114 PLANT STRUCTURES 
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. Some- 
times support is obtained by twining (compare Con- 
volvulus, 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 move- 
ments 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 fre- 
