CHAP. II. 
LEAVES. 
131 
margin of the leaf, curving towards the apex in their course, 
and finally, at some distance within the margin, forming an 
anastomosis with the back of the primary vein, which lies next 
them. That part of the primary vein which is between the 
anastomosis thus described, having a curved direction, may be 
called the curved vein. Between this latter and the margin, 
other veins, proceeding from the curved veins, with the same 
curved direction, and of the same magnitude, occasionally in- 
tervene : they may be distinguished by the name of external 
veins (fig. 56. 1). The margin itself and these last are con- 
nected by a fine net-work of minute veins, which I would 
distinguish by the name of marginal veinlets. From the mid- 
rib are generally produced, at right angles with it, and alter- 
nate with the primary veins, smaller veins ; which may not 
improperly be named costal veins (fig. 56. 5). The primary 
veins are themselves connected by fine veins, which anastomose 
in the area between them ; these veins, when they immediately 
leave the primary veins, I would call proper veinlets (venulce 
proprioe) (fig. 56. 4) ; and where they anastomose, common 
veinlets (ven. communes). The area of parenchyma, lying be- 
tween two or more veins or veinlets, I name with the old 
botanists interveniurn. 
These distinctions may to some appear over-refined ; but I 
am convinced that no one can very precisely describe a leaf 
without the use either of them, or of equivalent terms yet to 
be invented. With respect to their venation only, leaves may 
be conveniently arranged under the following heads ; — 
1. Veinless (avenium), when no veins at all are formed, 
except a slight approach to a midrib, as in Mosses, Fuci, &c. 
Leaves of this description exist only in the lowest tribes of 
foliaceous plants, and must not be confounded with the fleshy 
or thickened leaves common among the higher orders of 
vegetation, in which the veins are by no means absent, but 
only concealed within the substance of the parenchyma. (See 
No. 10.) Of this De Candolle has two forms, — first, his 
folia nullinervia, in which there is not even a trace of a midrib, 
as in Ulva ; and second, his folia falsinervia, in which a trace 
of a midrib is perceptible. These terms appear to me un- 
K 2 
