FOUMS OF PETIOLES OR iEAF-STALKS. 



95 



united to the lamina, or where the midrib joins the leaflets of a com- 

 pound 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. 201). 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. 

 220. In this soar the remains of the vascular bundles, c, are seen ; 

 arid its form furnishes characters by which particular kinds of trees 

 may be known when not in leaf. In the case of many Palms and 

 Tree-ferns, the scars or cicatrices of the leaves are very conspicuous. 

 In fossil plants important characters are founded on them. 



The petiole varies in length, being usually shorter than 

 lamina, but some- 

 times much longer. 

 In some Palms it 

 is fifteen or twenty 

 feet long, and 

 so firm as to 

 used for poles 

 walking-sticks. 



the 



Fig. 199. 



Fig. 200. 



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

 surface being flattened or grooved. Sometimes it is compressed 

 laterally, 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. 199 p), as in Pontederia and Trapa, 

 so as to float the leaf At other times it is. winged, or has a leaf-like 

 appearance, as in the pitcher plant (fig. 200 p), orange (fig. 201 p), 



Fig. 199. Leaf with a quadrangular toothed lamina or blade, I, and an inflated petiole, p. 

 containing air-cells. Fig. 200. Asoidium or pitcher of Nepenthes, p, Winged petiole 

 ■which heoomes narrowed, and then expands so as to form the pitcher a, by folding on 

 itself, (i, The operculum or lid, supposed to be formed by the blade of the leaf, and articu- 

 'lated to the pitcher. Fig. 201. Leaf of Orange, which some call compound. 5, DUated 

 or winged petiole, united by an articulation to the blade. In such a leaf, if the vessels of 

 the petiole were developed in a circular manner, so as to form a pitcher, the lamina or blade 

 would form the jointed lid. 



