;i4 CONCLUSION 



considerations support the conclusion that the Sporangiophoric Pteridophytes 

 constitute a brush of naturally related phylctic lines. 



It has been argued at length above (Chapter XXXI.) that the Ophioglos- 

 sales are an upgrade sequence, a view which accords with their homosporous 

 state : also that their spike illustrates various steps in the increasing complexity 

 of a bodv of the nature of the sporangiophore. The elaboration of the 

 subtending leaf runs parallel with it, while both leaf and spike show 

 branchings and fissions comparable with those recognised in the sporangio- 

 phoric Pteridophytes, but carried out here on a larger scale. On this view 

 the ivhole unbranched shoot is a simple strobihts bearing leaves, of which all 

 are potentially fertile, and the great majority actually so. But the large 

 size of the leaves, and their isolation in point of time (commonly only one 

 being expanded at once), disguises the real nature of the strobilus. All 

 the three genera have attained to great complexity, but in Ophioglossum. 

 and more clearly in Botrychium, the gradually increasing complexity of the 

 leaf in the individual life indicates what has probably occurred also in the 

 race. Along one line, that of Ophioglossum pendulum, intermedium, and 

 simplex, it seems probable that reduction of the vegetative system has 

 occurred ; but with this exception the Ophioglossaceae appear to have been 

 an upgrade sequence, sprung from some Sporangiophoric stock, and bearing 

 no near relation to the large-leaved Ferns. The anatomy here again points 

 to an origin from a protostelic structure, while the single leaf-trace strand 

 in all the simpler forms indicates a primitively simple structure of the leaf. 



The Filicales constitute a more isolated phylum than any of the smaller- 

 leaved forms. Their general comparison among themselves has been fully 

 discussed in Chapter XL., and the relations of their leading families graphically 

 indicated on p. 653. It is now recognised that true Ferns were represented 

 in the Primary Rocks by relatively few forms, while their derivative families 

 increased in number and extent in later periods. The Leptosporangiate 

 type is essentially modern : it is indeed doubtful whether any of the 

 Palaeozoic Ferns had an annulus composed of a single row of cells : on the 

 other hand, though Fusporangiate Ferns still survive, they were the leading 

 type of the Palaeozoic Period. Accordingly, it is in the latter and not in 

 the former that the features of interest for comparison with other phyla of 

 Pteridophytes are to be found. 



It has been shown that the construction of the shoot of the primitive 

 Eusporangiate Ferns is essentially strobiloid, maintaining constantly the same 

 relations of axis and leaf as in smaller-leaved forms : the axis is in some of 

 them permanently protostelic (Botryopterideae), while in the rest a protostelic 

 structure figures in the early seedling of the forms still living. The leaf-trace 

 is a single strand in primitive forms, though in the modern Marattiaceae it 

 may be broken up into separate strands. In addition to this the outcome 

 of anatomical comparison of the Ferns at large has been to show that the 

 axial structure is constantly referable in origin to a primitive protostele, a 

 construction which is held to be typical and primitive for strobiloid plants ; 



