Sinnott.— The Evohition of the Filicinean Leaf -trace. 179 
In Pellaea atropurpurea , which was examined by the writer, a some- 
what similar state of affairs was observed. The leaf-trace here is a single 
strand throughout its course, triarch at its base, and with the two lateral 
protoxylems sometimes mesarch ; but tetrarch and endarch higher up. 
In Adiantum , however, although the trace is a single arch at its 
base, it soon becomes double, and assumes the Onoclea condition in the 
petiole. 
These simpler Pterideae show a less highly developed structure than do 
the simplest forms of the other Polypodiaceae, but the more advanced 
members of the group may display extreme complication. The genus 
Pteris itself is an example of this. In P. aquilina , as is well known, the 
leaf-bundle is very intricate and in the shape of a much involuted arch. Its 
insertion upon the stele is complex, for it is derived from the medullary 
bundles as well as from the outer ring. It is interesting to note that at the 
base of the petiole the arch of bundles is much smaller and simpler than it 
is higher up, and that each strand is mesarch, as are the stem bundles, and 
not endarch, as is ultimately the case. 
Young plants of this species were investigated to discover what was the 
probable primitive condition of the leaf-trace. Professor Jeffrey ( 17 ) has 
described the early siphonostelic condition of the central cylinder in this 
species. In the youngest stage observed by the writer, the leaf-trace 
departed as a single roughly triangular strand with but one protoxylem 
group as yet developed, which was median and mesarch (Fig. 18). A root- 
trace was inserted with it. As it passed through the cortex, a protoxylem 
group appeared at each lateral corner and the trace divided into two, each 
of which was a small but complete bundle of the Onoclea type. This con- 
dition continued through perhaps half an inch of the base of the petiole, but 
higher up the bundles increased in size, became irregular in shape, and 
merging and separating several times through the length of the petiole, 
formed an arch somewhat comparable to that in the mature leaf. 
In a slightly older condition, the first indication of the departure of the 
leaf-trace was the appearance of a large island of parenchyma near the out- 
side of the stele, which caused a ‘swelling’ in the xylem. On either side 
of this was a protoxylem cluster, each representing half of the single median 
group of the earlier stage. These protoxylems with the surrounding seg- 
ment of the stele passed off as the leaf-trace and the peripheral swelling 
broke through, forming two xylem hooks, in the concavity of each of which 
was a cluster of protoxylem (Fig. 19). At each adaxial corner was a group 
of small cells which eventually became protoxylem also. The whole trace 
at this stage resembles two Onoclea bundles fused by their median masses 
of metaxylem. This is really the state of affairs, for the strand soon divides 
into two diarch traces. It would thus seem that in Pteris , at any rate, the 
very complicated petiolar condition is a derived one and has come through 
N 2 
