Behavioral and Morphological Adaptations of a Tropical Plant to High Rainfall

Abstract

Machaerium arboireum (Leguminosae), a liana of Panamanian lowland forests, sheds water from upper leaflet surfaces through thigmonastic and nyctinastic leaf movements. Experiments show that leaflets with acuminate tips have an improved water-shedding ability. A STUDY WAS CONDUCTED to test the effects of leaf shape and leaf movement on shedding of rain water by juvenile plants of Machaerium aorboreum (Jacq.) Vogel (Leguminosae), a liana of Panamanian lowland forests. The leaflets have acuminate tips (driptips), and exhibit both thigmonasty (movement in response to tacile stimuli) and nyctinasty (diurnal movement patterns). Thigmonasty also occurs in Mimosa pudica L. (Leguminosae), the sensitive plant (Roblin 1976), and many other tropical legumes The leaflets of Mimosa fold within a few seconds following gentle stimuli. Raindrops initiate thigmonastic responses in other tropical plants (von Denffer et al. 1976). The functional significance of this response has not been determined. Nyctinastic leaves may protect meristematic tissue from frost damage in some high-elevation plants (Smith 1974), but their adaptive significance in low-elevation tropical plants is undear (Darwin 1881, Satter and Galston 1971, Schwintzer 1971). Jungner (1891) was one of the first to regard drip-tips as an adaptation for rapid draining of the leaf surface. He observed that leaves with drip-tips dried more quickly, and were less overgrown with epiphyllae, than leaves without drip-tips. He believed that by facilitating water runoff, drip-tips kept the leaf surface relatively clear of spores of epiphyllae, and soluble substances which might encourage epiphyllae growth. Stahl (1898) showed that by removing the drip-tips he could increase the time required for a leaf to dry, but he doubted there was any correlation between drip-tips and the presence or absence of epiphyllae. He instead proposed that driptips functioned to prevent a lingering surface film of water, which would lower the surface temperature of the leaf, which in turn would slow the rate of transpiration by the plant. A low rate of transpiralPresent address: Department of Zoology, University of Iowa, Iowa City, Iowa 52242, U.S.A. tion, he reasoned, would slow the uptake of mineral salts by the roots, which would be disadvantageous to the plant. Shreve (1914), working in the understory in the montane forest of Jamaica, was unable to detect any difference in the rate of drying in leaves with driptips removed versus those with drip-tips intact. Richards (1966) maintains that Shreve worked with plants with atypical, abbreviated drip-tips, and that his results do not therefore disprove the hypothesis that drip-tips have a drying function. According to Richards and to Edmisten (1970), the controversy about the ecological significance of drip-tips remains unsolved.