170 THE LEAVES. 



has recently been made known by Dr. Torrey. In this the enlarged 

 summit of the tube is strongly arched hke a hood (as in Sarracenia 

 psittacina of the Southern States), and is abruptly terminated by a 

 singular two-lobed foHaceous appendage, resembhng the forked tail 

 of a lish. 



303. The Petiole, or Leafstalk, is usually either round, or half-cylin- 

 drical and channelled on the upper side. But in the Aspen, it is 

 strongly flattened at right angles with the blade, so that the slightest 

 breath of air puts the leaves in motion. It is not unfrequently fur- 

 nished with a leaf-like border, or ring ; 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 alate or winged. In many Umbelliferous plants, the 

 petiole is dilated below into a broad and membranaceous inflated 

 sheath ; and in a great number of Endogenous plants the petiole 

 consists of a sheath, embracing the stem, which in Grasses is fur- 

 nished at the summit with a membranous appendage, in some sort 

 equivalent to the stipules, called the ligvle (Fig. 237). 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 

 leafstalli when it falls off, and on the scar left on the stem, as so 

 many round dots (Fig. 153, 6), of a uniform number and arrange- 

 ment in each species. 



304. Phyllodla (Fig. 302, 303). Occasionally the whole petiole 

 dilates into a kind of blade, traversed by ribs, mostly of the parallel- 

 veined kind. In these cases the proper blade of the leaf commonly 

 disappears ; this substitute, called a PhyUodium (meaning a leaf-like 

 body), taking its place. These phyllodia constitute the whole foliage 

 of the numerous Australian Acacias. Here they are at once dis- 

 tinguished from leaves with a true blade by being entire and parallel- 

 veined ; wliile their proper leaves (of which the earlier ones uni- 

 formly appear in germination, and also later ones in casual instances) 

 are compound and netted-veined. They are also to be recognized by 

 their uniformly vertical position, presenting their margins instead of 

 their surfaces to the earth and sky ; and they sometimes bear a true 

 compound lamina at the apex, as in Fig. 303. 



305. Stipules (259, Fig. 229) are lateral appendages of leaves, 

 usually appearing as small foliaceous bodies, one on each side of the 

 base of the petiole. They are not found at all in a great number of 



