1022 



character of venation alone, viz. in Dictyoxiphium ; while in Schizo- 

 lonia he regards the venation as only of subgeneric value ; and he 

 treats it as a mere question of words, to be decided by convenience, 

 whether or not this character should be generically employed. In the 

 case for instance in reference to which Mr. Brown's remarks were 

 made, Polypodium (Dipteris) Horsfieldii, it seems to him, as a matter 

 of convenience, a much simpler and more easily comprehensible idea, 

 to regard Dipteris as a group of ferns with round naked sori, dichoto- 

 mous primary veins and reticulated venules, than to have to recog- 

 nize in Polypodium (a genus of ferns having round naked sori) an 

 included group called Dipteris, in which the primary veins are dicho- 

 tomous and the secondary reticulated. In most cases, indeed, he 

 regards subgenera as at the best but cumbrous contrivances. 



Looking at the question of venation, as illustrated in the great and 

 universally adopted natural divisions of flowering plants, he thinks its 

 generic importance in ferns rests on better grounds than convenience 

 alone. In the case of flowering plants the presence of complete floral 

 organs aff"ords the necessary diversity for generic distinction ; but as 

 an equivalent to these we have in ferns nothing more than certain 

 naked or covered aggregations of spore-cases, which in the great bulk 

 of the species scarcely afford any differential characters, or such only 

 as are microscopic, and therefore not to be resorted to until all more 

 obvious features are exhausted. But peculiarities in the venation of 

 ferns are for the most part associated with peculiarities of habit ; and 

 since it appears quite justifiable to employ other characters than those 

 derived from the fructification in distinguishing generically such groups 

 as the ferns, in which the fructification affords comparatively so little 

 variety, what is there so constant and unvarying, and at the same time 

 affording such diversities, as the peculiarities in the development of 

 the vascular structure .'' Experience, moreover, attests this character 

 of venation as one to be relied on with perfect confidence, because 

 (with very insignificant exceptions) whatever modification of vascular 

 structui'e is met with in a particular species, that and no other is found 

 in that species. The author concludes, therefore, that without lower- 

 ing the importance of the fructification of ferns in distinguishing 

 generic groups, the modifications of venation are properly as well as 

 conveniently admitted to share in the same oflSce. 



Passing to the question, whether a reticulated venation is in itself a 

 sufficient generic distinction among the ferns, he determines it in the 

 affirmative, inasmuch as a genus being in his view an arbitrary group, 

 all that is really required as a generic character is a constant diffe- 



