PTEROPSIDA 123 



as if it had been formed from a cambium, and many mor- 

 phologists describe it as secondary wood. Botrychioxylon 

 paradoxum had a very similar appearance, but in this stem 

 the cells of the inner cortex were also regularly arranged in 

 radial rows. It would seem, therefore, that the whole of the 

 growing point of the stem must have been organized in a 

 pecuharly regular manner and that great caution should be 

 used in describing even the outer xylem as secondary. 



Some species of Tubicaulis were tree ferns, while others 

 were epiphytes. They are characterized by a frond form 

 which approached closely to that of a present-day fern, for 

 the fronds coming off in spiral sequence from the stem were 

 pinnate and the pinnae were arranged in one plane. 



Stauropteris is represented by two species, S. burntislan- 

 dica from the Lower Carboniferous and S. oldhamia from 

 the Upper Carboniferous. Although the method of branching 

 of the frond was similar to that of many of the Zygopteri- 

 dales, differences in the vascular system are sufficient to 

 warrant the creation of a separate family. The most import- 

 ant of these is the absence of islands of parenchyma in the 

 xylem of the petiolar traces. It is beheved that, so far, only 

 portions of fronds have been found and that the stems have 

 yet to be discovered. Fig. 18A shows how the frond of S. 

 burntislandica was constructed, pairs of pinnae arising alter- 

 nately along the petiole, each associated with aphlebiae. 

 Then each pinna gave rise to secondary pinnae in the same 

 way and this pattern was repeated at all levels of branching 

 within the frond. The vascular system of the petiole of S. 

 oldhamia (Fig. 18B) consisted of four regions of xylem either 

 contiguous or separate from each other, each with a mesarch 

 protoxylem. The smaller branches, however, tended to have 

 a single tetrarch strand. 



Perhaps the most interesting feature of all about Stauro- 

 pteris burntislandica is the fact that it was heterosporous. 

 Its megasporangia (Fig. 18C), when found isolated, are 

 called Bensonites fusiformis. They were strangely fleshy at 



