Stilt Roots and Growth of Arboreal Palms

Abstract

Patterns in the of palms support the hypothesis that stilt roots enable arboreal palms to produce an axis early in their development. The height of stilt roots is positively related to axis diameter for Socratea durissima and Iriartea gigantea. Both species achieve greater heights per unit diameter earlier in development than the non-stilt root species Welfia georgii; however, at maturity all three palms exhibit similar relationships of height and diameter. Stilt root production and axis elongation are more rapid for S. durissima than for I. gigantea. Among immature palms, W. georgii has a greater percent of biomass underground than the stilt root species. Comparing the stilt root species, S. durissima produces a lower percent of leaves but a higher percent of stilt roots than I. gigantea. THE GROWTH OF PALM STEMS iS characterized by a lack of secondary growth, a result of the absence of a vascular cambium. Growth is confined primarily to the apical meristem. The adult diameter is produced by a stepwise process, termed establishment growth by Tomlinson and Zimmermann (1966), whereby successive nodes are progressively wider through gradual ontogenetic change. Therefore, the developing axis, devoid of secondary vascular tissue, assumes an obconical shape that is necessarily mechanically unstable. In most arboreal palms (e.g., Welfia and Washingtonia) this instability is obviated by initial development of the axis below ground (Halle et al. 1978). Consequently, arboreal palms spend years locked in the understory as a rosette while developing an axis of sufficient diameter to produce a stem capable of arboreal support (Putz 1983). Our intention is to demonstrate that one group of arboreal Arecaceae, the stilt root or iriarteoid palms (Moore 1973), exhibit a fundamentally different pattern. The obconical axis begins elongation very early in life, often immediately after germination. As a result, the diameter of the axis increases along the height of the stem. Instability would result were it not for the development of adventitious roots that form a cone of stilt roots. Thus, some arboreal palms may achieve more rapid vertical early in development. In order to verify these two different developmental patterns, stilt root versus rosette, we measured parameters on palms of different sizes.