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.) [Pg 78] 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. Fig. 46.—Climbing stem Fig. 47.—Climbing stem of Honeysuckle. (× ¼.) of Convolvulus. (× ¼.) The stem of the ivy climbs by means of little roots, which it puts out on the side furthest from the light (Fig. 45). [Pg 79] These give out a sticky liquid which, on drying, cements them to the wall or tree. Twining stems are much in advance of these. There seems to be something approaching intelligence in the manner in which the young stem of a hop, honeysuckle, or convolvulus, which at first grows straight up, begins to wander round and round, tracing a spiral path in the air until it touches a support. Then, however, as if the plant could feel, the movement below the point of contact stops; but the upper part of the stem still revolves and therefore twines round the support. The stems of the honeysuckle (Fig. 46) and the hop turn in the same direction as the hands of a clock. This is called the “clockwise” direction. On the other hand the convolvulus (Fig. 47) and most other twining stems are “counter-clockwise” climbers. The stem of the bittersweet revolves indifferently in either direction. Sensitive clasping organs in their simplest form are seen in the twining leaf stalk of the clematis and tropœolum; the stem itself revolves as if to give its leaf stalks every opportunity of finding suitable supports. The leaf stalks seize these and twine round them.