



20 THE BRITISH FERNS. 
of Ferns. This use of the venation has been objected to by some 
botanists of high authority, who think that the fructification alone 
should furnish the generic character. Such a rule may be quite 
proper as regards flowering plants, where the organs forming the 
parts of fructification are numerous and varied, but it does not 
appear to be equally so in the case of Ferns, where the so-called 
organs of fructification present few available differences, and where 
the species are nevertheless so numerous, that further distinguishing 
characters are desirable in order to break up into groups of moderate 
size the unwieldy genera of olden times. No auxiliary character 
that has yet been suggested, has proved so useful, nor so constant, 
nor as we believe, so important, as the venation. That such is 
` the case, is indeed practically admitted, even by those who object 
to its use in distinguishing genera, for they willingly employ it as 
a characteristic of sub-genera. Now the difference between a genus 
and a sub-genus is so very trifling, the limitation of genera being l 
a mere matter of fancy or convenience, and therefore varying in 
the estimation of different persons, that the admission of venation 
as a characteristic of the lesser conventional group—the sub- 
genus, may þe taken as admitting it in the case of the larger 
but equally conventional group—the genus. Much of the im- 
portance we are inclined to claim for venation as a feature of 
generic value, rests upon the fact that rr gives rise to the fruc- 
tification. The spore-cases which form this fructification spring 
out of it; the receptacle to which they are attached is part of it; 
and this intimate connection with the fructification must give the 
venation in the Fern a higher importance than could be properly 
attached to it in the case of flowering plants, though even among 
the latter, the important differences of free and reticulated venation 
run so exactly parallel with other features which mark the great 
primary groups of Exogens and Endogens, that they are the primá 
facie characters universally employed in distinguishing them. 
