94 FOEMS OF PETIOLES OE LEAF-STALKS. 



4. The conformation of leaves depends partly on the venation, and partly on the 



mode in which the parenchyma is developed. 



5. Leaves are either simple, i.e. composed of one piece, or componnd, i.e. com- 



posed of one or more articulated leaflets. 



6. Simple leaves are either entire or divided into segments. When the divisions 



are marginal, they are dentate, serrate, or crenate ; when the divisions are 

 deeper, cleft or partite. 

 1. Simple unicostate (one-ribbed) leaves having their parenchyma cut laterally 

 into various lobes, so that the divisions extend to about the middle of 

 each half of the lamina, may be referred to the Pinnatifid type, including 

 bipinnatifid, pectinate, pandurifoi-m, runcinate, and lyrate forms ; when the 

 divisions extend nearly to the midrib the form is pinnati-partite. 



8. Simple multicostate (many-ribbed) leaves, with the ribs divergent, when cut 



longitudinally into various lobes, the divisions extending to about the 

 middle of the lamina, may be referred to the Palmatifid type, including 

 trifid, quinquefld, pedate, and dissected forms ; when the divisions extend 

 to near the base the forms are palmately-partite or dissected. 



9. Simple leaves, with convergent ribs, are rarely divided deeply, and such is also 



the case with parallel-veined leaves, the margins of which are often entire. 



10. Simple leaves, whether unicostate or multicostate, with lobes or divisions at 



their base, exhibit rcniform, cordate, sagittate, and hastate forms ; with 

 lobes or divisions at their apex, emarginate and obcordate forms. 



11. Compound unicostate leaves, having lateral articulated leaflets, may be 



referred to the Pinnate type, including bipinnate, tripinnate, and decom- 

 pound forms. 



12. Compound multicostate leaves, with divergent ribs, divided longitudinally into 



articulated leaflets, may be referred to the Digitate type, including temate, 

 triternate, quaternate, and quinate forms. 



Petiole oe Leaf-Stalk. — This is the part which unites the limb 

 or hlade of the leaf to the stem (figs. 147 and 191 p). It is absent 

 in sessile leaves, and in many sheathing leaves is not well defined. It 

 consists of one or more bundles of vascular tissue, with a varying 

 amount of parenchyma. 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 

 epidermal 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. 148, n m), and sometimes several large bundles form 

 separate ribs (figs. 161, 177), whilst the ramifications of the smaller 

 bundles constitute the veins and veinlets. 



At the place where the petiole joins the stem there is frequently 

 an articulation, or a constriction with a tendency to disunion, and at 

 the same time there exists a swelling (fig. 220 p), called pulvinus 

 (pulvinus, a cushion), formed by a mass of cellular tissue, the cells of 

 which occasionally exhibit the phenomenon of contractility. At other 

 times the petiole is not articulated, but is either continuous with the 

 stem, or forms a sheath around it. At the point where the petiole is 



