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The silky hairs covering the whole surface of Lygeum spartum dispersal units are unicellular, unlignified epidermal structures whose length varies from 5 to 17 mm. The cell walls present a stratified texture after extraction of matrix material. Their composition depends on the geographical origin of the grasses. Polysaccharides extracted with boiling water represent approx. 18% of the cell wall dry matter in samples harvested in Algerian arid high-plateaus and only 3% in samples harvested in semi-arid coastal zones. In the latter case, hemicellulose and cellulose contents amounted to approx. 50 and 40%, respectively. Slightly lower values were found in high-plateau samples. Cinnamic acids were detected only in coastal region specimens which exhibited a higher mechanical resistance and a lower extensibility. These results support the existence of two distinct populations colonizing arid or semi-arid areas respectively. (C) 1998 Annals of Botany Company.