180 W. T. GORDON 
subsequent development of the petiole-trace long arms are gradually produced, and 
the trace then becomes H- or X-shaped. Primary pinne depart from the petiole 
in pairs, one pair alternately on each side, so that four rows of primary pinne are 
produced on the one petiole. The root-traces also leave the stem xylem in a 
protostelic manner ; they are diarch and barrel-shaped. 
As we have seen, the stele of JZ. duplez is also circular in outline, and the xylem 
consists of two kinds of tracheide. Both kinds are long, pointed, and reticulately 
thickened, except the smallest in the inner zone, which have scalariform thickenings 
on their walls. There is conjunctive parenchyma present, however. The stem 
branched dichotomously and was a rhizome, as in the last-mentioned species. 
The petiole-trace leaves the stem-stele in a protostelic manner as an elliptical mass 
with two sunk protoxylem groups, but its subsequent development is quite different 
from that of Diplolabis rémeri. No arms are produced, but both ends of the trace 
become dilated. In D. rémeri the island of parenchyma enclosed by the entering 
pinna-trace-bar is constant in size, whereas that in MZ. duplex gradually diminishes 
until it is exceedingly small, when it opens to the exterior, and a small groove is 
left in the petiole-trace instead of the wide V-shaped groove between the arms in 
D. vimerr. 
The primary pinne are borne in four orthostichies just as in Dzplolabis, but 
occasionally the trace becomes closed as in Clepsydropsis. The roots of M. duplex 
are also quite similar to those of D. rémerz. I have gone into considerable detail in 
this comparison, as these two species have many points of similarity and form two 
important links in a possible chain of evolution among the Zygopteridee. 
In comparing M. duplex with Ankyropteris corrugata, a great similarity in the 
shape of the stele must be noted. Both have two zones of tracheides, and there is con- 
Junctive parenchyma in the inner zone in each. But while only a few of the elements 
in the inner zone in M. duplex are scalariform tracheides, all the elements in both zones 
of A. corrugata have that type of thickening. It has also been shown that the arms of 
parenchyma and tracheidal tissue which radiate out from the inner zone of the stem- 
stele do not persist for any great vertical distance in M. duplex. In the case of A. cor- 
rugata, on the other hand, these arms seem to persist for a greater vertical distance—so 
much so, that the outer zone of the stele-xylem appears to consist of five groups of 
tracheides alternating with five parenchymatous arms which project from the inner zone. 
The largest tracheides in A. corrugata appear in the centre of each group, and in 
M. duplex the largest are in the central portion of the outer xylem ; but if the radiating 
arms broke up the outer ring as in A. corrugata the smaller tracheides would be situated 
along the sides of the arms, and then the large tracheides would occupy a central position 
in the resulting groups of the outer xylem zone. In both species these radiating arms 
of parenchyma and tracheides are intimately connected with the petiole-trace departure, 
and this emission is more distinctly protostelic in character in M. duplex than in A. 
corrugata. In the type of branching shown by both species there is a striking simi- 
ne ae ee 
