The role of cellulose microfibril orientation in determining cell wall mechanical anisotropy and in the control of the wall plastic versus elastic properties was studied in the adaxial epidermis of onion bulb scales using the constant-load (creep) test. The mean or net cellulose orientation in the outer periclinal wall of the epidermis was parallel to the long axis of the cells. In vitro cell wall extensibility was 30-90% higher in the direction perpendicular to the net microfibril orientation than parallel to it. This was the case for the size of the initial deformation occurring just after the load application and for the rate of time-dependent creep. Loading/unloading experiments confirmed the presence of a real irreversible component in cell wall extension. The plastic component of the time-dependent deformation was higher perpendicular to the net cellulose orientation than parallel to it. An acid buffer (pH 4.5) increased the creep rate by 25-30% but this response was not related to cellulose orientation. The present data provide direct evidence that the net orientation of cellulose microfibrils confers mechanical anisotropy to the walls of seed plants, a characteristic that may be relevant to understanding anisotropic cell growth.