1 86 Simiott. — The Evolution of the Filicinean Leaf trace. 
two diarch and mesarch bundles which enter the base of the leaf. Here 
they approach one another and fuse by their abaxial or outer ends into 
a tetrarch arch (Fig. 23) which is very similar to the leaf-bundle of certain 
modern ferns, except that it is mesarch instead of endarch. Its first-formed 
tracheides lie near the outer edge, and the whole structure is very similar to 
the bundle of Asplenium Filix- foemina shown in Fig. 20. The number of 
protoxylems increases considerably, and the strand may become divided 
into several in the upper portion of the petiole, but these all unite into such 
a triarch and mesarch bundle as is described and figured by Scott (21). 
The gross structure of the vascular system of the leaf in Lyginodendron is 
very similar to that in many of the simpler triarch ferns, and besides showing 
the close similarity in development between the Lyginodendreae and the 
modern Filicales, it throws considerable light on the origin of the triarch 
type itself from the monarch and diarch conditions. 
In conclusion, a word may be said as to the bearing of this whole 
matter on the important question of whether the stem in the Filicales is the 
leader in evolutionary development and the leaf lags behind it, retaining 
primitive characters as it does in the Lycopsida and Cycadaceae, or whether 
the reverse is the case and the leaf leads with the stem following. Impor- 
tant arguments in favour of the latter alternative have been made by 
Tansley and by G wynne- Vaughan, both of whom consider the siphonostele 
to have been produced by the influence of an arched leaf-trace on a proto- 
stele. The former also cites certain cases of medullary bundles as probably 
intrusions into the stele of the complications of the petioiar vascular 
system (28). 
The opposite view, however, seems to the writer much more convincing. 
The stem and the leaf, though doubtless interacting more or less on one 
another, serve very different purposes and seem to have developed inde- 
pendently, but of the two the leaf shows the greater conservatism and fixity 
of structure. This is evident in the first place from the position of its 
protoxylem. Tansley and Bower have emphasized the extreme irregularity 
and consequent unimportance of the position of these early tracheides in the 
Filicales as compared to its fixity in other vascular plants, and it might 
seem from this that the emphasis laid in the present paper upon the position 
of the first-formed elements of the wood was much too great. If we look 
carefully, however, we shall see that, no matter how it varies in the stele — 
for there are many cases of endarchy, of exarchy, and of mesarchy among 
the vascular cylinders of the Filicales— in every fern of which we know the 
anatomy except Lygodium , the protoxylem of the petioiar bundle is always 
endarch. This striking constancy, together with the fact that the leaf- 
bundles are almost without exception concentric, although internal phloem 
may be lacking in the stele, emphasizes the conservatism of foliar anatomy 
in the order and the apparent uniformity of conditions under which its 
