THE FOSSIL OSMUND ACE^E. 



457 



contained others which were of a more resistant nature, which occur scattered through- 

 out its substance. These resistant elements are unmistakably tracheal in form and 

 appearance (figs. 4 and 14, M. P.). They sometimes occur singly, but are often found 

 more or less collected together into groups (fig. 7). Their walls are well preserved and 

 strongly thickened, and show a well-defined porose or reticulate pitting (figs. 5 and 6). 

 They are very irregular in form, but are all more or less elongated. They vary greatly 

 in diameter, often swelling out at some points into local inflations (figs. 6 and 7), having 

 the appearance of short, blunt lateral lobes ; at other points, again, they become very much 

 narrower. It is possible, however, that this variation in form may be due, to some 



Fig. \. — Osmuiuiite$ Kulbci. Diagram of the ring of xylem strands seen from the outside, showing how the leaf-traces 

 depart without at once breaking through the xylem ring. 



extent, to the varying pressure they experienced during the flattening out of the stele. 

 In addition to these undoubtedly tracheal elements, numerous other irregular vermiform 

 cells occur in the pith which have fairly firm walls, but, so far as we were able to observe, 

 have no pits or markings. All these elements are imbedded in a cellular matrix too 

 much disintegrated for any certain determination as to its original structure. 



There is no doubt whatever that the tracheal elements are true and real constituents 

 of the central tissue. It is impossible to explain them away as due to the accidental in- 

 trusion of elements from the normal xylem ring. It is evident, in fact, that the central 

 tissue of the stele was of the nature of a " mixed pith," and, we believe, essentially 

 similar to that occurring in the stele of Zygopteris Grayi, Z. corrugata, etc. It is 

 possible, however, that various other tissue-forms were also present, transitional between 

 the well-developed tracheae and the probably parenchymatous matrix. 



