96 LEAF-ARCHITECTURE OF FERNS [CH. 



gained by intimate vein-fusions in promoting equal water-distribution over 

 large leaf-areas gives added probability to this conclusion. 



Among modern Ferns it is quite a common thing to find single genera 

 or species that have reticulate venation in families or genera characterised 

 for the most part by open venation. Sometimes the vein-fusions are only 

 occasional, but frequently the reticulum may be of an advanced type. This 

 occurs even in the most primitive groups. For instance, while Helmintho- 

 stachys and Botrychium have open venation as a rule, in Ophioglossum it is 

 reticulate. All the genera of the Marattiaceae have open venation except 

 Christensenia, in which the few broad segments are reticulate. The Schizae- 

 aceae are mostly open-veined, but Anemidictyon and Hydi'oglossum are 

 reticulate sections respectively of the genera Anemia and Lygodiuin. Hypo- 

 derris is reticulate among the open-veined Woodsieae, and Onoclea sensibilis 

 is reticulate while Matteuccia {Onoclea) Struthiopteris has open venation. 

 Instances might be multiplied which indicate that among living Ferns there 

 has been frequent progression within narrow circles of affinity from the 

 more primitive open to the more advanced reticulate venation. 



The detailed arrangement of the veins in the leaf-blade or its parts has been widely 

 used in the comparison of Ferns, and in their systematic arrangement. This has led to a 

 classification and terminology of the various venations, together with elaborate verbal 

 descriptions of those several types which were recognised by Mettenius. It will not be 

 necessary here to enter into these details. It should suffice to illustrate them by a series 

 of figures, as selected by Luerssen(8i). The names by which the several types of venation 

 are designated refer sometimes to living Ferns, but mostly to fossils, in which the venation 

 is often very well seen (Figs. 93, 94). 



An examination of the several types will show that they may all be traced back along the 

 lines explained above to a simple dichotomous venation, equally or more often sympodially 

 developed. This is combined with various degrees of fusion of veins, sometimes marginal, 

 sometimes nearer to the midrib : in other cases quite generally distributed. In leaves with 

 a broad surface the appearance is sometimes given as though a repetition or duplication 

 of the meshes from the midrib outwards had occurred, and this may often go along with 

 a repetition of the sori which are associated with them. It is particularly seen in Nervatio 

 GoniopJdebii and Cyrtophlebii (Figs. 93, / ; 94,/). Such factors as those mentioned will 

 account for the genesis of even the most complicated reticulate systems, as a careful 

 analysis and_ comparison will show. 



Beyond the fact that these complex venations are characteristic for the most part of 

 relatively late and advanced types of Ferns, their detailed study cannot be used as a con- 

 sistent or trustworthy basis for the phyletic seriation of the Filicales at large. This con- 

 clusion, which robs the subject of much of its interest, naturally follows from the fact now 

 demonstrated for so many isolated genera that a closed venation has originated repeatedly 

 in distinct stocks. It is in fact a widely homoplastic character, determined in great measure 

 by physiological necessity or convenience. 



