Water loss from roots back into drying soil is a problem of practical importance in plants growing under conditions of very low substrate water potential, such as dry or saline areas. Root exodermis is relatively impermeable and has been suggested to play a protective role against water loss. The relative water retention ability was compared in root segments from exodermal (maize, onion, sunflower, Rhodes grass and sorghum) and non-exodermal species (Pisum satiaum, Vicia faba and wheat). Apical and basal segments from exodermal roots, with dierent degrees of exodermis development, were also compared, as were segments from sorghum roots in which the exodermis thickness had been modified by subjecting the plants to a 30 d water stress treatment. Water retention was significantly higher in segments from exodermal roots. In each root, water loss was higher in apical than in basal segments, regardless of the presence of exodermis. In sorghum, prolonged drought treatment increased exodermis thickening in nodal roots, however, no dierences in rates of water loss were observed in segments obtained from control and droughted plants. Soil sheaths formed around roots of Rhodes grass growing in very dry soil with the epidermis adhering tightly to the sheath. In plants growing in the field, soil sheaths may be more eective than the exodermis in preventing root water loss. # 1999 Annals of Botany Company