Twining Leaf Petiole Climbers

It might be wondered why the fern Lygodium, with its twining leaves, is not included here. Leaf climbers are defined as plants whose stems are supported as they grow upwards by the sensitive petioles (stalks) of their otherwise unmodified leaves, which wind round any slender supports with which they make contact. In Lygodium the true stems remain on the ground, but the primary axis of the leaf is like a twining stem in its indefinite growth and indefinite ability to twine. Indeed it takes a botanist to appreciate that with Lygodium we are dealing with twining leaves rather than twining stems, so it seems more realistic to treat it as a twiner rather than a leaf climber.

New Zealand representatives in this category are all species of Clematis, a genus which is widespread in temperate regions and also found in the montane tropics. The best known and largest New Zealand species is Clematis paniculata, which is found in lowland forests throughout, particularly marginally, and is greatly appreciated in the spring when its sprays of large, pure-white flowers stand out against the dark foliage of the forest. The adult leaves are divided into three leaflets, which are broad, dark-green, and smooth-margined. Leaves on young plants on the forest floor are very different. The first leaves are long, narrow, membranous in texture and undivided. These are succeeded by compound leaves with three narrow leaflets. In subsequent leaves, the leaflets become deeply lobed, broader, and, where light is adequate, gradually trend to the adult form. Similar juvenile leaves have been reported for Clematis species in eastern Australia. Seedlings rotate in an anticlockwise direction and are able to twine around slender supports. Similar twining ability, at least when young, has been recorded for some other leaf climbers outside New Zealand; nevertheless the primary mechanism for climbing is the clasping leaf petiole.

When a leaf is formed at a stem apex it is first erect, then gradually bends downward until it projects at right angles to the stem. The petioles of the leaf as a whole, and of the leaflets, are well developed at this stage and if any of them touch a suitable support they are stimulated, by a process not yet understood, to wind round it. The portion of the petiole in contact with the support enlarges and becomes page 62strengthened. Clearly the leaves cannot attach to large supports, so where a large stem of Clematis paniculata up to 10 cm in diameter ascends to a tree crown 10 m or more above the ground, it must have attained that position via smaller trees and shrubs.

The four or five other climbing New Zealand species of Clematis are smaller than C. paniculata. They are incompletely known and in some cases not yet clearly defined. Several grow at forest margins, including C. forsteri and C. foetida, and some also grow in shrubland.