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 reins, 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 veiy 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. 
