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The physiology and growth of Lygeum spartum and Stipa tenacissima, two perennial tussock grasses which dominate wide areas of semi-arid south-eastern Spain, were compared at times of high and low availability of water, in autumn and summer respectively, to study the adaptation of this growth form to arid environments. The two species differed in morphological and physiological trails. Stipa tussocks were larger and had opportunistic growth, and Stipa leaves had a smaller specific area and lower diffusive conductance to water vapour. The two grasses were similar in maximum photosynthetic rate, leaf nitrogen concentration, and in the response to high light conditions but had different tissue water relations. Stipa was better suited to cope with drought and erratic rainfall, because of its more effective control of water loss and its growth patterns. Lygeum seems to be adapted to less dry conditions and more saline soils. The tussock growth form provides an adaptive advantage in these infertile environments by reducing radiation absorbance. (C) 1996 Annals of Botany Company