1853.] Linnean Society. 211 



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 oeconoray 

 even in reference to propagation, it being not at all infrequent to 

 meet with extraordinary means of development in connexion with it, 

 viz. adventitious buds ; and in ferns particularly those points of the 

 veins which serve in normal cases as the receptacles to which the 

 sori are attached, in other cases become viviparous and develope 

 gemmae from which new plants are produced. He believes, more- 

 over, that characters derived from this system of vessels, when taken 

 in connexion with the fructification, though sometimes forming 

 groups of considerable extent, and occasionally separating species 

 having some external similarity, nevertheless in no case bring to- 

 gether obviously ill-assorted species, but rather associate those of 

 obvious similarity and affinity. 



For these reasons he is not prepared to follow Sir W. Hooker in 

 setting aside the genus Hewardia of Mr. John Smith. He regards 

 the difference as broad and important between the accidental anasto- 



