THE FOSSIL OSMUNDACE^. 227 



" Knorria" or partially decorticated condition of the actual stem. The true external 

 surface of the stem, and still more so that of the coating of leaf-bases, would present a 

 very different appearance. Numerous leaf- traces and root-steles are cut through on 

 their way out through the cortex. The former arise in a close spiral upon the stele. 



Structure of the Stele and Cortex. 



As in Zalesskya gracilis, the xylem forms a perfectly continuous ring and the leaf- 

 traces depart in a protostelic manner. The break in the xylem seen to one side of the 

 stele (fig. 22) is obviously due to the crumbling away of the stone. This also accounts 

 for the absence of tissue in the centre of the stele. Fig. 23 shows clearly that the 

 broken inner margin is not the natural limit of the xylem, but the same tissue extended 

 further in towards the centre of the stele. The whole mass of the xylem is 6 '4 mm. 

 in diameter, but the widest part of it still preserved only measures 2*5 mm. across. If, 

 however, an equal amount of it had been preserved all round, the central area still 

 unaccounted for would only be 1 '4 mm. wide. 



As in Zalesskf/a gracilis, xylem parenchyma is entirely absent, and two distinctly 

 different regions can be observed in the xylem — a central and a peripheral (figs. 23 and 

 24). The contrast between the two kinds of xylem appears to be much more marked in 

 Zalesskya diploxylon than in Z. gracilis, but this is perhaps due to the fact that we 

 have so much more of the central xylem present in Z. diploxylon than in the other 

 species. The elements of the outer xylem zone are very elongated, with more or less 

 pointed ends. Those at its extreme periphery are not much smaller than the rest, but 

 just where the outer zone borders on the central xylem there is a somewhat sudden and 

 fairly distinct decrease in the general size of the elements (fig. 23, at x). The elements 

 of the central xylem are much wider than those of the outer zone, and in longitudinal 

 section the contrast between the two kinds of xylem is even more strongly marked than 

 in the transverse (fig. 24). This is chiefly due to the fact that, in addition to being- 

 wider, the elements of the central xylem are also much shorter than those of the outer 

 zone, and further, their terminal walls are transverse, or at most only slightly oblique 

 (figs. 24 and 25). In fact, the tracheae of the central xylem might well be described as 

 inflated, oblong, or sack- shaped ; some of the inner ones not being more than twice as 

 long as broad. 



The elements of both kinds of xylem have multiseriate pits, those of the outer 

 zone having usually two vertical series of oblong pits ; but in the central xylem as many 

 as five regular series were observed. Where the pits are more numerous than this they 

 are more or less irregularly arranged and the trachese have a reticulate or porose 

 appearance. In spite of their parenchymatous and irregular form there is no doubt that 

 even the inmost of the elements preserved belonged to the xylem, being in fact true 

 pitted tracheae. Further, we are strongly of the opinion that in the living plant similar 

 elements filled up the whole of the vacant space that now occurs in the centre of the 

 stele. In fact, we believe that the xylem was solid throughout. 



