The mechanism of formation of fossil plant ‘pith casts’ has been investigated experimentally using a small laboratory flume tank. The extent to which filling occurs is dependent on stem length and diameter, and on the current velocity. Stem fills produced by sediment carried in suspension show a distinct structure of two wedges of sediment deposited at either end of the stem cavity. These wedges are deposited from small flow vortices which form when the flow around the stem separates, as the flow through the stem becomes restricted. Stems filled by bedload currents do not show such a uniform structure. The experimentally produced stem fills are comparable with fossil pith casts, e.g. Catamites. The formation of cortical casts of Stigmaria is not simply explained in terms of the processes operating in open-ended stem segments. This investigation of the mechanics of filling of hollow plant organs, and the structure of the fill produced, offers a basis for interpreting the sedimentary environment in which the fossil fills have formed, and leads to a fuller understanding of the mode of formation of such fossils.