Most wonderful of all climbing organs are the tendrils. They are well seen in Fig. 48. A part of the plant—sometimes a leaflet, as in the pea (Fig. 28); sometimes a branch, as in the passion flower; or a flower stalk, as in the vine—becomes modified into a thread, slight but strong. When the end of the thread touches and then twines round a support, the whole tendril forms itself into a spiral which, like a wire spring, draws the plant up to the support, and can yet lengthen and yield to the wind when necessary. In the middle of the tendril the direction of the spiral is reversed, so that the tendril can be straightened without being twisted. [Pg 80] The tendrils of the Virginian creeper do not twine, but on meeting a wall they form round red suckers at the end, and attach the plant (Fig. 49). Fig. 48.—How a tendril grasps a support. Fig. 49.—Virginian Creeper. The spiral is reversed at x. R, R, stem tendrils. (× ½.) 14. CREEPING AND UNDERGROUND STEMS. 1. A creeping stem.—Examine a plant of the ground ivy. Is the stem strong enough to stand upright? How does it spread out its leaves to the light and air? The stem grows along the ground, and at intervals it gives off a pair of leaves which grow upwards, and a tuft of roots which grow down to the ground. 2. A runner.—Is the “runner” of the strawberry of the same nature, i.e. is it a continuous stem like that of the ground ivy? When the plant is carefully examined, the creeping “stem” is seen to be a branch arising in the axil of a leaf. The branch runs along the ground for a little distance, and then roots itself and gives off a [Pg 81] number of leaves. In the axil of one of these another branch arises and runs on in the same direction. The same branch does not run on and on. 3. A stolon.—Follow carefully the underground part of the couch grass and make out its connection with the main shoot. It is a branch like the runner of the strawberry, which arises in the axil of a leaf, and extends only to the next shoot. Compare the stolons of the cinquefoil. 4. A potato tuber.—Examine a potato tuber (the part which is eaten). Notice the “eyes.” These are buds, with scale leaves. Leaves never occur on roots, so that the potato must be an underground stem. Put a pin into every eye, and wind a thread round the tuber along the bases of the pins. It forms a spiral. Cut the potato into halves, and pour a drop of iodine solution on the cut surface. What is the meaning of the blue dots which at once make their appearance? Plant a potato in warm, moist earth, and when it has sprouted notice that each bud (eye) has given rise to a branch.