ON THE STRUCTURE AND AFFINITIES OF METACLEPSYDROPSIS DUPLEX. 169 
traces—the petiole-trace, two primary pinna-traces, and two pairs of aphlebia-traces 
(text-fig. 2, No. 3, 4, and 5, and PI. IV. fig. 40). 
Lower down in the petiole the four aphlebia-traces unite to form two, or, if we 
momentarily change our point of view and follow the aphlebia-traces upwards, the 
single strands each bifurcate (Pl. IV. fig. 40, aph. tr.). In Pl. IV. fig. 39 a section 
at a still lower level is shown. The aphlebia-traces have almost joined on to the outer 
ends of the pinna-traces. In Pl. III. fig. 38 the junction is complete, but the four 
protoxylem groups on each combined-trace indicate that there has been a fusion of two 
distinct strands. ‘lhe two inner protoxylem groups die out downwards, and at the 
level of Pl. III. fig. 37 each combined-trace has two protoxylem groups, one at each end. 
Proceeding still lower, the two combined-traces unite to form the pinna-trace-bar, on 
which there are four protoxylem groups. A short distance below this the bar joins on 
to the hourglass-shaped xylem of the petiole. In Pl. II. figs. 19 and 18 the bar (bd) is 
shown immediately before and immediately after its union with the petiole strand. 
(These figures are not, however, from the same specimen.) An island of parenchyma 
thus appears at the end of the petiole-trace (Pl. II. fig. 18, 7s), and round the periphery 
of the island there are six protoxylem groups (PI. Il. fig. 21). Four of these belong to 
the pinna-trace-bar and two to the petiole-trace. These latter lie on the sides of a 
| small bay situated at the end of the hourglass-shaped petiole strand (Pl. II. fig. 21, 
prx,, prx’,). Still continuing downwards, the island gradually becomes smaller and the 
| protoxylem groups of the pinna-trace-bar unite in pairs to form two groups (PI. I. fig. 
20, pra,, prx,). The small bay with its two protoxylem groups also gradually dis- 
| appears, and the island assumes an elliptical form with a protoxylem group at each apex 
| (PL II. fig. 21, pra’, pra’). 
As we descend still further, the island becomes circular; then it opens to the ex- 
| terior again, and an open bay with two protoxylem groups results (Pl. II. figs. 17, 18, 
| 19, 22, 23, and 24, g). This bay is vertically below the one which was mentioned 
‘| above, and so the mistake has arisen that there was a permanent groove running down 
| the petiole-trace on each side. As has been shown above, the groove ultimately dies out 
and is re-formed lower down; indeed, it is the last vestige of the wedge of parenchy- 
matous tissue shut in between the entering pinna-trace-bar and the petiole-trace. The 
| disappearance of the groove and a diagrammatic representation of the reduction of the 
island are represented in text-fig. 3. 
So far we have seen that two primary pinna-traces and their “spreads”* of pinne 
of a higher order have joined the petiole-trace and gradually disappeared into it. A 
similar sequence is shown at the opposite end of the petiole-trace, but the two ends are 
never in the same phase of pinna-trace emission. Thus four orthostichies of pinna 
“spreads” are borne by each petiole. Up to this point every stage has been ascertained 
by reference to numerous series. 
* A pinna spread is the whole assemblage of secondary pinne_borne by a primary:pinna and spread out into 
one plane. 
