18 
HISTORY OF BRITISH FERNS. 
rachis, which is, as it were, quite bared of the contiguous 
leafy expansion ; and when this occurs, the frond is said to 
be pinnate; in this case, each of the distinct leaf-like 
divisions is called a pinna. When these pinnae are 
divided again upon precisely the same plan, the frond 
becomes bipinnate i or twice pinnate; hut if the pinnae 
are only deeply lobed, they are, like the frond when simi¬ 
larly divided, said to be pinnatifid. 
When the fronds are thrice pinnate, they are called 
tripinnate ; and in all other more intricate forms they are 
called decompound; but this seldom occurs in any of the 
native kinds, the nearest approach to it being in very 
vigorous plants of the common Bracken, and in some of 
the Lastreas, when very largely developed. 
The young fronds of the ferns, before being developed, 
are arranged in a very curious manner, the rachis being 
rolled inwards volutely from the point to the base. In 
the compound sorts, the divisions are each again rolled up 
in a similar way. This arrangement is what is called 
circinate. All the British species, with two exceptions, 
are folded up in this way, so that their development 
consists of an unrolling of the parts of the fronds. The 
exceptions mentioned are the Moonwort and the Adders- 
