THE VASCULAR SKELETON 689 



within the cortex (Cheirostrobus), giving sometimes a median bundle 

 (Ophioglossuni), sometimes a paired trace (Botrychium). All the more 

 primitive types of Ferns, including the fossil Psaronius, have a single 

 more or less horseshoe-shaped trace ; but the modern Marattiaceae and 

 the bulk of the Polypodiaceous Ferns 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 Psaronius; in most Mixtae 

 as compared with the Gradatae (p. 648), and in the section Ophioderma as 

 compared with Euophioglossum (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. 1 



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 



1 In 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/^V^w Phytologist, 1907, p. 77). It is not proposed here to 

 criticise those arguments, but merely to point out from the side of the Pteridophyta that 

 there is no constant relation between foliar dichotomy and a double leaf-trace. In SigiHaria, 

 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 Tmesipteris 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., PI. 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 Sigillaria. 



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