Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Tendrils are similar to the petioles of leaf climbers in that they are sensitive to touch and respond by twining round a support. They differ in that they are derived from plant organs — branches, inflorescences, leaves or leaflets — which have completely lost their original function and are used solely for climbing. Further, once a tendril has attached to a support, it coils into two opposed helices in its free part, which increases its elasticity and also draws the stem closer to the support.
In New Zealand we have only one forest liane which climbs by tendrils. This is the native passion vine, Passiflora tetrandra, which ranges through the North Island and down to Banks Peninsula on the east of the South Island. The leaves are dark green and shiny and drawn out to a point at the tip. The flowers are much smaller and less colourful than those of the cultivated species and less elaborate in their form, but the fruit compensates for this by being bright orange and 2-3 cm in diameter; it is greatly sought after by birds.
The tendrils arise in leaf axils and are considered to be modified inflorescences. They are at first erect then bend downwards; if they encounter a slender support they wind round it. The part in contact gradually becomes thickened, until it is about twice the diameter of the free part of the tendril. The native passion vine is most common in the lower marginal parts of forests, but it spreads so effectively over the forest roof that it frequently reaches the tops of taller trees. The woody stems can be up to 12 cm in diameter and in their lower parts often form tortuous coils on the forest floor.