50 



[SFXTION 7. 



The whoic surface is covprcd by a transparent skiu, tlic Epidermis, not 



unlike that whicli covers tiie surlace 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 tlie 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 fibrous and lough material 



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

 form of parallel threads or bundles of fibres ; 

 and iu the blade these spread out iu a hori- 

 zontal direction, to form the ribs and oeitis 

 of the leaf. The stout main branches of 

 the framework are called the Ribs. When 

 there is only one, as iu 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, Veinlets. The latter 

 subdivide again and agaiu, until they be- 

 come so fine that they are invisible to the 

 naked eye. Tlie fibres of which they are 

 composed are hollow; forming tubes by 

 which the sap is brought iuto the leaves 

 and carried to every part. 



12G. Venation is the name of the mode 

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

 veins are distributed in the blade. This is 



of two principal kinds ; namely, i\ie parallel-veined, and the mtied-veined. 



127. Iu 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 

 Quuice-leaf, iu Fig. 112, shows this kind of vciniug in a leaf witli a single 

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

 leaves of several ribs. 



128. In parallel-veined leaves, tlic whole framework consists of slender 

 ribs or veins, which ruu 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 furnisli a good illustration. Such 

 parallel veins Liiuiaeus called Nerves, and parallel-veined lea'.'es are still 

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



Fig. 112. Leaf of the Quince: h, blade; p, petiole; st, stipules. 



