10 
BRITISH FERNS. 
woody tissue surrounding the mass of cellular tissue 
which fills the centre of the stem. In this cellular 
tissue are imbedded seven bundles of vascular tissue, 
rounded, of different sizes, and placed near the woody 
border. In Nephrodium Thelypteris there are only two 
such bundles, Polypodium vulgare has four, Polypodium 
Phegopteris only two, and Asplenium Ruta-muraria has 
but one. Some of these bundles of tissue are round, 
some oval, some oblong, and some crescent-shaped. The 
outer circle of woody tissue varies in colour and thick- 
ness. Sections of stems show these vascular bundles 
now as the steps of a ladder, now as a spread-eagle, now 
as an oak-tree ; their number and shape vary at different 
elevations in the same stem. This structure is dis- 
cernible in the fossil ferns. The leafy portion of the 
frond is either simple or compound. When formed of 
one entire leaf, as in the hartVtongue, it is simple ; 
when it is divided nearly, but not quite, as far as the 
rachis , it is pinnatifid ; when divided quite to the rachis, 
it is pinnate . It is compound when the rachis is fur- 
nished with branches, each of which is called a secondary 
rachis ; the branches traversed by the secondary rachis 
are pinna , as are also the leaflets of a simple pinnate 
frond, and the leaflets bordering the secondary rachis 
are pinnules : — when a frond is thus divided, it is called 
bipinnaie ; when the pinnules are divided into leaflets, 
the frond is tripinnate. The character of the divisions 
of the fronds is an important point of distinction. The 
outline of the frond varies greatly; we generally consi- 
der its outline exclusive of the naked part of the stem. 
Many ferns have lance-shaped fronds, broadest in the 
central part, and narrowing to a point at the summit, 
narrowing also in a less degree towards the base, so as 
