THE STRUCTURE OF FERNS. 15 



In some species the leafy portion of the frond is un- 

 divided, that is to say, the margins are not scalloped or 

 cut away at all : an example of this occurs in the common 

 Hart's-tongue. The margin is, however, much more com- 

 monly more or less divided. In the simplest mode of 

 division which occurs among the British species, the margin 

 of the frond is deeply divided or scalloped out at short 

 intervals, the divisions extending inwards nearly to the 

 rachis, bat not reaching it : this slightly divided form is 

 called pinnatiftd. 



The fronds are sometimes divided quite down to the 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 pinnse are divided again 

 upon precisely the same plan the frond becomes lipinnate, 

 or twice pinnate, but if the pinnse are only deeply lobed 

 they are said to be pinnatifid. 



When the fronds are thrice pinnate, 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 is in very vigorous plants of the common 

 Bracken, and in some of the Lastreas, when very largely 

 developed. 



