THE FOSSIL OSMUNDACE.E. 221 



There is no reason to believe that this actually represents an empty space in the living 

 plant ; on the other hand, it was doubtless occupied by some kind of tissue. 



The xylem is composed of tracheides alone, without any trace of xylem parenchyma. 

 The peripheral elements of the ring (figs. 3 and 4, o. xy.) are most distinctly smaller 

 than the more central (figs.* 3 and 4, i. xy.), and their average size increases gradually 

 towards within until a certain point is reached not far from the inner margin of the 

 ring. At this point a distinct change is noticeable in the general aspect of the xylem, 

 owing to the presence of a number of conspicuously larger tracheides. In fact, two 

 definite zones may be distinguished in the xylem ring, although the difference between 

 the two kinds of xylem is not very pronounced in transverse section. In longitudinal 

 section, however, the outer xylem zone is seen to consist of very elongated elements 

 with slowly tapering and often very finely pointed ends (fig. 4. o. xy.), which appear to 

 have undergone much sliding growth. On the other hand, the elements of the inner zone, 

 and especially the larger ones, are comparatively short, and they are somewhat contorted in 

 form (fig. 4, i. xy.). They are also more or less square-ended, with transverse or at most 

 slightly oblique terminal walls. Wherever the flattened elements at the inner margin are 

 sufficiently well preserved, they show the same characteristics as those of the inner xylem. 



Close within the external periphery of the xylem ring occur some twenty to twenty- 

 five well-defined mesarch groups of protoxylem, evenly distributed all round the stele 

 (fig. 3, prx., and fig. 11). These protoxylems are really the downward prolongations of 

 the protoxylems of the leaf- traces, which are decurrent for some distance into the xylem 

 of the stem, gradually dying out below. It is possible to regard the small elements 

 that line the outer periphery of the stele as a continuous exarch protoxylem proper to the 

 stem itself, and apart from the decurrent mesarch protoxylems of the leaf- trace. At the 

 same time, however, they are rather large for protoxylem elements, and they do not differ 

 in form or in the nature of their pitting from the contiguous elements of the metaxylem. 



The mesarch protoxylems decurrent from the leaf-traces consist of a few very small 

 tracheides, which are typically scalariform ; that is to say, each of their walls has only 

 one series of pits. On the other hand, the elements of the rest of the xylem have two 

 or more vertical series of pits on almost every wall. It is only on the narrower walls ; 

 up to about 40 m broad, that a single series is found (fig. 5 a). A more frequent 

 marking is that of two regular vertical series of more or less horizontally elongated 

 oblong pits (figs. 5 b and c). The unpitted region separating the two series varies 

 considerably in breadth, and should it exceed a certain limit, a third series of pits 

 appears in. the middle of the wall (fig. 5 d). The pits of this middle series are 

 usually smaller and more irregular than those of the lateral, being oval or almost 

 rounded in outline. More rarely three or even four regular series of pits, all similar in 

 size and form, are met with (figs. 5 e and/"). When the number of series is still greater 

 the pits are usually quite irregular in arrangement and oval or elliptic in outline 

 (figs. 5 g and h). In the large elements of the inner zone of xylem this irregularity in 

 arrangement is so great and the pits are so numerous that the thickened part of the 



TRANS. ROY. SOC. EDIN., VOL. XLVI. PART II. (NO. 9). 33 



