were so, then the 

 phyllosiphonic structure would not necessarily indicate affinity, and the 

 Pteropsida, as based on the structural point, could not be held to be a 

 natural group. The question will therefore be whether there is any evidence 

 of the origin of a phyllosiphonic from a cladosiphonic state. It might be 

 expected either in a shoot, with increasing proportion of the leaves, or of 

 decreasing proportion of the axis. The latter is the state of the distal 

 region of the shoot of Tmesipteris, and Fig. 268 shows the condition there 

 seen : the two larger tracts of xylem are separate ; but isolated elements 

 showing imperfect lignification link them together : the cauline stele is here 

 seen in course of disintegration into mere leaf-traces : these enter the 



