574 
DR JOHN M‘LEAN THOMPSON ON 
It frequently occurred that when the liberation of a leaf-trace made a heavy 
drain on the tracheides of the outer cylinder, so that the parenchymatous inner 
xylem was locally in direct parenchymatous connection with the parenchyma beneath 
the leaf-trace-xylem, a parenchymatous bridge appeared in transverse section passing 
athwart the xylem-ring. But many chains of parenchyma traversed the inner xylem 
from side to side at points other than the insertions of leaf-traces, and in not a few 
instances were continued almost to the periphery of the outer xylem-cylinder. 
Where the latter was locally parenchymatous or thin these rays were sometimes seen 
in transverse section to be continuous from the inner to the outer surface of the 
xylem. The rays were usually narrow and followed varying courses, and, in branch- 
ing, ran in obliquely longitudinal directions, both backwards and forwards in the 
stem. In no case observed did they disturb , or threaten , the unbroken continuity of 
the inner and outer endodermal bands. 
From the above account it will be apparent that no leaf-gaps were found in the 
length of stem examined, and that the local depletion of the outer xylem-cylinder, 
or a parenchymatous development within it, led to the formation of rays which 
traversed the xylem from side to side, and that since such local depletions of the 
outer xylem-cylinder were unavoidable at the points of departure of leaf-traces, it 
was at these points that continuous rays were most frequently seen in transverse 
sections of the stele. 
No special interest was presented by the origins of the root-traces. The latter 
were restricted to the lower surface and lower portions of the sides of the stele. 
They originated like the leaf-traces by a protrusion of the outer xylem-cylinder, and 
frequently coincided with xylem-rays, but in departing they created no gaps in the 
stelar ring by which direct continuity of pith and cortex could be established. 
The facts thus summarised have been fully illustrated in Plates I, III, and IY of 
the memoir mentioned above (5). With the facts recorded by Dr Poiratjlt and 
Mr Boodle, they provide the only record of stelar structure which has been made 
public for Platyzoma. They bear only one interpretation, namely, that in the 
materials so far investigated the stele comprises a conductive cylinder with both 
outer and inner endodermis. It is- devoid of leaf-gaps and perforations , and 
includes centrally a pith completely isolated from the cortex. The phloem is present 
only on the outer face of the xylem. The stelar structure is of the so-called 
ectophloic-siphonostelic type. 
This curious stele has been the subject of some discussion, and it will be well to 
draw together and briefly consider the theoretical statements which have been made 
regarding it. In the memoir already referred to (3), Dr Poiratjlt considered that 
the stelar structure of Platyzoma might perhaps be brought in line with that of a 
Eugleichenia. In explanation of this view he suggested that the pith is the result 
of confluence and final isolation of a series of foliar sclerotic pockets. This con- 
ception arose from the fact that in the leaf-trace of certain Gleichenias a mass of 
