Abstract: A treatment plant for web material which, when bent, exhibits stiffness transverse to the direction of bending. The treatment plant comprises a series of blow boxes arranged in sections with the boxes in each section having parallel plane perforated surfaces on opposite sides of the web path. The perforations are graduated in degree with a high degree of perforation on one side of the web path being opposed by a low degree of perforation on the other side of the web path, and vice versa, so that the web assumes a wave shape between the first and second planes, the wave staying with its nodes in definite positions closer to the one plane opposite the highly perforated group in the second plane and closer to the second plane opposite the highly perforated group in the one plane.