Contrast a prickle with the thorn of the hawthorn. The thorn does not come off easily, and it contains a woody core which is continuous with the wood of the branch. Cut lengthwise through the thorn and the branch which bears it to see this. The thorn is a short pointed branch; it arises in the axil of a leaf, and sometimes bears leaves itself. 2. The ivy.—Observe how the climbing stem of the ivy is attached to a wall or tree. It puts out a line of roots on its [Pg 76] shaded side. These roots give out a sticky fluid, which, on hardening, fixes them to the wall. 3. Twining stems.—Watch the growth of a convolvulus seedling. At first the young stem grows straight up, but soon the tip begins to move round and round. Try to find out how long it takes to describe one revolution. Put a long stick in the ground near the plant and notice how, when the revolving stem touches the stick, the spiral is henceforth described round the support and the stem consequently clings to it. Lay your watch face-upward, and notice whether the stem moves in the same direction as the hands (the “clockwise” direction), or in the opposite (“counter-clockwise”) direction. Make similar observations on the hop, honeysuckle, and scarlet runner, and note the results. 4. Leaf climbers.—Examine a climbing tropœolum (often, though wrongly, called “nasturtium”). Which parts of the plant clasp the support? Watch a young plant coming up. Does the stem revolve before the leaf stalks come in contact with the support? Compare the clematis. 5. Tendril climbers.—Examine plants of sweet pea, bryony, vine, passion flower, and cucumber. Try to find out in each case which part has been modified to form the tendrils. Watch a plant day by day until a free tendril grasps a support, and notice how it becomes spirally coiled. Can you straighten the tendril by pulling, without leaving any kinks? Why? Is the spiral of the tendril continuous, or does it change its direction in the middle? Make a continuous spiral with wire, and notice the kinks formed when the wire is straightened by pulling the ends. Which is better for the plant in a gale of wind, a continuous or a reversed spiral? Why? Notice the sucker-like tendrils of the Virginian creeper, and the way in which they fix the plant to the wall. Plants whose stems are not strong enough to stand erect without support must adopt some special means of spreading out their leaves to the light and air. One of the commonest devices of such plants is that of climbing up other and stronger plants, walls, trellis-work, etc. [Pg 77]