AFFINITIES OF LY.GINODENDREAE 419 



in the vegetative structure, including habit as well as 

 anatomy, between Heterangium and Lyginodendron is 

 sufficiently close to leave no doubt that they belonged 

 to the same family, and presumably bore a similar type 

 of fructification. Nevertheless the fact of the proto- 

 stelfc structure of the Heterangium stem, apparently 

 so unlike anything known in other Spermophyta, will 

 give a -peculiar interest to the discovery of the repro- 

 ductive organs of this genus (compare Sutcliffia, p. 447). 



But while Heterangium was in all probability a 

 Seed-plant like Lyginodendron, it is important to recall 

 the points in which it shows affinity with the Ferns. 



The external habit was altogether that of a Fern, 

 so much so that no modern botanist would have 

 hesitated* in placing it, so far as these characters are 

 concerned, among the Filices. The internal organisa- 

 tion to a great extent confirms this conclusion. The 

 concentric structure of the vascular bundle in the 

 petiole is quite Fern-like, while an even more im- 

 portant character, the primary structure of the stem 

 agrees wonder(ully closely with that of some of the 

 simpler Filices. We must, of course, go to a protostelic 

 Fern for an analogy, and it is with the simpler species 

 of Gleichenia that the correspondence is closest. In most 

 Gleichenias the single stele consists of a central mass of 

 xylem, surrounded by a zone of phloem. The xylem 

 is* made up of intermixed tracheides and conjunctive 

 parenchyma, just as in Heterangium, and the position 

 of the protoxylem is the same in the recent as in the 

 fossil genus, for in both, the spiral tracheides form a 

 number of groups* a little within the periphery of the 

 wood, of which the greater part is consequently centri- 



38 



