48 FERNS : BRITISH AND FOREIGN. 



a continuation or branch of the ultimate rachis with which it is 

 either adherent or articulated. It is generally central ; but is 

 sometimes excentric, or even quite on one side (unilateral), or some- 

 times there is no costa at all. From the sides of the costa veins are 

 produced at more or less distance from each other, generally equal 

 on each side, except when the costa is excentric or the frond or 

 segment has a radiating axis. The direction of the first or primary 

 veins is, as in the leaves of other plants, towards the margin and 

 apex of the frond or segment, forming a more or less acute or 

 obtuse angle, or sometimes nearly a right angle with the costa. 



In describing venation the words veins, venules, and veinlets are 

 employed, each successive one of which is intended as a diminutive 

 of the preceding ; " veins " being applied to the first ramification 

 of the midrib, "venules" to the branches, and "veinlets" to 

 the branches of the venules. Some fronds have veins only, others, 

 veins and venules, and others again all three. 



Terms are occasionally employed to express the relative distinct- 

 ness of the venation, particularly when any marked peculiarity 

 exists : thus it is said to be 



Elevated, or external, when they are so thick that they are readily 

 seen and felt on the under surface of the frond ; and 



Internal when very much sunk in the substance of the frond. 



The primary veins are 



Costceform when very strong and well defined, more or less re- 

 sembling the costa in general appearance ; 



Undefined when of the same size as and not distinguishable from 

 the venules and veinlets ; and 



Evanescent when they gradually disappear towards the margin. 



Veins are spoken of as 



Free when each vein proceeding from the midrib, however much it 

 may be divided, is entirely unconnected with the neigh- 

 bouring ones ; and 



Anastomosing when the venules of one vein are in some way con- 

 nected with those of the next. 



A fascicle comprehends a single vein with all its venules and 

 veinlets. 



