12 ON GENERA AND SPECIES, 
(rachiform), in others they are involute, in the form of a ` 
pod or siliqua (siliquiform) ; in others plain, the under E. 
surface being densely occupied with sporangia ; in some, 
the lower portion of the frond is sterile, while the upper 
portion is contracted and fertile. 
The duration of fronds is variable, some remaining per- 
manent for several years, but the greater majority, espe- 
cially those in temperate and cold climates, develope and ` 
come to maturity in one year. 
It is, however, otherwise with the genera Gleichenia, Lygo- 
dium, and Salpichlena; in the first the fronds are dicho- 
tomous; and from the axis of the forks are successively 
produced other dichotomous branches, and the fronds thus 
continue long permanent. 
As regards Lygodiwm, the fronds so called continue to 1 
lengthen and branch (indéfinite), rambling over bushes or _ 
climbing the tops of lofty trees; such is also the case with 
the blechnoid Fern Salpichlena. The permanent nature 
of these climbing fronds (?) may be considered analogous 
to climbing flowering plants, such as Clematis; to these 
peculiar growing Ferns I apply the term Pierampelids. 
VENATION. 
The vascular structure of Ferns consists of cords of 
indurated tissue termed scalariform, which branches from 
the base of the previous frond into the nascent bud of the - 
forthcoming frond, and lengthen upwards with the growth ` 
of the frond. It consists of one, two, or more cylindrical, ` 
. flat, or channeled cords, which, on reaching the leafy part - 
~ ofthe frond, divide and ramify through all its parts, the ul- ` 
_ timate ramifications ending in the leafy part (lamina), being 
