Sinnott . — The Evolution of the Filicinean Leaf- trace. 187 
modifications have been developed. Perhaps more important even than 
this evidence is the fact that in every case the simplest and most primitive 
structures and relations of the vascular system occur at the node or in the 
base of the leaf-trace. It is a noteworthy fact that in very many ferns the 
leaf becomes slender at its attachment, and that in almost all of them the 
vascular supply, besides being much less complicated at this point than 
higher up in the petiole, is also composed of a much smaller number of 
cells. This is strikingly shown in Osmunda and in the Marattiaceae, but is 
very evident in almost all non-degenerate ferns. As the bundle ascends the 
increasingly wide petiole, it shows a strong tendency to broaden laterally, 
with the result that a single compact trace may become much stretched 
and sometimes divided, or that two bundles, at first close together, may be 
widely separated and broken up. It has often been pointed out by Gwynne- 
Vaughan, Tansley, and others that as all the water going to a leaf must 
pass through the base of its petiole, it is at this point that the greatest 
modifications for efficiency in water-conduction will appear. That increased 
size and complexity are evident in the lower portion of the leaf-stalk is 
quite clear, but the fact that this condition does not continue to the stele 
itself, but that the structure becomes continually more simple towards the 
very base of the leaf, makes it extremely doubtful if the size of the tran- 
spiration current has had much influence on the development of the vascular 
supply, for the stream is as well accommodated by the small double trace 
of Danaea , for example, as by its large petiolar bundle. From this fact, 
the argument for the derivation of a siphonostele from a protostele by the 
influence of a continually enlarging leaf-trace — a theory in which the shape 
of the trace and the appearance of the pith are accounted for by changes 
in the size and course of the transpiration current — loses much of its 
weight. This is further emphasized by conditions in many ferns with the 
Onoclea type of leaf-strand, where the two traces as they leave the stele are 
united or close together, but where they, as well as the two bundles from 
which they departed, gradually draw far apart from one another. The 
influence of the wide petiolar system can surely not have caused the widening 
of the dictyostele, for both systems are constricted when they meet. 
Against the argument that medullary bundles in ferns are caused by 
the influence on the stele of the complication of the petiolar bundle, it may 
be pointed out that in the Marattiaceae the intricate vascular systems of 
stem and leaf communicate with one another by only a double trace, and 
that in young plants of Pteris a medullary bundle appears in the stele, 
while the leaf-trace is still a very simple strand. 
It appears much more probable that the stele and the leaf-bundle have 
increased in size and complexity independently, as suggested above, and 
that the base of the leaf-trace, occupying an intermediate position and sub- 
jected to no great mechanical stress, has preserved most fully a primitive 
