THE VASCULAR SKELETON 689 
within the cortex (Cheirostrobus), giving sometimes a median bundle 
(Ophioglossum), sometimes a paired trace (Botrychium). All the more 
primitive types of Ferns, including the fossil Psavonius, have a single 
more or less horseshoe-shaped trace; but the modern Marattiaceae and 
the bulk of the Polypodiaceous Fens have a trace. composed of many 
strands: these are, however, arranged in series corresponding to the 
horseshoe outline of the undivided trace. The facts indicate with no 
possible uncertainty that there has been a disintegration of the leaf-trace 
by fission: it finds its origin in branching of the strands in an enlarged 
upper region of the leaf, and has been phyletically progressive from a region 
lying above towards the base. A comparison of Fig. 97 will make this clear : 
leaf-traces are there shown each of which consists at the base of a broad 
strap-shaped strand: this breaks up distally into numerous strands. But in 
Cyathea, which is structurally a more advanced type, the breaking up has 
been continued down to the base, and the leaf-trace comes off initially as 
numerous separate strands (Fig. 337). The same has‘probably happened 
in the modern Marattiaceae as compared with Psaronzus; in most Mixtae 
as compared with the Gradatae (p. 648), and in the section Opfhioderma as 
compared with Zwophioglossum (p. 462). Thus in several distinct phyla 
it is shown that a progressive disintegration of the leaf-trace has been 
effective; and goes always with leaf-enlargement just as disintegration of 
the axial stele has followed on expansion of the axis. But in both cases 
the enlargement has phyletically preceded the consequent disintegration.! 
The present interest in these complex structures in axis and leaf-stalk 
does not lie in their detailed study, so much as in the fact that in all 
cases they appear only in the plant when advanced towards full develop- 
ment. In the young seedling a stelar structure, little removed from or, in 
most cases, actually showing a protostelic state, is constantly found; and 
from it the various steps may be traced to the more complex condition. 
So far as the development of the individual can be held to reflect the 
-1In certain Pteridosperms and Gymnosperms a double leaf-trace has been found to be 
prevalent, and it has been suggested that it finds its origin in the bifurcation of the leaf. 
Arguments based on the existence of a double leaf-trace have recently been extended to 
Flowering Plants (Miss Thomas, Mew Phytologist, 1907, p. 77): It is not proposed here to 
criticise those arguments, but merely to point out from the side of the Ptcridophyta that 
there is no constant relation between foliar dichotomy and a double leaf-trace. In Sigi/laria, 
Kidston (Proc. R.S., Edin., vol. xxvii., p. 203) has shown that. the double leaf-trace, 
already recognised by Renault, exists in a leaf of simple-form; on the other hand, the 
bifurcate sporophyll of. Zmesipéeris ‘has only a simple leaf-trace. In the Ophioglossaceae, 
Euophioglossum and Helminthostachys have a simple leaf-trace, which soon branches, 
Botrychium has a double leaf-tree, Ophioderma a trace of several strands, not arranged 
in any binary scheme (Ann. of Bot., xix., Pl. xv., Figs. 6-29). ‘Lastly, in many primitive 
Ferns, where dichotomous and other branching of the leaf is prevalent, the leaf-trace is a 
single ‘strand. Such facts suggest the propriety of extreme caution in applying arguments 
based on the vascular structure at the base of the leaf. It would seem not improbable that 
a double leaf-trace might appear in any broad flattened organ which’ is bilaterally sym- 
metrical, whether branched or not. This may very well have been the case in Sigzd/aria. 
2X 
