LAMINA.—VENATION OR NERVATION. 157 
bent or rolled, we shall describe four varieties :—1st, when in- 
volute leaves are applied together in a circle without over- 
lapping (fig. 304), they are said to be induplicate ; 2nd, if the 
leaves are conduplicate, and the outer successively embrace and 
sit astride of those next within them as if on a saddle (fig. 
305), as in the Privet, and the leaves of the Iris at their base, they 
are equitant ; 3rd, if the half of one conduplicate leaf receives 
in its fold the half of another folded in,the same manner (jig. 
306), as in the Sage, the vernation is half-equitant or obvolute ; 
and 4th, when a convolute leaf encloses another which is rolled 
up in a like manner (jig. 307), as in the Apricot, the vernation 
is supervolute. 
The terms thus used in describing the different kinds of ver- 
nation are also applied in like manner to the component parts 
of the flower-bud, that is, so far as the floral envelopes are con- 
cerned, under the collective name of exstivation or prxfloration. 
We shall have therefore to refer to some of them again, together 
with others, not found in the leaf-bud, when speaking of the 
flower-bud. 
4. LAMINA OR BLADE. 
We have already seen that the leaf (jigs. 273 and 274) in its 
most highly developed state consists of three parts ; namely, of 
a lamina or blade, a petiole or stalk, and of a stipular portion. 
We have now to describe each of these portions in detail, com- 
mencing with the lamina or blade. 
VENATION OR NeERvatIon.—The term venation is applied 
generally to indicate the various modes in which the veins are 
distributed throughout the lamina. These veins have also been 
called nerves, and their distribution nervation ; but the latter 
terms, by indicating an analogy which does not exist between 
them and the nerves of animals, are better avoided ; hence we 
shall in future always use the terms veins and venation. 
In some plants, as Mosses, and those living under water, &c., 
the leaves have no fibro-vascular skeleton, and consequently no 
true veins, and are hence said to be veinless ; while in succulent 
plants the veins are hidden more or less from view, in consequence 
of the great development of parenchyma, in which case the leaves 
are termed hidden-vined. 
In those leaves where the veins are well marked, they are 
subject to various modifications of arrangement, the more im- 
portant of which need only be mentioned here. Thus, when 
there is but one large central vein, proceeding from the base to 
the apex of the lamina, and from which all the other veins pro- 
ceed, such a vein is called the midrib or costa (fig. 308); or 
when there are three or more large veins, which thus proceed 
from the base to the apex (fig. 309), or to the margins (fig. 310), 
of the lamina, the separate veins are then termed 7.bs. The 
