To explain the massiveness of a tree trunk we have to remember that, while the cross-section of any structure varies as the square of its linear dimension, the volume varies as the cube of the same. If we double the dimensions of a tree, we increase its weight eight times, but the strength of the trunk is increased only four times. If a tree 100 feet high is supported on a stem 6 feet in diameter, a tree 200 feet high of the same proportions would need a stem not 12 feet, but over 17 feet in diameter, to be supported equally efficiently. This proportion increases rapidly: a similar tree 300 feet high would need a stem 30 feet in diameter; a tree 1,000 feet high would require a stem 180 feet in diameter, or 32,400 square feet in cross-section. We see, then, why a limit of tree growth is rapidly reached, at about 300 feet, and why the trees which grow to that height have trunks which are one of the wonders of the world, exceeding 30 feet in diameter, or about 100 feet in circumference. Climbing stems represent efforts on the part of plants to economize material by utilizing the rigidity of neighbouring plants, and by reaching to the light on their shoulders. Here, as in aquatics, the _rope_ type of stem is in evidence; it resembles a garden hose, offering great flexibility and conducting capacity, but without rigidity to support its own weight, much less that of the leaves and flowers which it bears. To secure support, the stem itself (or branches of it), the leaves, or the stipules (leafy projections on either side of the junction of leaf and stem), are used. Sometimes support is obtained by twining (compare Convolvulus, Grape-Vine, Vetch), sometimes by adherent discs (Virginia Creeper), or aerial roots (Ivy), often by mere scrambling, often aided by reflexed hooks on leaf and stem (Bramble, Cleavers). The mechanism by which twining is accomplished is of great interest. It is an effect of unequal growth of the different sides of the stem. If the unequal growth were confined to one side, the stem would eventually form a coil, or series of circles. But the region of greatest growth keeps shifting round the stem, with the result that the tip of the shoot describes a circle or ellipse, like the hand of a clock pointing successively in all directions. The stimulus is due, as in the case of the erect growth of ordinary stems (which usually display similar movements in a less degree) to gravity. Sometimes the movement, or _nutation_, is in the same direction as that of the hands of a clock (_e.g._, in the Hop); more frequently it is in the opposite direction, as of a clock-hand moving backwards. The result of this movement is that if the shoot encounters, say, an upright stem, it will lap round it in a spiral manner, and unless the said stem be quite smooth and unbranched, the twining shoot will be eventually supported by it. How effective the twining habit is as regards economy of building material may be seen