The cells abutting the vessels in the primary xylem of healthy melon exhibit the characteristics of contact cells. These cells also produce a protective layer and transfer-cell-like thickenings. The primary wall is partially if not totally hydrolysed at the level of the pits. After infection by Fusarium oxysporum f. sp. melonis Sn. ' H., the contact cells of resistant plants show a strong cytoplasmic activity leading to the growth of tyloses into the lumen of abutting vessels. Such tylosis formation occurs rapidly and is helped by the material stored in the ingrowths of the protective layer which is reorganised into the single wall of the tylosis. The primary wall bursts and disappears during tylosis formation.