The Lolium multiflorum roots were studied. The plants were taken from the research field of the Dublany Experimental Station of the Lviv State Agrarian University. Before sowing the seeds, a solution of CdCl₂ in amounts of cadmium 0 (control), 3, 15, 30 mg kg^-1 soil was added to the soil (humus forest-steppe, leached, formed from loess, with pH = 6.8). Plants were harvested at flowering stage and at full maturity of seeds. The apparent surface area of the roots of the plants were determined from adsorption isotherms of water vapour. Measurements of adsorption-desorption isotherms of water vapour were carried out according to the Polish Standard PN-Z-19 010-1. To describe the experimental data of adsorption-desorption of water vapour the BET equation model was used. The apparent surface area of the roots in the flowering stage and in the maturity stage was studied. Under the influence of Cd⁺² ions the apparent surface area decreased. The apparent surface area of the roots in the flowering stage decreased significantly under the effect of Cd⁺² in the concentration of 15 and 30 mg kg^-1 while the apparent surface area of the roots in the maturity stage decreased under the effect of Cd⁺² in the concentration of 30 mg kg^-1. Cadmium added to soil at 3 mg kg^-1 soil caused only a slight impact, reducing the value of the test surface of the roots of ryegrass. No matter what stage of development, cadmium concentrations used in the experiment had a similar influence, causing a decrease in the apparent surface area of the roots of Lolium multiflorum. Doses of cadmium added to soil in an amount of 30 mg kg^-1 resulted in a greater relative changes in the size of the root surface of the older roots (full seed maturity phase) than of roots harvested in the flowering stage of the plant. The intensity of changes in the apparent surface area is likely to depend not only on the resistance of plants and the concentration of stressor but also on the exposure time of plants to stress. The observed changes in apparent surface area of the roots of Lolium multiflorum are probably associated with changes in physiological processes that occur in the plant under the influence of prolonged stress.