Cavity formation in the S2 layer of the secondary cell wall by soft rot fungi is initiated following the establishment of a T-branch by the extension growth of a proboscis hypha. This growth is inhibited after a time and lateral dissolution of the cell wall takes place to form a cavity and enlarge the hypha within it until cavity and hypha are large enough or mature enough to overcome the inhibition to extension growth and a new proboscis hypha is formed and the process repeated. The pointed ends and restrictions to the cavities are brought about by the inhibition of enzyme activity at the ends of the proboscis hypha and the possible deposition of inhibitory material along the pointed ends of the cavity formed.