50 



LEAVES. 



[SECTION 7. 



The whole surface is covered by a transparent skin, the Epidermis, not 



unlike that which covers the surface of all fresh shoots. 



12 i. Note that the leaf-blade expands horizontally, that is, normally 



presents its faces one to the sky, the other to the ground, or when the 



leaf is erect the upper face looks toward the stem that bears it, the lower 



face away from it. Whenever this is not the case there is something to be 



explained. 



125. The framework consists of wood, a flbrous and tough material 



which runs from the stein through the leaf-stalk, when there is one, in the 



form of parallel threads or bundles of fibres ; 

 and in the blade these spread out in a hori- 

 zontal direction, to form the ribs and ceins 

 of the leaf. The stout main branches of 

 the framework are called the Ribs. When 

 there is only one, as in Fig. 112, 114, or a 

 middle one decidedly larger than the rest, 

 it is called the Midrib. The smaller divi- 

 sions are termed Veins ; and their still 

 smaller subdivisions, Vemlets. The latter 

 subdivide again and again, until they be- 

 come so fine that they are invisible to the 

 naked eye. The fibres of which they are 

 composed are hollow ; forming tubes by 

 which the sap is brought into the leaves 

 and carried to every part. 



126. Venation is the name of the mode 

 of veiuing, that is, of the way in which the 

 veins are distributed in the blade. This is 



of two principal kinds ; namely, the parallel-veined, and the ndted-veined. 



127. In Netted-veined (also called Reticulated} leaves, the veins branch 

 off from the main rib or ribs, divide into finer and finer veinlets, 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 

 Quince-leaf, in Fig. 112, shows this kind of vcining in a leaf with a single 

 rib. The Maple, Basswood, Plane or Buttonwood (Fig. 74) show it in 

 leaves of several ribs. 



128. In 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 Lily of the Valley (Fig. 113) will furnish a good illustration. Such 

 parallel veins Linnaeus called Nerves, and parallel-veined leaves are still 

 commonly called nerved leaves, while those of the other kind are said to be 



Fio. 112. Leaf of the Quince: b, blade; p, petiole; st, stipules. 



