172 



THE LEAVES. 



quently furnished with a leaf-like border, or wing ; which, in the 

 Sweet Pea of the gardens, extends downward along the stem, on 

 which the leaves are then said to be decurrent ; or the stalk or 

 stem thus bordered is said to be alette or winged. In many Um- 

 belliferous plants, the base of the petiole is dilated into a broad and 

 membranaceous inflated sheath ; and in a great number of Endoge- 

 nous plants, especially in Grasses, the petiole consists of a sheath, 

 embracing the stem, which in the true Grasses is furnished at the 

 summit with a membranous appendage, in some sort equivalent to 

 the stipules, called the ligule (Fig. 195). In the proper Pea tribe, 

 the apex of the petiole is often changed into a tendril (Fig. 216) ; 

 and in one plant of that tribe (Lathyrus Aphaca), the whole petiole 

 becomes a tendril, the office of the leaf being fulfilled by a pair of 

 large stipules. Sometimes, as in one section of Astragalus, the 

 petioles harden into spines after the leaflets fall off. 



299. The woody and vascular tissue runs lengthwise through 

 the petiole, in the form usually of a definite number of parallel 

 threads, to be ramified in the blade. The ends of these threads are 

 apparent on the base of the leafstalk when it falls off, and on the 

 scar left on the stem, as so many round dots (Fig. 130, 127, Z>), of 

 a uniform number and arrangement in each species. Sometimes 

 they are so close as to be confluent into a continuous line or bundle. 



300. Phyllodia (Fig. 226, 227). Occasionally the woody sys- 

 tem spreads and the whole petiole dilates into a kind of blade, 



FIG. 223. Pitchers of Heliamphora; 224, of Sarracenia purpurea ; 225, of Nepenthes. 226. 

 A phyllodium of a New Holland Acacia. 227. The same, bearing a reduced compound blade. 



