THE LEAF 



77 



leaf stalk, -wlien there is one, in the form of parallel threads or bundles 

 of fibers; and in the blade these spread out in a horizontal direction, 

 to form the ribs and veins of the leaf. The stout main branches of 

 the framework are called the ribs. When there is only one, as in 

 Fig. 01, or a middle one decidedly larger than the rest, it is called the 

 midrib. The smaller divisions are termed veins; and their still 

 smaller subdivisions, ceinlels. The latter subdivide again and again, 

 until they become so fine tliat they are invisible to the naked eye. 1'he 

 fibers of which they are coijiposed are hollow ; forming tubes by which 

 the sap is brought into the leaves and carried to every part. 



121. Venation is the name of the mode of veining; that is, of the 

 way in which the veins are distributed in the blade. Tljis is of two 

 principal kinds; namely, i\ie parallel-veined, and tlie nelleil-veined. 



122. In neited-veined (also called reticulated) leaves, the veins 

 branch off from the main rib or ribs, divide into finer and finer vein- 

 lets, and the branches unite with each other to form meshes of network. 

 That is, they anastomose, as anatomists 



say of the veins and arteries of the body. 

 The Willow leaf, in Fig. (jl, shows this 



61. Reticulated veration of a 

 AVillow leaf. — Ettings- 



HAUSEN. 



02. Parallel venation of the 

 Lily of the Valley leaf. 

 — Ettinoshaosen. 



kind of veiniTig in a leaf with a single rib. The I\Iaple, Basswood, 

 and Plane or P>tittonwood show it in leaves of several ribs. 



123. Tn parallel-veined leaves, the whole framework consists of 

 slender ribs or veins, which run parallel with each other, or nearly so, 

 from the base to the point of the leaf, — not dividing and subdividing, 

 nor forming meshes, except by minute cross veinlets. The leaf of any 

 grass or that of the Lilly of the Valley (Fig. 62) will furnish a good 



