in the ‘ Seedlings ? of Certain Leptosporangiate Ferns. 381 
a considerably lower level than the other, as is so often the case (Figs. 109, 
no). The two leaf-strands pass gradually outwards through the cortex, 
and, in doing so, approach one another and finally fuse, leaving the stem as 
a single bundle. The fifth leaf, therefore, is supplied with a single trace 
which has an obviously double origin. The sixth leaf-trace is also double 
in origin, and formed exactly as before, being preceded by the usual 
closing of the elongated gap and subsequent splitting. Although the 
mature trace is single, its double origin is hinted at in the bilobed character 
of its xylem. 
After the gap is closed, the subdivision of the vascular strands 
proceeds apace. Instead of a single strand dividing into two, the strands 
divide, one into two, the other into three parts, each being surrounded by 
its own endodermis. The division of the actual strand is nearly always 
preceded by the constriction of the xylem into the required number of 
parts, the portions of the xylem being separated by phloem parenchyma 
and occasionally sieve-tubes. Fig. 118 represents the appearance of the 
stem in transverse section after the sixth trace has passed out. The next 
two or three leaf-traces are formed in exactly the same way as before, but 
retain their double character for successively longer periods. Soon, how- 
ever, the leaf-traces have an apparently simple origin. At about the eighth 
or ninth leaf, the stem in transverse section presents six or seven strands 
embedded in the ground-tissue. The leaf-trace is formed by the nipping 
off of a vascular strand from each of two neighbouring cauline strands, one 
being separated from the cauline bundle at a considerably lower level than 
the other. Both soon leave the stem to enter the petiole, but the exit 
is gradually delayed in succeeding leaf-traces. It is this fact which 
explains the apparent anomaly in the formation of the leaf-strands in older 
plants. 
In a moderately sized rhizome there are from twelve to fifteen vascular 
strands, and at the node two of these are seen to pass out bodily into the 
petiole. This apparently is quite different from the process obtaining in 
young plants, but a complete series of sections through two of the long 
internodes shows that the leaf-trace formations are essentially the same in 
both cases. In the later internodes, the vascular strands divide and fuse 
several times, but strands from which the last leaf-trace was derived remain 
separate for some considerable distance. Finally, however, they subdivide, 
and the lesser strands fuse among themselves, so that the leaf-gap is closed. 
The true siphonostelic character of the vascular system is therefore 
maintained. Two neighbouring strands then divide into two, one much in 
advance of the other, and the two daughter strands run through the 
remainder of the internode and apparently belong to the cauline system. 
At the next node, however, they pass outwards to the petiole and form 
the leaf-trace as described above. It will thus be evident that the later 
