Differences in the shape of ribs can be observed in transverse sections of leaf blades of Italian ryegrass (<i>Lolium multiflorum</i> Lam.) var. 'Szelejewska', a diploid, has ribs similar to triangles, and 'Kroto', a tetraploid, has ribs which are more rounded, dome-like. A coefficient expressing width and surface increase of the leaf blade in relation to unribbed leaves has been determined. This coefficient (W) was calculated from the ratio of upper, wavy part of the leaf blade (g), to its width W=g/l. For Italian ryegrass the coefficient is from 1.5-1.46 and thus the width and the surface of the upper leaf blade is increased on the average by about 30%. With basic fertilization (1N) this coefficient is smaller for the diploid form as compared to the tetraploid. An increased dose of nitrogen (3N) causes increase of the coefficient in the diploid and a decrease in the tetraploid. Thus the joint action of the variety and fertilization in respect to the examined character is not equally significant to the two varieties.