Introduction

The xylem of plants has three major functions: transport of water and minerals, mechanical support of the plant body, and storage of water and nutrients. Since vines depend upon other plants (or trellises) for mechanical support of their plant body, the mechanical demands on the stem xylem of vines are much reduced compared with the situation in free-standing growth forms.

Vines in general have thin stems and a high ratio of supported leaf weight to transverse stem area (Schenck, 1893; Hallé, Oldeman & Tomlinson, 1978; Putz, 1983; Ewers, 1985). Putz (1983) showed that although lianas (woody vines) made up only 4.5% of the total above-ground biomass of a rainforest in Venezuela, they constituted 19% of the total leaf area.

In vines without secondary growth, the original primary xylem system must supply the leaves with water and minerals throughout the life of the plant. In contrast, most liana stems can add further xylem for water transport as the plant ages. Old liana stems can, in some instances, become quite wide. For instance, there is a report of a 433-year-old stem of Hedera helix that was over 60 cm in diameter (Schenk, 1893). Similarly, The Carpinteria Vine, a 51-year-old individual of Vitis vinifera, had a trunk that was over 80 cm in diameter (Winkler et al., 1974).

Despite these extreme examples, the present evidence suggests that liana stems normally have much lower rates of secondary growth than do trees or shrubs.