ON GENERA AND SPECIES. ] 3 



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 

 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 

 being a diminutive of the preceding. In many cases the 

 primary vein that rises direct from the midrib is very short, 

 and branches sometimes close to the midrib, or even within 

 the substance of the midrib, as in Neottopteris australiaca ; 

 in others they terminate at or near the margin, and when 

 parallel to one another and very evident are termed costce- 

 form, and the diminutive costulce is applied to the primary 

 veins of laceanae. 



Veins are termed free when all the branches of each 



