394 Chandler . — On the Arrangement of the Vascular Strands 
The root is of the usual diarch character, and the passage to the 
protostele is of the normal type. One or two parenchyma cells appear in 
the xylem, but do not form a central core as is usually the case. Sieve- 
tubes are not differentiated. The xylem does not long retain its rod-like 
character, but quickly becomes irregularly angled as seen in transverse 
section (Fig. 37) ; protoxylem elements are not distinguishable at this 
stage. At a higher level, the irregularities have largely disappeared, and 
the vascular tissue is in the form of a slightly curved band with somewhat 
sharply pointed horns (Fig. 38). As soon as the vascular strand has 
reached this condition, the first leaf-trace is differentiated in a manner 
remarkably different from that occurring in other ferns. From each of the 
sharp horns of the arc a concentric strand is constricted off, the two strands 
gradually passing outwards. The distance between them is maintained 
until just before the base of the petiole is reached, when they suddenly 
unite, entering the petiole as a single concentric rod. 
The early appearance of such a double leaf-trace is very different from 
the usual state of affairs, the more so as it appears spontaneously, as it were, 
without any intermediate stages being passed through. Again, the method 
of formation of the trace is very unusual, reminding one of the simple process 
obtaining in Polypodium aureum (q. v.). During the exit of the trace, the 
vascular arc becomes attenuated at its middle, the ground-tissue c encroach- 
ing upon ’ it at this point to such an extent that, on the completion of the 
leaf-trace, the cauline system consists of two separate concentric strands 
(Figs. 39, 40). One of the strands quickly divides into two parts (Fig. 41), 
but later the three strands thus formed unite into a band of vascular tissue, 
which at a higher level becomes broad and sharply bent (Figs. 42-44). 
Fig. 45 shows in ground-plan the differentiation of the next leaf-trace. It 
is formed in the same way as its predecessor, and like the latter enters the 
leaf-stalk as a single strand. One of the leaf-strands separates from the 
cauline system at a slightly lower level than the other (Figs. 44, 45). 
Subsequent changes are very interesting in relation to what has gone 
before, since the third leaf-trace, instead of being double like those pre- 
ceding it, is single at its origin, although the xylem is slightly bilobed ; 
indeed, the double and, in some cases, multiple character of the later-formed 
leaf-traces is only arrived at after the preceding traces, from the third 
onwards, have passed through the usual progressive stages of complexity, 
starting with a petiolar bundle in which the xylem is merely bilobed, and 
ending with a leaf-trace composed of two or more separate concentric 
strands, each surrounded by a pericycle and endodermis. The chief stages 
in the process are as follows : — After the exit of the second trace, the 
vascular band enlarges slightly, and its central portion passes out as the 
third leaf-trace ; the xylem of the latter is, as mentioned above, distinctly 
bilobed. After the differentiation of the third trace, one of the cauline 
