ON GENERA AND SPECIES, 18 
called veins; these veins, which either radiate from the base 
of some fronds as also from segments, in a flabellate 
manner; but are more generally produced from a midrib 
(costa), which in simple fronds is a continuation of the 
vascular cords of the stipes; and in segments of fronds, a — 
branch of the rachis from which they are produced. 
The costa is generally central, or in some excentric, or 
even quite on one side of the segment (unilateral); it 
generally terminates at or near the apex of the frond or 
segment, or it becomes undefined before reaching the apex, 
or is very short, rudimentary, or absent. It is generally 
elevated in the form of a ridge on one or both sides of the 
lamina, or it is in the form of a channel on the upper side, 
or it is imbedded in the substance of the frond, and then 
often obscure. 
From the sides of the costa veins are produced at a more - S | 
or less distance from each other, and generally equal or 
nearly so, on both its sides; they form a more or less 
acute, oblique, or nearly a right (patent) angle with the 
| Costa, and when free always directed outwards towards the 
margin or apex of the frond or segment. They are either 
straight, curved or flexuose, and are either simple or branch 
in various ways, the first are termed primary veins, their 
branches venules, and the branches of these veinlets, each — 
form, and the diminutive costule is applied to the Loc > 
veins of laceane. us 
Veins are termed free doi all the branches of cach 
being a diminutive of the preceding. In many cases the - | 
primary vein that rises direct from the midrib is very short, 
` &nd branches sometimes close to the midrib, or even within —— 
the substance of the midrib, as in Neottopteris australiaca ; x 
in others they terminate at or near the margin, and when ` ` 
parallel to one another and very evident are termed cosic- e 
