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.
Scramblers and climbers.—In the simplest cases the plant simply scrambles over other plants. Many brambles and roses are merely scramblers, but more often they are true climbers, weaving themselves among their neighbours by the help of hooked prickles ([Fig. 76]). The prickles point backwards, and therefore anchor the twigs firmly, as is very evident on trying to pull a branch out of the hedge. A prickle may easily be broken off by a side push, for it is merely an outgrowth of the rind, and does not contain any woody core. It is so attached, however, that it resists much greater force in the lengthwise direction—a manifest advantage to the plant.
Fig. 45.—Ivy climbing up a wall. R, aërial roots. (× ⅓.)
The differences between such a prickle and a thorn like that of the hawthorn should be carefully noticed. A thorn is really a little twig which has remained short and become pointed at the end. It has a core of wood which is continuous with the wood of the branch bearing it. That a thorn is really a little branch is shown by its origin in the axil of a leaf, and by its often giving rise to leaves and buds.
