1 88 Sinnott . — The Evolution of the Filicinean Leaf -trace, 
state of affairs. In any fern the structure of the base of the leaf-trace 
represents a condition which probably once obtained throughout the entire 
petiole. 
But why, if the smaller bundle is sufficient for the conduction of the 
necessary water, should it be later increased in size and pulled apart ? In 
answer to this it may be pointed out that the lower and slender portions of 
the petiole are almost always either subterranean and thus supported by 
the soil or are immersed in a packing of old leaf-bases. The widest part 
of the leaf-stalk is where it becomes free, for at this point the strain of sup- 
porting the leaf is greatest, and there is therefore the most need of strength. 
The increase in size of the whole petiole here is probably a response to this 
need, and the amplification of the vascular bundle is either for increased 
rigidity or is simply complementary to the enlargement of the whole and 
serves to distribute the vascular elements more uniformly throughout the 
leaf-stalk, and also, perhaps, to facilitate the departure of the pinna-trace. 
The simplified conditions found in the upper part of the rachis may 
show a retention there of ancestral characters, as suggested by Gwynne- 
Vaughan, but as the lamina is subject to change in external influences more 
than any other part of the plant, it would not be safe to conclude that the 
simplicity found in its upper portion was always a primitive simplicity. 
In several cases observed there seemed to be an influence exerted by 
the stele upon the histology of the base of the leaf-trace. Its mesarchy in 
the mature leaves of P ter is aquilina and Woodwardia virginica , as noted 
above, is probably due to the carrying up for a little way of the mesarch 
condition of the stelar bundles. This influence is also felt in such cases 
of reduction as Platyzoma and the Lindsay a type. In these forms the 
degenerate state of the stele has influenced the base of the leaf-trace, and 
here it is the petiolar bundle which is least affected and therefore nearest 
the original condition. 
From the conservatism of their general foliar anatomy, therefore, and 
from the fact that at the very base of the leaf-trace are found the simplest 
and most primitive structures and relations of the plant body in this order, 
it is clear that the Filicales form no exception to the general principle that 
the leaf of vascular plants is the seat of ancestral characters. 
Summary. 
1. The base of the leaf-trace in living ferns presents three main types 
of structure : the primitively monarch, with one group of protoxylem ; the 
primitively diarch, with two ; and the primitively triarch, with three. 
2. The first type is characteristic of the Osmundaceae and the Ophio- 
glossaceae, where the base of the leaf-trace is a single monarch strand, 
which is often mesarch as well. It is more strikingly so in the fossil 
