INTRODUCTION. 17 
mainly consists of this vascular tissue, definitely arranged, and this 
is continued onwards throughout the leafy part of the frond, 
becoming branched more or less according as the frond itself is 
more or less divided, the branches, according to their position, 
forming the rachis, costa, veins, venules, or veinlets. 
When the frond is simple, that is undivided, as in the Scolopen- 
drium vulgare, the rib or continuation of vascular tissue from the 
stipes through the frond, forms the Costa or midrib. From the 
costa are given off, laterally, branches more slender than itself, of 
which the first series are called Veins; if these veins are branched, 
the branches form the Venules, and if the venules are again branched, 
their branches are Veinlets—the last series, whenever the venation 
is very compound, being distinguished as the ultimate veinlets. If the 
frond is pinnate, that is, divided into separate leaflets, or Pinna, as 
in Asplenium marinum, the analogue of the rib, which in the simple 
frond formed a costa, then becomes the Rachis, and each separate 
pinna has its own costa and series of veins as the undivided frond 
had. If the frond is bipinnate, that is, having the pinne again 
divided into separate leaflets called Pinnules, the part corresponding 
with the costa of the undivided frond becomes the primary rachis, 
or rachis of the frond; and that which corresponds with the costa of 
the pinna in the once-divided frond, becomes the secondary rachis, 
or rachis of the pinna; the pinnules each acquiring their respective 
costa and veins. If the frond is tripinnate, that is, having the 
pinnules distinctly divided, it then acquires a tertiary rachis and 
secondary pinnules, or Pinnulets, and these latter are furnished as 
before with a costa, and the series of veins proceeding from it. Once 
more divided, the fronds become quadripinnate, and these and. such as 
are still more divided are called decompound. The several series of 
pinne and pinnules in compound fronds may in any case be distin- 
guished as primary or secondary, dc. ; and the last series of divisions 
in a very much divided frond, are most readily and distinctly referred 
VOL. I. 0 

