572 
DR JOHN M'LEAN THOMPSON ON 
an uninterrupted endodermis which was immediately followed by a few layers of 
large-celled parenchyma. This parenchymatous zone was followed on its outer 
surface by a ring of xylem which varied in thickness as one passed from the 
upper to the lower surface of the stele. On its upper side the xylem-ring 
was thin ; on its lower side it was relatively thick. The xylem was composed 
of scalariform tracheides together with chains and groups of parenchymatous cells 
which followed irregular courses between the tracheides, and which fused and 
branched repeatedly. The xylem was roughly divisible into two concentric cylinders 
according to the dimensions of the tracheides and the distribution of the 
parenchyma. The inner cylinder was composed of wide tracheides associated with 
numerous chains and masses of parenchyma, and formed the essentially parenchy- 
matous part of the xylem. The outer cylinder was composed mainly of small 
tracheides which were closely packed together. Parenchyma did not bulk largely 
in the organisation of this outer cylinder, the chains and groups being 
usually short and narrow, and their component elements were few. This outer 
cylinder supplied the xylem to the traces of the leaves and roots. The difference 
in xylem-thickness between the lower and upper sides of the stele was due mainly 
to a massing of parenchymatous inner xylem towards the lower surface of the 
stele. The inner xylem fluctuated from point to point either in the direction 
of increase or of decrease. The greatest increases were in those regions where 
the departing traces were most crowded ; on the other hand, the inner xylem 
was usually less prominent in those portions of the stele where the traces were 
large and scattered. 
The phloem followed almost immediately upon the outer surface of the xylem, 
and was separated from the xylem locally only by a single layer of small parenchy- 
matous cells. It varied in bulk from point to point, but seldom exceeded five 
layers in breadth. It was composed entirely of typical fern sieve-tubes, and was 
interrupted only at those points where leaf- and root-traces were departing. The 
phloem was followed externally by a large-celled pericycle of three to four 
parenchymatous layers, and beyond this was a continuous outer endodermis. These 
features are in complete harmony with those described by Poiratjlt and Boodle. 
In the formation and departure of both large and small leaf-traces one general 
method was adopted. The first indication of the formation of a leaf-trace was the 
marking-out of a peripheral tangential band of tracheides from the outer xylem- 
cylinder. Beneath this tracheidal band a definite mound of parenchyma appeared, 
while outside it the phloem, pericycle and outer endodermis were gently curved. 
As the band of xylem became more distinct and arched, the phloem upon its convex 
face became thinner. The xylem-band then freed itself from the stelar-xylem, 
and passed out as the arched xylem of a leaf-trace. In some cases it severed its 
connections with the stelar-xylem simultaneously on both sides ; in others it freed 
itself more rapidly on one side than on the other. In either case, while the phloem 
