BOTANY. • 17 



in other cases it is continuous ^vith the stem. When leaves fall off annually, 

 or about the same time, they arc called deciduous ; when they remain for sev- 

 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, givinix off lateral 

 veins (pritnari/ 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, Avhich, in turn, 

 furnish tertiär}', all apparently anastomosing, and giving a reticulated ap- 

 pearance to the surfiice 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 oblique, 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 paiiitions for the 

 segments. 



Should the divisions occur in a feather-veined leaf, this becomes pinnatifid 

 when the divisions extend about to the middle, and are rather broad ; 

 pectinate, when they are narrov/, like the teeth of a comb. Should the 

 incisions extend to the midrib, the leaf becomes jiinnatipartite. These 

 primary divisions may again be subdivided, forming a bipinnatifid, or bi- 

 pinnatipartite leaf; tripinnatifid indicates a still further subdivision. A 

 pinnatifid leaf is runcinate when the divisions are sub-triangular, with the 

 extremities pointed slightly backAvards, 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 lyratc. It is pan- 

 duriform, when, with a rounded apex, there is a concavity on each side, like 

 that of a violin. 



ICONOGRAPHIC ENCYCLOPiEDIA. — VOL. It. 2 17 



