q + 
zn ert ‘ é,, WA nah i, ‘ 
The External Form of Plants. 105 
All the terms now defined in reference to the attachment, 
form, incision, and division of simple leaves apply equally to 
the leaflets of compound leaves. 
The disposition and course of the ribs and veins, or the 
venation of leaves, is greatly dependent on the form of the 
leaf (or perhaps the converse is rather the case). The term 
principal nerve or mzd-rzd is given to the one which 1s most 
_ strongly developed, and which passes through the centre of 
the leaf ; the larger branches which go out from it are the 
lateral rzbs, and the smaller 
ramifications the vezs (see 
Pas 181, p. 102). If aleaf 
has only a single unbranch- 
ed mid-rib, as in many 
Conifer, the venation is 
said to be simple. If the 
principal vein, when it en- 
ters the lamina of the leaf, 
divides at once into several 
branches, the terms fa/- 
menerved (Fig. 191), pedate- 
Be 83, p. 102), one oh Malnterel leat of Core 
pelitnerved (Fig. 162, p. 96), 
are given, according to the shape of the leaf. When the - 
mid-rib runs through the leaf to its apex, sending out lateral. 
branches from each side, the leaf is pznnatinerved (Fig. 163,. 
_p.97). When several nerves enter the leaf from the stem 
or petiole, they may be nearly straight and parallel to one 
another, as in grasses, or somewhat curved asin the lily of the 
valley, but are said in either case to be parallel-veined. The 
writers. They may be combined in a variety of ways, as Linear-lanceo- 
late, cordate-ovate, &c. The following, not mentioned in the text, are 
also in use among English writers : deltoid, with three nearly equal 
angles ; swbudate, nearly cylindrical and awl-shaped ; apiculate, ending 
in a short sharp point ; czsfédate, with a slenderer sharp point. The 
margin is revolute when turned backwards on to the under side.—ED. ] 
