116 Sinnott.—Foliar Gaps in the Osmundaceae . 
on physiological grounds, of the strong tendency for a foliar bundle to cause 
a gap in the stele from whence it arises, seems most easily attributable to 
the persistence, in a reduced condition, of an ancestral feature. The 
Osmundaceae, therefore, must apparently be included in Jeffrey’s phylum 
Pteropsida. They give strong support to the general principle pro¬ 
posed by him that in all the members of this great group, the departure 
from a tubular stele of a vascular strand, no matter how much reduced, 
which supplies a foliar organ, causes a break in the continuity of the xylem 
of the stele. 
Summary. 
i. In the mature stem of the six species of the Osmundaceae which 
were studied, a foliar gap, or break in the continuity of the stelar ring of 
xylem, was formed at the departure of the leaf-trace from the cylinder. A 
number of cases were observed, however, in all the species but one, where 
the gap did not become complete for some time after the departure of the 
leaf-trace, which thus at first seemed to go off in a gapless manner. In no 
case which could be thoroughly investigated by complete serial sections, 
was there found to be the real absence of a foliar gap. 
z. None of the fossil Osmundaceae, so far as we know them, which 
possessed a true parenchymatous pith, present a clear instance of the 
departure of a leaf-trace which does not cause a break in the xylem-ring. In 
Osmundites Dunlopi , which seems at first sight to be an exception to this 
statement, we probably have a form with very short and narrow gaps, which 
have been largely obliterated in the process of fossilization. 
3. In all the young plants of the Osmundaceae which have been 
investigated by the writer and others, foliar gaps have been observed from 
the very youngest condition, with the barely possible exception of the very 
early stages in Todea hymenophylloides . Our evidence in this species, 
however, is as yet insufficient. 
4. In the seven species studied, the departure of the trace to the pinna, 
or primary division of the frond, was always found to leave a gap in the 
arch-shaped leaf-bundle. In three species this gap affected only the xylem, 
but in the remaining four a complete break in the vascular tissue was 
made. In only three of the latter, however, was the gap wide enough to 
permit of the connexion through it of the fundamental tissue on the inside 
with that on the outside of the leaf-bundle. The width and character of 
the gap in all instances seemed to depend largely on the relative sizes 
of the pinna-trace and the main bundle. 
5. From such fossil evidence as is available, therefore, and from the 
structure of the young plant and of the foliar strands, both of which are 
known to be conservative of ancestral characters, it seems quite clear that 
the presence of foliar gaps is a primitive feature in the Osmundaceae. 
