38 LinncBan Society. 



modifications of the vascular structure, or the various ramifications 

 of the bundles of vessels or veins of the frond, combined with the 

 relation of the sori to their trunks or branches." He notices an in- 

 stance in which Sir "William Hooker has given generic importance 

 to this character of venation alone, viz. in Dictyoxiphium ; while in 

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

 he treats it as a mere question of words, to be decided by conve- 

 nience, whether or not this character should be generically em- 

 ployed. 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, dichotomous primary veins and reticulated ve- 

 nules, than to have to recognize in Polypodium (a genus of ferns 

 having round naked sori) an included group called Dipteris, in 

 which the primary veins are dichotomous and the secondary reti- 

 culated. 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 affords 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 differen- 

 tial 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 diver- 

 sities, as the peculiarities in the development of the vascular struc- 

 ture ? Experience, moreover, attests this character of venation as 

 one to be relied on with perfect confidence, because (with very in- 

 significant exceptions) whatever modification of vascular structure is 

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

 species. The author concludes, therefore, that without lowering the 

 importance of the fructification of ferns in distinguishing generic 

 groups, the modifications of venation are properly as well as conve- 

 niently admitted to share in the same office. 



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 

 difference from established genera in the structure of some import- 

 ant organ or system of organs. Now the vascular system must be 

 regarded as of the highest importance in the vegetable economy 



