172. The leaf of Nepenthes (Fig. [175]) combines three structures and uses. The expanded part below is foliage: this tapers into a tendril for climbing; and this bears a pitcher with a lid. Insects are caught, and perhaps digested, in the pitcher.
Fig. 176. Leaves of Dionæa; the trap in one of them open, in the others closed.
[173.] Leaves as Fly-traps. Insects are caught in another way, and more expertly, by the most extraordinary of all the plants of this country, the Dionæa or Venus's Fly-trap, which grows in the sandy bogs around Wilmington, North Carolina. Here (Fig. [176]) each leaf bears at its summit an appendage which opens and shuts, in shape something like a steel-trap, and operating much like one. For when open, no sooner does a fly alight on its surface, and brush against any one of the two or three bristles that grow there, than the trap suddenly closes, capturing the intruder. If the fly escapes, the trap soon slowly opens, and is ready for another capture. When retained, the insect is after a time moistened by a secretion from minute glands of the inner surface, and is digested. In the various species of Drosera or Sundew, insects are caught by sticking fast to very viscid glands at the tip of strong bristles, aided by adjacent gland-tipped bristles which bend slowly toward the captive. The use of such adaptations and operations may be explained in another place.
[§ 3.] STIPULES.
174. A leaf complete in its parts consists of blade, leaf-stalk or petiole, and a pair of stipules. But most leaves have either fugacious or minute stipules or none at all; many have no petiole (the blade being sessile or stalkless); some have no clear distinction of blade and petiole; and many of these, such as those of the Onion and all phyllodia ([166]), consist of petiole only.
175. The base of the petiole is apt to be broadened and flattened, sometimes into thin margins, sometimes into a sheath which embraces the stem at the point of attachment.
