ON THE STRUCTURE AND AFFINITIES OF METACLEPSYDROPSIS DUPLEX. 167 
GENERAL STRUCTURE. 
In order to obtain a general idea of the organisation of this fern—Metaclepsydropsis 
duplex—it will be most convenient to follow its tissues from above downwards. By 
proceeding in this way we shall find that we pass from what is already known to what 
is unknown. In the first place we shall concentrate our attention on the xylem tissue, 
partly because it is better preserved and more continuous than any other, but chiefly 
because it changes very considerably as we descend. 
In the smallest divisions of the pinne with which we are acquainted—namely, 
tertiary pinnee—the trace is curved in form, with tapering, incurved ends. In the 
sinus, formed between the hooked ends and the body of the trace, the protoxylem 
elements may be found. These bundles then are C-shaped or horseshoe-shaped, and 
they are emitted, alternately on each side, from the ends of a similarly shaped secondary 
pinna-trace. At the point of emission of the tertiary pinna-trace the combined 
trace of tertiary and secondary pinna has four protoxylem groups. Lower down the 
two inner groups die out, and the resulting trace is again C-shaped, with hooked ends, 
and two protoxylem groups in the sinuses formed by these hooks. 
The smallest traces known are distinctly curved, and have parenchyma in the 
concavities. In one or two examples, however, the secondary pinna-trace is not open, 
although it gives off open tertiary traces. In this case the emission of the small strand 
strongly resembles the departure of the secondary pinna-trace from the primary one in 
the genus Clepsydropsis. PI. IV. fig. 42 is a good example of this emission. 
The occurrence of a similar “closed” bundle has also been noted in several primary 
pinnee (Pl. IV. fig. 43). This probably only takes place some way along the pinna, 
because, as such primary pinne are followed downwards into the petiole, each pinna- 
trace becomes like a much flattened C with hooked ends. No primary pinna-trace has 
been observed which was “closed” while still passing through the cortex of the petiole. 
Apart from these few exceptional examples, the pinna-traces of one order join those of 
a lower order alternately on opposite sides as described above. 
When we come to the primary pinnz, however, a change takes place. They do not 
enter the petiole separately but in pairs (Pl. IV. fig. 40, pen. t.), and these pinna-pairs 
enter alternately on each side of the petiole. Two primary pinna-traces, then, enter 
the cortex of the petiole at the same level and pass downwards to join the petiole trace. 
(Before joining the petiole-trace, however, each pair of pinna-traces unite to form one 
xylem are, as will be explained later (PI. Il. fig. 19, b).) They are placed symmetri- 
cally one on each side of the principal plane of the petiole, and are thus “ mirror images ” 
in this plane of symmetry. A short distance below the point where the two primary 
pinna-traces enter the cortex of the petiole, two pairs of very small traces may be seen 
to pass into the petiole. These are the aphlebia-traces. These also are “ mirror 
images” of one another in the principal plane of the petiole. They are situated out- 
side the pinna-traces. A transverse section through a petiole at this level shows seven 
