BOTANY. 17 
in other cases it is continuous with the stem. When leaves fall off annually, 
or about the same time, they are called deciduous ; when they remain for sey- 
eral years, or only fall singly through the year, they are evergreen. 
By the venation or nervation of a leaf is meant the distribution of the 
fibro-vascular bundles in it. This may be traced in most cases, but 
instances occur, as in sea-weeds and other submerged plants, where true 
veins do not exist ; such are said to be veinless. There is generally a more 
or less central vein larger than the rest, called the midrib, giving off lateral 
veins (primary veins), which either end within the margin, or else go 
entirely out to the edge. Smaller veins, given off by the midribs, are called 
costal, these giving origin to veinlets. Sometimes, instead of one central 
midrib, there are several large ribs diverging from the part where the 
petiole enters the blade. These give off secondary veins, which, in turn, 
furnish tertiary, all apparently anastomosing, and giving a reticulated ap- 
pearance to the surface of the leaf. Such leaves are called reticulated, or net 
veined. Parallel veined leaves are those in which there is a central rib, 
giving off a single series of well defined veins, parallel to each other ; or else 
there are several ribs which run from base to apex, nearly parallel to the edge 
of the leaf or to each other. Leaves of this kind are of usual occurrence in 
endogenous plants. 
Leaves are either simple or compound. The simple leaf has but one ar- 
ticulation with the stem, and the incisions in the margin do not reach the mid- 
rib. The compound leaf has other articulations beyond that with the stem, 
or consists of leaflets separately attached to the petiole. All leaves at first 
are simple, and the nature of the compound character is intimately connected 
with the venation. 
A simple leaf is equal or Dati, according as the midrib bisects the blade 
symmetrically or not. Should the margin be even, without divisions, the 
leaf is entire. When the projections are more or less irregular and pointed, 
the leaf is dentate ; when arranged regularly, and like the teeth of a saw, 
we have a serrate leaf; crenate, when the serrations are rounded. Should 
the indentations of the margin extend about half-way to the midrib,-the 
leaf is cleft (fidus), the segments are fissures. A continuation of the 
division to the midrib gives us a partite leaf, with partitions for the 
segments. 
Should the divisions occur im a feather-veined leaf, this becomes pinnatifid 
when the divisions extend about to the middle, and are rather broad; 
pectinate, when they are narrow, like the teeth of a comb. Should the 
incisions extend to the midrib, the leaf becomes pinnatipartite. These 
primary divisions may again be subdivided, forming a bipinnatifid, or bi- 
pinnatipartite leaf; tripinnatifid indicates a still further subdivision. A 
pinnatifid leaf 1s eesti when the divisions are sub-triangular, with the 
extremities pointed slightly backwards, as in the Dandelion. When the apex 
consists of a large rounded lobe, and the somewhat rounded divisions be- 
come gradually less and less towards the base, the leaf is Jyrate. It is pan- 
duriform when, with a rounded apex, there is a concavity on each side, like 
that of a violin. 
ICONOGRAPHIC ENCYCLOPEDIA.—VOL. I. 2 17 
