VI 
INTEODTTCTOEY EEMAEKS. 
the preceding by the fronds being developed from the apex 
of the progressing axis either in a single alternate or a 
continuous fasciculate series, their united and permanent 
bases forming an erect or decumbent cormiform caudex, or 
a creeping or scandent sarmentum. This mode of growth 
is common to the majority of Ferns, and is readily distin- 
guished from Eremobrya by the fronds not being articu- 
lated with the axis. 
On viewing the various modifications of these two modes 
of growth in combination with the character of the fructi- 
fication and vascular structure of the frond, we obtain data 
to enable us to arrange the naturally allied forms or spe- 
cies in groups or genera (if they may be so called). 
With regard to the characters that define the limits of 
specific forms, much uncertainty prevails amongst authors. 
This is owing to several causes, such as many species being 
normally heteromorphous, presenting at the same time 
different forms, which again vary at another period of 
growth ; and in many instances authors have described the 
different states as distinct species ; and in some cases por- 
tions of the same species or individual plant have even 
been placed under separate genera. It also frequently 
happens that two or more presumed species present so 
many intermediate gradations of form, that only the most 
extreme states appear sufficiently distinct to warrant their 
adoption as species, the numerous intermediate forms 
seeming to set specific distinctions at defiance. It there- 
