[Illustration: FIG. 119.—THE COMMON PITCHER PLANT (_Sarracenia purpurea_) showing the tubular leaves and the odd, long-stalked flowers.] =Carnivorous Plants.=—Certain plants capture insects and other very small animals and utilize them to some extent as food. Such are the sundew, which has on the leaves sticky hairs that close over the insect; the Venus’s fly-trap of the Southern States, in which the halves of the leaves close over the prey like the jaws of a steel trap; and the various kinds of pitcher plants that collect insects and other organic matter in deep, water-filled, flask-like leaf pouches (Fig. 119). The sundew and the Venus’s fly-trap are sensitive to contact. Other plants are _sensitive to the touch_ without being insectivorous. The common cultivated sensitive plant is an example. This is readily grown from seeds (sold by seedsmen) in a warm place. Related wild plants in the south are sensitive. The utility of this sensitiveness is not understood. =Parts that Simulate Leaves.=—We have learned that leaves are endlessly modified to suit the conditions in which the plant is placed. The most marked modifications are in adaptation to light. On the other hand, _other organs often perform the functions of leaves_. Green shoots function as leaves. These shoots may look like leaves, in which case they are called _cladophylla_. The foliage of common asparagus is made up of fine branches: the real morphological leaves are the minute dry functionless scales at the bases of these branchlets. (What reason is there for calling them leaves?) The broad “leaves” of the florist’s smilax are cladophylla. Where are the leaves on this plant? In most of the cacti, the entire plant body performs the functions of leaves until the parts become cork-bound. =Leaves are sometimes modified to perform other functions than the vital processes=: they may be tendrils, as the terminal leaflets of pea and sweet pea; or spines, as in barberry. Not all spines and thorns, however, represent modified leaves: some of them (as of hawthorns, osage orange, honey locust) are branches. [Illustration: FIG. 120.—EXCLUDING LIGHT AND CO_{2} FROM PART OF A LEAF.] [Illustration: FIG. 121.—THE RESULT.]