ON THE STRUCTURE AND AFFINITIES OF METACLEPSYDROPSIS DUPLEX. 177 
is one peculiarity, the broad waist, which links up the trace of fig. 20 with those of 
figs. 17 and 18. 
While it would be absolutely impossible to pass from the trace of Pl. II. fig. 17 to 
that of Pl. I. fig. 7, or even that of Pl. II. fig. 24, yet when all the intermediate stages 
are shown it becomes not only possible but quite simple. The whole development may 
be summed up thus :—The demands made on the xylem of the petiole by more and more 
robust pinna-traces have caused the petiole-trace (1) to become larger, and (2) to increase 
the size of the ends so as to accommodate the much stouter pinna-trace-bar. A closely 
similar conclusion was arrived at in the case of the petiole-trace in Diplolabis rémeri, 
but instead of the ends of the trace becoming inflated they were drawn out into two 
long arms at each end. 
In Pl. III. fig. 36 an interesting example is shown. The specimen was very short, 
but it happened to include the portion of the petiole to which the primary pinne are 
attached. ‘These pinnze were seen to be quite normal though small. This has led me 
to believe that the specimen must have been a portion near the top of a petiole. In 
that region we would naturally expect the trace to be smaller, and, if the hypothesis be 
correct that the hour-glass shape was impressed on the petiole-trace in order that it 
might accommodate more robust pinne, we would also expect that the petiole-trace 
near the apex would revert to its elliptical form, since the pinne diminish in size near 
the top of the frond. The trace in this example is exactly similar to that in Pl. II. 
fig. 21, except that it is smaller, and the latter figure represents a section well down 
towards the base of the petiole. 
Turning now to examine the primary pinna-trace, we find that one of the earliest 
obvious stages in its development on the petiole-trace is seen in Pl. LV. fig 39, where, at 
g, a round island of parenchyma is seen. At each side of this island there are groups 
of small elements—the protoxylem. Such a stage, however, is not the earliest that may 
be seen, for the protoxylem groups may be observed before the island is formed, z.e. 
when there is a groove at the end of the petiole-trace and not an island (PI. II. figs. 
17, 18, 19, 22, 23, 24, prx*,, prx*,). (The island has, of course, been formed by the 
production of xylem elements at the horn-like extremities of the sides of the groove. 
When the xylem outgrowths from the two sides meet in the centre the groove becomes 
an island of parenchyma.) The protoxylem groups may be distinguished at a still 
lower level, where the groove has become very shallow; indeed, the groove is a mere 
depression between the two protoxylems (Pl. II. fig. 21, pra‘), pra’, and fig. 17, pra’, 
prx’,). Pl. Il. fig. 18 is probably just below the level at which the last vestiges of 
these protoxylem groups are visible, although the bay or groove which results from the 
island opening to the outside is still seen to persist at C. 
A stage beyond that of fig. 39 is shown in PI. II. fig. 21, at the lower end of the trace. 
The island of parenchyma which was circular in the former figure is elliptical in the 
latter, and has still two protoxylem groups, one at each end of the major axis of the 
ellipse. At a still higher level (Pl. IV. fig. 39, lower end of trace) the island is much 
