Abstract: A foldable cell structure is formed from water-permeable flexible strips whose width represents the height of the cells and which are connected to adjacent strips at intervals. An integral part of said cell structure is formed by strong stabilizing bands which, in the unfolded state of the cell structure, are arranged in straight lines and are at least predominantly parallel to one another, the cell structure preferably being formed by the unfolding of a water permeable sheet material provided with off-set, mutually parallel incisions of finite length. A further preferred embodiment of the cell structure has some of the water-permeable strips replaced or supplemented by stabilizing bands.