Abstract
Plants in the Orchidaceae (the orchid family) have evolved in diverse and physically stressful habitats (Benzing et. al, 1982). Epiphytic orchids, which grow on other plants in tree canopies, are especially susceptible to dehydration because their roots are on bare branches or in leaf litter rather than actual soil. In seasonally dry rainforests these epiphytic orchids face the challenges of water stress and nutrient acquisition. In response to these constraints the aerial roots of epiphytic orchids have developed a specialized spongy ?multilayered epidermis? called the velamen. The velamen is formed from epidermal cells and is extremely porous allowing rapid water movement towards the exodermis, where water can next enter the cortex (Figure 1). This structure assists with water uptake and is speculated to aid in water retention and possibly nutrient acquisition. In order to determine the role of the velamen in water uptake, aerial roots of various orchid species with the velamen intact and with the velamen removed were tested for saturation and water loss. The number of velamen layers was also compared to leaf thickness, an index of shoot desiccation tolerance. Hand-cut sections were examined microscopically for a variety of orchid species from Costa Rica and a shaded greenhouse at Occidental College in order to determine if there is a correlation between leaf thickness and the number of velamen layers
