80 



FORMS OP COMPOUND LEAVES. 



other, the leaf is alternately pinnate (fig. 175); and when the pinna? 

 are of different sizes, the leaf is interruptedly pinnate (fig. 180). 



154. In the case of a leaf with radiating venation, if the ribs are 

 separately covered with parenchyma, and each leaflet is articulated to 

 the petiole, the leaf becomes ternate (figs. 141, 181)if there are three 

 divisions; quaternate, if four (fig. 142); quinate, if five; septenate, if 

 seven (fig. 176), and so on.* If the three ribs of a ternate leaf sub- 

 divide each into three primary veins, which become covered with 

 parenchyma so as to be separate articulated leaflets, the leaf is biternate; 

 and if another three-fold division takes place, it is triternate (fig. 182). 



.1S2 



155. Petiole OP Leaf-stnlk. This is the part which unites the limb 

 or blade of the leaf to the stem (figs. 132 and 175 p). It consists of one 

 or more bundles of vascular tissue, with a varying amount of paren- 

 chyma. The vessels are, spiral- vessels connected with the medullary 

 sheath in Exogens and with the fibro-vascular bundles in Endogens, 

 porous vessels and other forms of fibro-vascular tissue, woody tissue, 

 and laticiferous vessels. These vessels are enclosed in an epider- 

 mal covering, with few stomata, and are more or less compressed. 

 When the vascular bundles reach the base of the lamina, they separate 

 and spread out in various ways, as already described under venation. 

 A large vascular bundle is continued through the lamina to form the 

 midrib (fig. 133, n m), and sometimes several large bundles form 

 separate ribs (figs. 146, 162), whilst the ramifications of the smaller 

 bundles constitute the veins. 



156. At the place where the petiole joins the stem, there is fre- 



* Some apply the term digitate to radiating compound leaves with five or seven leaflets. 



Fig. 180. Impart -and alternately pinnate leaf. Leaflets or pinnas sessile, and serrated at the 

 margin. 



Fig. 181. Ternate leaf of Strawberry. Margin of leaflets, toothed or dentate, p, Petiole with 

 projecting hairs. I, Lamina divided into three leaflets. 



Fig. 182. Tritemate leaf. Leaflets cordate. 



