The Filicinean Vascular System. 
35 
In the frond-like dorsiventral type of branch-system seen in 
some species of Selaginella we have in fact a kind of working model 
of the hypothetical thallus of the “ pro-Lycopod” alluded to in the 
first lecture, the leaves representing the ultimate assimilating 
branchlets, and the whole showing a convergence with a fern-frond 
hypothetically derived by integration of a whole thalloid branch- 
system. But whether the Lycopod leaf be an ultimate assimilating 
branchlet,and whether the fern-frond bean integrated branch-system 
increase of a thallus, or not, both systems are capable of considerable 
in complexity and modification, both of external form and in vascular 
structure, and both have apparently found in this plasticity the means 
of successfully establishing themselves, though in comparatively 
subordinate positions, in the struggle with the flexible angiosperms. 
The Ferns and the Seed-Plants. 
Finally, let us briefly compare the Ferns witn the great group 
of the Cycadofilices which lead up to the Cycadophyta and the 
Flowering Plants. 
The seed-bearing fern-like plants are undoubtedly derivatives 
of the great megaphyllous fern-stock, though they must have 
branched off from the true ferns long before the rocks were laid 
down in which the earliest records at present available are enshrined. 
Nevertheless, since the possible lines of evolution from any given 
point are limited, we find among the primitive Ferns fairly 
close parallels with the structures whose evolution marks the 
beginnings of the lines of descent some of which culminated in the 
production of the typical vegetative structure of the Flowering 
plants. 
In the first place let us consider the series Heterangium, 
Lyginodendron, Poroxylon, Cycads, the demonstration of which by 
Scott was the first convincing proof of the possibility of the evolution 
of the vascular skeleton of the higher plants from something like that 
which we find in the more primitive Ferns. 
What are the essential features of this progression ? First 
there is the disappearance of the central primary xylem involving 
the localisation of the xylem of the stele in peripheral strands which 
are directly continuous with petiolar strands, i.e. are leaf-traces. 
Secondly there is the change of these peripheral strands from 
mesarch to endarch structure. Thirdly there is the increase in 
importance of the secondary xylem relatively to the primary, till in 
