SPECIES OF PERMIAN OSMUNDACEJE. 351 
Leaf-lruces. 
The leaf-trace leaves the stele in the same manner as in a protostelie stem. 
The place of origin of a leaf-trace appears at the periphery of the stele as a 
prominence in the central part of which is one small group of protoxylem 
elements. At a somewhat higher level the base of this prominence becomes 
separated from the periphery of the xylem by a narrow-celled tissue of the 
xylem-sheath. Ata still higher level the section of the leaf-trace, its cells 
filled with dark contents, is seen in the inner cortex as an oval strand in 
which the protoxylem has a mesarch position and is rather nearer to the 
adaxial side of the bundle (Pl. 32. fig. 4). The xylem is surrounded by : 
sheath of cells in which the elements of the second layer and sometimes of 
the third layer are characterized by brown contents. These ceils are to be 
observed round the whole vascular bundle, but are especially well developed 
on the adaxial and abaxial sides. _ Immediately behind these cells, and 
generally separated from them by one or two layers of empty cells, occurs 
the phloem, readily recognized by the comparatively large sieve-tubes, which 
are usually arranged in the form of ares three or four cells in width, both on 
the adaxial and abaxial sides of the bundle. In addition to the sieve-tubes, 
the phloem includes protophloem elements which occur on the outside as one 
or two layers of cells. Each bundle is surrounded by a series of cells with 
black contents which enclose it as a sheath uniting on the adaxial side of the 
bundle with a sheath of the same kind of cells with black contents that 
surrounds the stele. On the abaxial side of the bundle there may be about 
s. This sheath is clearly limited on the outside by : 
eight layers of such cel 
layer of empty cells of the inner cortex, disposed sometimes with the longer 
axis at right-angles to the leaf-trace. The cells with black contents are 
described by Kidston and Gwynne-Vaughan in 7 hamnopteris Schlechtendalii 
as resistant cells, because as a rule only these cells are preserved round the 
leaf-trace of that species as it passes through the outer cortex. In Thamno- 
pteris Sehlechtendalit the resistant cells of the endodermis are clearly seen as a 
black line, bounding on the outside the cells of the perieyele which abut on 
the protophloem. But in Thamnopteris Kidstoni, in spite of the excellent 
preservation of the tissue, it is not possible to distinguish with certainty the 
endodermis and perieycle from the protophloem. — This is especially difficult 
in the case of those leaf-traces where the phloem zone of the vascular bunale 
is feebly developed. Prosenchymatous cells appear on the adaxial side of 
the bundle; their number gradually increases until at a higher level they 
entirely replace the centripetal xylem; the oval mesarch vascular bundle 
becomes endarch, with a bay of parenchymatous tissue opposite the proto- 
xylem which gives it a falcate form. On the adaxial side the cells of the 
xylem-sheath with their brown contents appear behind the empty paren- 
chymatous cells of the bay, and at a higher level cells with black contents 
make their appearance ; these cells represent the mucilaginous sacs which 
