INTRODUCTION. 39 
more diffieult to overcome, than those presented by all other methods 
Which have been proposed for the classification of the Ferns. 
We have already recorded* some instances in which the sori of 
dorsiferous and other Ferns were aberrantly situated. The inferences 
to be drawn from the examples referred to are, we think, confirma- 
tory of the importance of the venation. It would thence appear 
that the veins are important structures in the economy of Fern 
development, since they are capable of originating the receptacle and 
spore-cases from their surface in any part—even in unusual parts— 
of the frond. This being so, sufficient importance would appear 
to attach to them, to justify their being employed for the purpose of 
assisting in the definition of genera, in a family of plants where 
something more than the so-called fructification itself is confessedly 
needed to supply distinctive characters. 
Mr. J. Smith has recently proposed t to classify the Ferns accord- 
ing to their mode of development from the caudex or rhizome. 
Taking advantage of the apparent difference between the growth of 
those Ferns which have a caudiciform and a rhizomatous stem, he 
proposes to bring the polypodiaceous Ferns under two groups, which 
he calls Eremobrya and Desmobrya. 
In the Eremobrye “the fronds are developed from the sides of a 
special rhizome which has its axis of growth always in advance of - 
the nascent frond (exeurrent) ; the fronds are produced from nodes 
more or less distant from each other, each node producing a single 
frond, which after having arrived at maturity separates by a special 
articulation formed between the node and the base of the stipes. 
After the frond has fallen the node remains in the form of a round 
concave cicatrix generally more or less elevated. The rhizome is 
solid, fleshy, and brittle, varying from long and slender to more or 
less short and thick, and is always covered with scales which unless 
* Ante, p. 22. 
+ J. Smith, Botany of the Voyage of H.M.S. Herald, 226. 

