The simplest case of modified circumnutation is that offered by climbing plants, with the exception of those which climb by the aid of motionless hooks or of rootlets:  for the modification consists chiefly in the greatly increased amplitude of the movement.  This would follow either from greatly increased growth over a small length, or more probably from moderately increased growth spread over a considerable length of the moving organ, preceded by turgescence, and acting successively on all sides.  The circumnutation of climbers is more regular than that of ordinary plants; but in almost every other respect there is a close similarity between their movements, namely, in their tendency to describe ellipses directed successively to all points of the compass—­in their courses being often interrupted by zigzag lines, triangles, loops, or small [page 265] ellipses—­in the rate of movement, and in different species revolving once or several times within the same length of time.  In the same internode, the movements cease first in the lower part and then slowly upwards.  In both sets of cases the movement may be modified in a closely analogous manner by geotropism and by heliotropism; though few climbing plants are heliotropic.  Other points of similarity might be pointed out.

That the movements of climbing plants consist of ordinary circumnutation, modified by being increased in amplitude, is well exhibited whilst the plants are very young; for at this early age they move like other seedlings, but as they grow older their movements gradually increase without undergoing any other change.  That this power is innate, and is not excited by any external agencies, beyond those necessary for growth and vigour, is obvious.  No one doubts that this power has been gained for the sake of enabling climbing plants to ascend to a height, and thus to reach the light.  This is effected by two very different methods; first, by twining spirally round a support, but to do so their stems must be long and flexible; and, secondly, in the case of leaf-climbers and tendril-bearers, by bringing these organs into contact with a support, which is then seized by the aid of their sensitiveness.  It may be here remarked that these latter movements have no relation, as far as we can judge, with circumnutation.  In other cases the tips of tendrils, after having been brought into contact with a support, become developed into little discs which adhere firmly to it.

We have said that the circumnutation of climbing plants differs from that of ordinary plants chiefly by its greater amplitude.  But most leaves circumnutate [page 266] in an almost vertical plane, and therefore describe very narrow ellipses, whereas the many kinds of tendrils which consist of metamorphosed leaves, make much broader ellipses or nearly circular figures; and thus they have a far better chance of catching hold of a support on any side.  The movements of climbing plants have also been modified in some