STKUCTCRE. 
and development in the same species. In most of the 
creeping-stemmed ferns, the base of the stipes is articulated 
with the stem ; that is, furnished with a natural joint or 
interruption of the woody fibres, so that in age it sepa- 
rates spontaneously. This is less frequently the case with 
the tufted growing kinds. In the more highly compound 
fronds, the rib which runs through their centre is called 
the primary rachis, and that which runs through the 
pinna?, the secondary rachis, and so on. 
The upper portion of the frond, extending more or less 
downward, is leafy. This leafy portion offers many states 
of division, the parts being much influenced in size and 
number by external circumstances. It is sometimes simple 
or undivided ; sometimes pinnatifid, or more or less 
deeply cleft ; sometimes pinnate, or divided into distinct 
leaf-like divisions, called pinnce; sometimes bipinnate, 
that is the pinnae themselves pinnate, this second series 
of pinnae being called pinnules; sometimes still more 
compoundly divided, the pinnules being either pinnatifid, 
or even again pinnate. When the pinnae themselves are 
only deeply cleft, they are said to be pinnatifid. The 
character of the division of the frond is much employed 
in distinguishing the species, and is tolerably constant. 
The outline of the fronds varies greatly, and is dis- 
tinguished by the terms which are applied to the same 
forms in other plants ; the most common being the lance- 
shaped, triangular, and oval. In their magnitude also 
the fronds of the British species vary greatly, from two 
or three inches to five or six feet in length, and from le"ss 
than an inch to two feet or more in width. 
In the form of the divisions there is an almost endless 
variety. Their texture and colour afford other differ- 
ences, some being thin and almost transparent, others 
thick and leathery, and some even rigid ; some pale-green, 
some deep-green, some blue-green, some brown-green ; 
some smooth and shiny, others opaque or hairy. Like 
the leaves of other plants, the fronds of ferns are variable 
in their duration. In some species they are persistent, 
