FORMS OF PETIOLES OB LEAF-STALKS. 



81 



quently an articulation or a constriction with a tendency to disunion, 

 and at the same time there exists a swelling (fig. 203 JP), called pulvmus 

 (pulvmw, a cushion), formed by a mass of cellular tissue. 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 united to the lamina, or where the midrib joins the leaflets of a 

 compound leaf, there is occasionally a cellular dilatation called struma 

 (struma, a swelling), with an articulation. This articulation or joint is 

 by many considered as indicating a compound leaf, and hence the leaf of 

 the orange is considered as such, although it consists of one undivided 

 lamina (fig. 185). In articulated leaves, the pulvinus may be attached 

 either to the petiole or to the axis, and may fall with the leaf, or remain, 

 attached to the stem. When articulated leaves drop, their place is marked 

 by a cicatrix or scar, seen below the bud in fig. 203. In this scar, the re- 

 mains of the vascular bundles, c, are seen; and its form furnishes charac- 

 ters by which particular kinds of trees may be known when not in leaf. 

 157. The petiole varies in length, being usually shorter than the lamina, 

 but sometimes much longer. In some palms it is fifteen or twenty 

 feet long, and is so firm as to be used for poles or walking-sticks. 

 In general, the petiole is more or less rounded in its form, the upper 

 surface being flattened or grooved. Sometimes it is compressed later- 

 ally, as in the Aspen, and to this peculiarity the trembling of the leaves 

 of this tree is attributed. In aquatic plants, the leaf-stalk is sometimes 

 distended with air 

 (fig. 183 />), as in 

 Pontederia and Tra- 

 pa, so as to float 

 the leaf. At other 

 times it is winged, 

 or has a leaf-like ap- 

 pearance, as in the 



183 184 185 



pitcher plant (fig. 184 p), orange (fig. 185 p), lemon, and Dionsea 



Fig. 183. Leaf with a quadrangular toothed lamina or blade, /, and an inflated petiole, p, con- 

 taining air cells. 



Fig. 184. Ascidium or pitcher of Nepenthes, p, Winged petiole which becomes narrowed, and 

 then expands so as to form the pitcher by being folded on itself. , The operculum or lid, formed 

 by the blade of the leaf, and articulated to the pitcher. 



Fig. 18-5. Leaf of Orange, which some call compound, p, Dilated or winged petiole, united by 

 an articulation to the blade. In such a leaf, if the vessels of the petiole were developed in a cir- 

 cular manner, so as to form a pitcher, the lamina or blade would form the jointed lid. 



G 



