THE LEAF, 55 



themum) ; and similar external forms are presented by the cylindrical or 

 flattened fistular leaves of the Onion, &c. 



If the blade stands alone upon an undivided petiole, or is sessile 

 on the stem, it is called simple (figs. 54, 55). Where the petiole is 

 branched, and bears more than one distinct blade, the leaf is com- 

 pound (fig. 64), and its separate blades are called leaflets. Both 

 simple leaves and leaflets may be entire that is, the blade may be 

 undivided at its margins ; or it may be more or less deeply incised 

 or lobed. The divisions or branchings of such leaves are analogous 

 to the monopodial branching of the stem (p. 38). 



Form. The general form of simple and compound leaves, and 

 the character of the subdivisions of the blade of simple leaves and 

 of leaflets, are associated with the plan of arrangement of the ske- 

 leton of the leaf. The solid framework of leaves is composed of 

 woody structures which when large are usually termed ribs (costce), 

 the small divisions being called indifferently nerves or veins. The plan 

 of arrangement of the framework is called the nervation or venation ; 

 the ordinary custom is to call the principal ribs nerves, and the 

 smaller branches veins. When a distinct principal rib, continuous 

 with the petiole, exists, it is called the midrib. 



The superabundance of terms is an inconvenience here as in many 

 other departments of Botany. Where it is necessary to select, it is 

 advisable to choose those terms which are least objectionable as not 

 involving hypothetical notions of function. 



Nervation or Venation. The modes of nervation of leaves may 

 be classed under four principal heads : 



1. Straight- or parallel-nerved (folia par allelinervia), when (with 

 or without a midrib) the principal ribs run in more or less 

 parallel lines from the base to the summit (fig. 66). 



2. Curvinerved (/. curvinervia), when the principal ribs run in 

 curves from the base to the summit (fig. 67), or from the mid- 

 rib to the margin (fig. 68) differing little from the foregoing, 

 but occurring in broader leaves. 



3. Palminerved (/. palminervia), when the principal ribs radiate 

 from a point at the base of the leaf (fig. 69). 



4. Penninerved (/. penninervia), when the strong midrib gives 

 off the side-ribs at a more or less acute angle, like the blades 

 on the shaft of a feather (figs. 68 & 70). 



The term triple-nerved (triplinervia) is sometimes used for a modifica- 

 tion of No. 4, approaching to No. 3, when the midrib gives oft' on each 

 side near the base a strong side-rib, wliich runs up within the margin 

 towards the summit. Feather-ribbed (penninerved) and hand-ribbed (pal- 

 minerved) leaves are most common among the Dicotyledons, but they 



