SPECIES OF PERMIAN OSMUNDACEÆ. 355 
bundles near the stele are generally cut obliquely because of the wide angle 
at which they are given off. They bend sharply towards the periphery, and 
are seen as oval strands with an almost central protoxylem. At a higher 
level these bundles become somewhat elongated tangentially and, in conse- 
quence of this, they are cut more or less transversely in a transverse section 
of the stem (PI. 33. fig. 4). The metaxylem group of small tracheids, which 
was previously almost central in its position, is shifted towards the adaxial 
side of the bundle and eventually passes from a mesarch to an endarch type 
of structure. At the same time the oval bundle becomes crescentic owing 
to curvature on the adaxial side. Examination of a transverse section of 
the stem shows that this change takes place very rapidly, and it is not 
possible to follow the gradual reduction in the centripetal xylem and its 
replacement by the parenchyma that has been observed also in the leaf-traces 
of T. Kidstoni and T. Schlechtendalii, This is explained by the sharp 
deviation of the leaf-trace from the stele and by the imperfect preservation 
of the tissues of the xylem-sheath and phloem which form round the vascular 
bundles a band composed of an imperiectly preserved mass of cells ; this 
band abuts on a space left by the pericycle and is limited towards the outside 
by a brown line of endodermal cells. At a higher level the vascular bundles 
of the leaf-trace become more and more crescentic, and the single group of 
protoxylem branches into two (Pl. 38. fig. 5) : one protoxylem group occurs 
at each end of the median line on the concave, adaxial half of the trace. 
Before the entrance of the leaf-trace into the outer cortex three of the proto- 
xylem groups become differentiated ; one group is situated on the median 
line and the other two on the flanks, one on each side. It is important 
to note that on the abaxial side of the vascular bundle at each horn of the 
crescentic xylem, which consists of tracheæ of the ordinary size, their occur 
along the edge linear groups of protoxylem. This occurrence of linear 
groups of narrow tracheæ on the flanks of the abaxial side of the bundle 
appears to be characteristic of 7. Gwynne-Vaughan, as in T. Kidstoni and 
T. Schlechtendalii nothing similar has been observed. At its entrance into 
the outer cortex the leaf-trace is accompanied by an enveloping sheath of 
parenchymatous cells the tissue of which is badly preserved. Among the 
parenchymatous cells of this sheath, on the abaxial side of the trace and 
abutting on the phloem, there occur elements which correspond in structure 
and position to the secretory sacs that occupy a corresponding position in 
T. Kidstoni. The seeretory sacs are accompanied by a group of cells with 
brown contents, and these cells appear to correspond to the elements in 
T. Schlechtendalii called by Kidston and Gwynne- Vaughan resistant cells. 
Similar cells are seen in certain leaf-traces on the abaxial side in the form of 
an are which passes across to the adaxial side and envelops the whole leaf- 
trace. It is possible to observe also in certain leaf-traces secretory sacs on 
the abaxial side, where they occur as a narrow band immediately adjacent to 
