648 GENERAL COMPARISON OF THE FILICALES 



former, though its soral structure has remained virtually unaltered, shows 

 the highest condition known of the solenostelic development. 



Parallel with such structural progression of the axial system goes an 

 elaboration of the leaf-trace. In protostelic, and usually in solenostelic 

 forms, it is represented by a single strand, which may, however, be widened 

 into a broad strap, and curved in transverse section into a horse-shoe outline: 

 and this may again be contracted into a pseudo-stelic condition (compare 

 Fig. 98, p. 194). But with dictyostely comes usually a division of the 

 single strand into many. It is interesting in Dicksonia to see a middle 

 condition illustrated ; for in D. Culdta and D. Barometz the leaf-trace at 

 its base is still a single strand, but at a point above the base, varying in 

 different leaves, it breaks up into many separate strands : it thus represents 

 the various stages of the probable phyletic sequence (Fig. 97). The 

 complete subdivision is seen in the larger species of Dicksonia and in 

 Cyathea, as well as in most of the Mixtae, and it is held to be an 

 advanced and derivative state. 



The seedling structure gives a strong support to the view of elaboration 

 here put forward : in all observed cases the stele of the axis is either 

 protostelic or very closely related to that structure, and the leaf-trace is a 

 single vascular strand. In the primitive forms this may remain permanently 

 so ; but in others there may be a quick transition to the more complex 

 and presumably derivative state. The example of Alsophila excelsa 

 (p. 608) shows that the individual life, after the first stages are past, 

 reflects the probable story of development of the complex adult condition 

 in the race. 



It is in this way, through the seedling, that the Marattiaceae may best 

 be approached. They have in the mature stem a still more complicated 

 system of vascular strands than other Ferns ; but in their seedlings the 

 ontogeny opens in all cases with a monostelic state, with a solid xylem-core. 

 Complications soon arise : in Kaulfussia and Archangiopteris a cylindrical 

 dictyostele is formed, not unlike that of other Ferns ; but in Aiigioptcris 

 there may be as many as three or four concentric, meshed zones in the 

 stock, while the leaf-trace is also disintegrated into numerous strands. It 

 is important to note, however, that in the related fossils the leaf-trace is 

 habitually a single connected strand, while greater coherence is also seen 

 in the vascular tracts of the axis in the fossils than in the living species. 



These facts all indicate that in the evolution of Ferns there has been 

 a progressive amplification and disintegration of the vascular tissues ; and 

 they lead back towards a type, which seems to have been a common one : 

 the original type was characterised by a radial shoot traversed by a protostele, 

 from which the successive leaf-traces came off each as a simple strand, and 

 with the minimum of disturbance of the axial stele. If this were the 

 original type of shoot in the Filicales, it is plain that the foliar gap, to 

 which Jeffrey attaches so much importance as the distinctive character of 

 his phyllosiphonic type, must have been a secondary development : it is 



