368 Chandler . — On the Arrangement of the Vascular Strands 
‘dips into’ the leaf-gap. As we pass upwards, the ground-parenchyma 
encroaches more and more upon the vascular tissue, so that the internal 
phloem loses its strand-like character, and appears as a layer of sieve-tubes 
and parenchyma lining the internal concavity of the xylem, the horns of 
which gradually approach one another as the gap is closed (Figs. 1-3). 
While these changes are taking place, and before the closing of the gap, 
the xylem, at a point almost exactly opposite the latter, breaks into two, 
sieve-tubes and parenchyma appearing between the two portions ; the 
continuity of the endodermis is not affected. As in many similar cases, to 
be described below (an especially good example occurring in Lomaria 
gibba ), this new break in the xylem is in reality the first preparation for 
the exit of the next leaf-trace, for very soon after its appearance a con- 
centric strand of vascular tissue is nipped off from each of the cauline arcs, 
the changes involved in the process being indicated in Figs. 3-5. The two 
concentric strands, each of which is surrounded by its own endodermis, 
pass outwards as the leaf- trace, which is thus double at its junction with 
the cauline system. Long before the strands enter the petiole, however, 
union has taken place, so that the leaf-stalk itself possesses a single 
concentric vascular strand. 
In most other cases of ferns possessing multiple leaf-traces it has been 
found that the latter do not appear suddenly, as in the case just described, 
but only after the few preceding leaf-traces have shown a very complete 
series of transitions between the primary single leaf-trace and the more 
complex system supplying the later leaves. The only hint at such a 
transitional series occurring in Doodia aspera was the occasional slightly 
bilobed character of the xylem of the previous leaf-strand. 
As soon as the exit of the double leaf-trace is complete, the former 
gap closes and a root enters. As will be seen from Figs. 6 and 7, the 
cauline system at this region of the stem consists of but a single curved 
strand. A consideration, however, of the vascular system as a whole up 
to this level will establish its essentially tubular (siphonostelic) character. 
This is rendered more obvious by subsequent changes, for very quickly 
the single strand divides into two parts, one much larger than the other 
(Figs. 8 and 9). From each a small concentric strand is nipped off, the 
two forming the next double leaf-trace, which, as before, is single before it 
reaches the petiole, though in this case the two xylem patches are separated 
by phloem for a short distance at the base of the leaf-stalk. The larger 
of the two cauline strands then divides into two (as in Fig. 12), and the 
next leaf-trace is formed in exactly the same way as its predecessors. 
It is unnecessary to follow the further development of the vascular 
system ; its final siphonostelic condition has been reached, and subsequent 
changes are merely an elaboration of the stage just described. 
