Bower . — Studies in the Phytogeny of the Filicales . VI 7 . 39 
it separates again, moving across the pith to a point opposite the next leaf- 
trace, attaching itself as before on the basal lip of the leaf-gap. So far the 
leaf-gaps do not overlap. But a transition to dictyoStely occasionally 
makes its appearance (iv), though this is not a common feature, and does 
not appear in the adult stem. 
The next step is the expansion of the medullary strand into a second 
ring, which is brought about by a process of ‘ pocketing ’ similar to that seen 
in the original stele (Fig. 27 bis , v). It results in a second ring within the first. 
The further elaboration to form a second ring with a vascular strand again 
within it has not been actually observed ; but there can be no doubt that 
it is a mere repetition of the earlier steps. The result is seen in vi, where 
there are two concentric solenosteles and a central strand. A leaf-trace is 
being formed from the outer, while the vascular extension at the extreme 
right supplies an abaxial bud which is formed at the base of the petiole, as 
in Lophosoria and other Ferns. In vii, which shows a section in a higher 
plane, the leaf-trace is detached, while a broad compensation-strand is 
forming from the inner ring to fill the leaf-gap. The single central strand 
has meanwhile branched. A section in a still higher plane (viii) shows the 
leaf-gap filled- by the compensation-strand, while the central strands have 
fused to form a sort of secondary compensation-strand, and joined up with 
the inner solenostele. 
As the stem grows larger, still greater elaboration of the vascular 
system may be seen. This is illustrated in Fig. 28, A-D, which shows 
the essential features of the very elaborate system, and the relation of the 
supply of axis and leaf. Three leaf-bases are included (a, b , c), and the 
drawings are in alphabetical order from below upwards. They demonstrate 
the steps in giving off the trace a . In Fig. 28, a, wide dilatations of the 
outer unbroken solenostele are shown corresponding to the leaf-bases a , b , 
and a ‘smaller one for leaf c. The inner ring is also complete, while within 
it a somewhat irregularly interrupted third ring is seen, with two central 
vascular strands. B shows the formation of the leaf-trace a more ad¬ 
vanced, while the compensation-strand which is to fill its leaf-gap is partly 
detached from the second solenostele. A similar bar of vascular tissue 
parallel to the first compensation-strand is being formed from the third 
ring. In section C the leaf-trace is detached, and forms a completely 
closed, though fluted ring. After what has been seen in the petiole such an 
occurrence will not cause surprise; but it has only been observed in fully adult 
leaves. Section C shows that the outer solenostele is completed by the fusion 
of the compensation-strand with the margins of the leaf-gap, while a similar 
replacement has taken place so as almost to complete the second solenostele. 
The third ring and central strands are here irregularly interrupted. But in 
section D they have again settled down to constitute a third, almost com¬ 
plete solenostele, and central vascular core. The second and outermost 
